Lawn mower engine stuck. How To Troubleshoot a Lawn Mower That Won’t Pull
How To Troubleshoot a Lawn Mower That Won’t Pull
If yes, it must have kept you wondering whether it is possible to troubleshoot such failure, and, if possible, you must have wondered how to do so.
First, measure the cord length and circumference of the winder, wind the cord 3 times, and test. Look for blade obstruction and recoil starter failure. Take the necessary action.
If you follow the instructions thoroughly provided in today’s article, you can easily fix your mower. So, learn how to troubleshoot a lawn mower won’t pull.
What Causes the Problem?
Your lawn mower might seem hard to pull or won’t pull due to wrong cord adjustment, stuck rope, or the blade dragging on the ground.
In most cases, simple cord adjustment works perfectly to solve the hard-to-pull problem, but in some cases, other faulty parts also lead to experiencing similar issues.
A lawn mower becomes hard to pull due to blade obstruction, disengaged spark plug wire, or recoil starter failure.
Steps To Troubleshoot A Lawn Mower That Won’t Pull:
Encountering such commotions every once in a while, is very common for every lawn mower user, but it’s still annoying to deal with such problems out of the blue.
If you follow the available online consumer forums, blogs, and YouTube vlogs, you will find several DIY ideas to fix such problems. Unfortunately, not all of those ideas are reliable, and most of them are not even confirmed by the users whether those fixing techniques worked or not.
Thus, I have enlisted an easy-to-do yet effective troubleshooting method to fix the problem quickly when your lawn mower fails to pull:
Mower Cord won’t pull
Things you will need to troubleshoot the lawn mower won’t pull:
Look at the checklist of the required tools or materials:
| Tools |
| ¼ Nut driver |
| Measurement Tape |
| Screwdriver |
| Matchesbox |
Step 1- Measure the length of the Cord:
First, remove the recoil assembly with a ¼ nut driver to fix the lighters.
You will see a couple of quarter-inch screws so, carefully excuse them with the nut driver.
Now you can lift the upper cover off and wind it back to correct tension, you need to know the accurate length of the cord.
If you see, you will find the cord mounted up on your lawn mower handle just like most mower models.
Tip: Remember that while you are calculating the cord length, you are not going to wind the entire cord.
If you measure the cord length the way it was done in the earlier picture it will measure around 85 inches (it can differ from model to model). The entire cord length might be 35 inches.
Step 2- Measuring the Winder Circumference:
Next, you need to know the winder’s circumference, and to measure it use a measurement tape shown in the picture.
You might see a 17 and three-quarter measurement in there (it can also vary depending on your mower model and size).
Let’s do the calculation properly before proceeding further:[Overall winder – Outside winder] = 50/Circumference (17).So, our result will be 2.9 which means the wind is three times three ones.
Step 3- Winding the Cord:
Now you need to wind the cord three times. But first, get the hole lined up outside the exit and wind the cord three times by using your hand.
Next, insert a screwdriver to lock it up and hold it there.
After that prepare the cord end by cutting the damaged or worn end.
Tip: Melting the cord end will keep it safe from fraying and will help to feed through there too if it’s nice and flat
Now, thread the cord end through from the outside to the hole, which will look like threading a needle in.
When the cord gets fully through the hole line, make a simple knot by hand.
Next, put the recoil assembly again in place securely. Let’s try to wind again and see whether the cord is responding or not.
Step 4- Test the Cord:
If the three winds do not seem enough, you have to take it back out and again just wind it. Back up all the way and jam the screwdriver in for the cord lined up.
Next, undo the simple knot and let it go back. Also, take off the screwdriver line that you used before.
You can do it again and this time you can try 4 if three seem too much slack. So, this time follow the same procedures to wind 4 times.
Now mount the cord first in its original place on your mower. Then, just swivel it around and line up the holes.
Grab a quarter-inch nut driver and reinsert the nuts or connecting screws in both slides securely.
Now you need to pull the cord couple of inches before its agency engine and let’s check out the cord
Step 5- Blade Obstruction Recoil Starter Failure:
Next, you should check the condition of your mower’s blade.
If any strands of grass or debris get trapped in there, it will prevent the blade from spinning due to obstruction between the mower deck and the blade.
If you do not clean the obstruction out of the way on time, it will eventually cause the pull cord to get stuck.
To fix the blade obstruction trouble, thoroughly follow the Correct way to Tip a Mower for Maintenance.
If the blade is not the main culprit of your issue, check the recoil starter, which might be worn out or damaged.
In that case, open the blower housing first to inspect the recoil starter more closely. If it appears faulty, you need to replace the Recoil Starter.
Besides that, watch this video to learn how to fix the recoil starter easily.
Undertaking such maintenance can be hazardous. Thus, always remember to read your lawn mower’s instructions manual first before operating, servicing, or troubleshooting it. Make sure you have the minimum expertise to do this task alone.
Frequently Asked Questions: lawn mower won’t pull
Why does my lawn mower pull cord not catching?
The main reason behind a lawn mower cord catching failure is the failed flywheel starter assembly components like the broken/ worn out pawls or a damaged pulley system. They mostly fail or turn defective due to regular stress of use.
What causes the starting failure of my lawn mower pull start?
Such failure might occur due to a loose, dirty, or disconnected Spark Plug and dirty Air Filter. It can also arise if fuel is not reaching the mower engine properly and causing the motor to starve for fuel.
Why can’t I pull the string on my lawn mower?
Excess crankcase oil and storing the mower while it’s standing on its front wheels can cause the oil to bleed into the piston cylinder. This action will restrict the movement of the piston, and the piston will fail to move.
As a consequence, the crankshaft will not turn, and you won’t be able to pull the rope.
What causes the cord to get stuck on my lawn mower?
Such commotion can arise due to problems with the Recoil Mechanism. The pull cord sometimes crosses over itself while rewinding and gets stuck.
How to determine if my lawn mower engine is locked up?
Some obvious signs can help you to determine when your lawn mower engine is locked up or seized. You will see symptoms such as motor sounds rough, hard to start, stuck blades, the piston, as well as cylinders won’t move, insufficient or old oil in the tank, and deteriorated fuel.
Final Verdict
Now that I have explained the simplest way to troubleshoot your lawn mower that won’t pull, you can easily get your mower back in perfect mowing condition.
But remember, you can only get the best result if you thoroughly follow all the instructions without skipping a single detail.
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Lawn Mower Engine Surging – Check this easy fix first!
Lawnmower engine surging is a right pain in the Jacksie; it’s an engine that runs erratically and revs up and down by itself uncontrollably. In some cases, it may only happen under certain conditions, for example, only after the mower gets hot or only when the fuel level gets low.
So what causes the lawnmower engines to surge? The most common reason for a surging lawn mower engine is a blockage in the fuel supply, but there are other possibilities:
Often you’ll find playing around with the throttle helps or applying some choke. You are not on your own; this is a regular complaint. In this guide, we will cover the diagnosis, likely causes, and solutions.
Try the easy fix first – replacing/cleaning gapping the spark plug before attempting carburetor work. If your mower engine is a Honda or Kohler, the fix is simple. Honda and Kohler’s surging is commonly caused by a blocked idle jet see “Gas starvation” towards the end of the page.
If you need more help, check out the “How to fix a surging mower video.”
If your surging mower is a Honda, check out the “Honda mower surging video.”
For many mowers, the fix is to replace the carburetor, and as carburetors are inexpensive, it just makes sense to swap it out and save a ton of messing around. You can check out the quality carburetors available and conveniently delivered to your door by Amazon.com.
Need more info on the fuel system, carburetor components, and how they work, you can check them out here.
Briggs Stratton Surging
Surging BS Classic 450, 500, or 550Some engine types are famous for surging; the Briggs Stratton 450, 500, and 550 series engines are fitted with a metal fuel tank and priming bulb-style carburetor. If you have one of these types of engines and it’s surging – You’re in the right place.
If you don’t have this type of carburetor, skip this section and jump to “Surging Test” below. These engines are fitted with a metal fuel tank and carburetor combination. The gasket sandwiched between the tank, and carburetor distorts over time, allowing a vacuum leak.
The vacuum leak causes the surging; replacing the gaskets and cleaning the carburetor/tank will leave it like new, I promise. In this tutorial, we’ll remove the tank/carburetor unit, clean it and replace the gaskets. Just some basic tools are needed, but get yourself a can of carburetor cleaner; it makes the job a lot easier.
In the workshop, I use WD40 carb cleaner, and you can check out all the tools and parts I use here on the “Small engine repair tools” page.
Tools You’ll Need
Here’s a short list of tools you’ll find useful to complete the task of fixing your surging mower. These tools aren’t essential, but they do make the whole job a ton easier; you’ll need:
Fuel treatment – Every small engine owner should use gas treatment. Most people don’t know gas goes off, and gas left in small engines can cause real problems, as you already know.
Using a gas stabilizer will keep the gas in your mower and your gas can fresh for up to two years.
Carburetor gasket – If you’re fixing the BS Classic engine, then you’ll need this gasket set.
Complete carburetor – As an alternative to replacing your BS Classic carburetor gasket, replace the complete carburetor instead; it includes the replacement gasket.
Manifold – This will only be needed if you have confirmed it has failed. Note there are a few different types of manifold pipe, so be sure to check before ordering.
You can check out all these tools on this page “Carburetor Surging Repair Tools.”
This carburetor style is fitted to a few engines and is prone to gasket failure. The job of replacing is simple and will solve the surge. The process is as follows:
Remove the spark plug wire – prevents the mower from starting.
Remove – Remove and clean the air filter and filter housing – Clean it using soapy water, and when dry, smear some engine oil over the surface of the foam. This helps trap dirt.
Remove tank bolts – They hold the fuel tank to the engine.
With fuel tank bolts removed – pull the tank unit straight out gently and remove the governor control link.
Remove the black rubber elbow crankcase breather pipe. Remove the manifold seal and keeper ring. Sometimes they will come loose and get stuck on the manifold pipe.
Remove – Remove carb screws from the carburetor and set aside.
Using a can of carburetor cleaner – clean all the ports on the surface of the fuel tank.
Empty the tank and rinse it out with fresh gas.
Pull the Siphon from the carburetor; they can be stubborn. Remove both gaskets and use carburetor cleaner to clean the siphon metal filter and all ports of the carburetor. Check the primer bulb for damage; mice like to eat them.
Spray – Spray the carburetor with carb cleaner.
Remove – Remove old gaskets and discard them.
Careful of this spring; it lives under the gaskets, and it can drop off and be tricky to find, as I know only too well.
The gasket is a two-part kit; the rubber-type gasket faces the tank. (carb fitted here for demo only)
The Siphon pushes back into the carb with a click. If you don’t hear the click, it’s not right – try again.
Refit the carburetor to the tank. Don’t over-tighten the screws, as this will distort the gasket. Fit manifold seal and keeper. Smear a small amount of oil on the seal; it helps it seat.
Clean the intake manifold. The grey tube in this shot is manifold. Inspect it for any signs of damage; they are prone to cracking. This will also cause a surge.
To fully inspect the pipe, you need to remove the pull assembly.
I would only do this if there was obvious damage to the manifold or if I had replaced the carburetor gasket and the engine was still surging.
This manifold is cracked and will cause a surge.
Before refitting the tank, fit the keeper ring and O-ring seal. Lube the seal before refitting the gas tank.
Offer the carb/tank unit up to the manifold and attach the governor link and spring. Now push the unit firmly onto the manifold. Fit both bolts.
Refit the air filter and spark plug wire. Use only fresh gas; make sure your gas can is clean. Gas older than three months is stale.
If, after fitting the gaskets, you still have a surge – Replace the Manifold.
Surging Test
As you know, gas starvation causes an inconsistent flow of fuel which in turn causes erratic running. And you also know a vacuum leak will cause erratic running, but it is a much less common cause; however, some carburetors are prone to vacuum leaks.
As engine manufacturers strive to make their engines more efficient, they have also made the carburetors more likely to clog; this has become a common issue.
To quickly diagnose which problem you have, a clogged carb or vacuum leak, follow this simple test.
You will need a helper to hold the bail lever or improvise with duct tape. CAUTION careful where you place your fingers and toes; the engine will be running, so the blade will be spinning.
Your mower will have a Manual choke, Auto choke, or a Primer bulb. Identify which type your mower has; the test is slightly different for each.
If you have a manual choke – apply half choke with the engine running.
If the engine now runs without surging – Gas starvation is the likely fault. If it runs just the same – A vacuum leak is a more likely fault.
If you have an Auto choke – Remove the air filter cover and filter – place a clean rag over the intake while the engine is running.
If the engine now runs without surging – Gas starvation is the likely fault.
If the engine runs just the same – A vacuum leak is the more likely fault.
If you have a primer bulb – you can still do the test – while the engine’s running (need a helper); give it some extra gas by pressing the bulb.
If the engine now runs without surging – Gas starvation is the likely fault.
If it runs just the same – A vacuum leak is a more likely fault.
Gas Starvation
If the test revealed gas starvation, it also showed that your problem is likely a dirty fuel jet in the carburetor, or the gas may be stale or contaminated by water.
Cleaning the main jet usually does the job.
Idle Jet Surging – Honda and Kohler use a relatively easy-to-access idle jet that clogs up and causes surging. The Kohler is easier to access than the Honda.
The Kohler is easier to access than the Honda.
Briggs has fitted a plastic carburetor to a range of engines which also clog up and cause surging.
All of these carburetors can be repaired by cleaning, which I’ve covered previously in videos (links below). Videos cover step-by-step removal, cleaning, refitting, and adjusting your carb.
Fixing this is not difficult. Sometimes you can get lucky by just draining and cleaning the gas bowl, which only takes a few minutes.
I have written a complete guide to Carburetor cleaning with pictures; it includes the gas bowl clean-out, which, as said, is worth trying first.
If cleaning doesn’t work out for you, go ahead and swap out the carburetor for a new one. Check out “New lawn mower carburetors page,” here, I’ve listed good quality replacement carburetors for all the most popular engines.
Carburetors aren’t expensive; messing around with them doesn’t make sense.
You might find this page helpful too – “Carburetor repair tools” I’ve listed some really useful tools that make the job easy. Some of these tools I’ll bet you already have some.
But do try cleaning the gas bowl before removing the carburetor.
Finding a Vacuum Leak
Air that enters the combustion chamber without passing through the carburetor is un-metered. This means the fuel-to-air ratio is unbalanced and, in turn, causes erratic engine performance.
When air sneaks in like this, it causes the engine to run lean (lacks gas). A lean engine runs hot, which isn’t good for an engine, especially an air-cooled one.
Vacuum leaks usually occur because of damaged gaskets. Gaskets are sealing materials fitted between the mating surfaces of engine components. Their function is to create an airtight seal.
They are commonly made from paper, felt, cork, Teflon, neoprene, metal, and rubber. The material type is dependent upon where the gasket is to be used.
Gaskets wear out and break down, and that causes surging.
Extreme Caution – You need to be careful, the engine will need to be running, and so the blade will be spinning when running this test.
A vacuum leak check is performed with the engine running and a can of carburetor cleaner; WD40 works, too, (is there anything WD can’t do?)
Spray the cleaner around all carburetor gaskets anywhere the carburetor meets the engine. The trick is to hear an instant change in engine note; that’s the sign of a vacuum leak.
This can be challenging; you must train your ear to notice the instant change in engine note (and not the surging).
Just do a small section at a time; this will allow you to pinpoint the failure area. Jumping the gun and replacing gaskets without finding the actual leak may work out for you or leave you with the same problem after the rebuild.
You’re right in thinking carburetor gaskets usually cause the problem, but other components, such as manifold pipes, can crack or become loose, causing surging.
Fixing A Vacuum Leak
If a leak is detected, replace all carburetor gaskets, and as you have the carburetor removed, go ahead and clean it. Replacement gaskets are available online; you will require the make and model numbers from the engine.
All manufacturers will have a model number printed on a sticker placed on the body or on the engine. Have a poke around; you’ll find it. Most engine manufacturers will stamp the model numbers in an accessible area. Briggs Stratton stamp their numbers on the metal engine cover.
A new carburetor comes with new inlet gaskets; I like to fit original parts where I can; they fit and are guaranteed.
If, after replacing the carburetor gaskets, the engine still surges, you’ll need to go a little further and replace the manifold intake and gasket.
It’s not a big job, and they don’t give a lot of trouble, but they do crack as they get older. I wrote a step-by-step guide showing you everything you need to know – “Briggs Manifold Replacing.”
Related Question
Honda lawn mower surging fix? To fix a surging Honda lawn mower engine, clean the carburetor, gas tank, and fuel filter. Use fresh regular gas or e10. What causes a lawnmower to run slowly? The most likely cause is a throttle linkage bent out of shape by bumping into the shrubbery or a throttle spring has detached itself.
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I’ve worked on all types of mechanical equipment, from cars to grass machinery, and this site is where I share fluff-free hacks, tips, and insider know-how.
And the best part. it’s free!
Lawn Mower Struggles To Turn Over: 6 Immediate Things To Check
Lawn mowers can occasionally struggle to turn over due to potential issues in the fuel system, filters (air fuel), or the spark.
If your riding lawn mower struggles to turn over, inspect the current state of the fuel, spark plug, filters, battery, starter, solenoid, and engine blades to either repair or replace the faulty elements.
Fortunately, it’s relatively easy and convenient to resolve when a lawn mower won’t turn over with simple DIY solutions.
Reasons Why Riding Lawn Mower Struggles to Turn Over
The most common reasons behind turnover failure are:
- Empty Fuel tank/does not have enough Fuel/Bad Fuel/Problem in the Fuel Flow
- Defective Spark Plug Lead
- Dead Battery or Battery Connection Error
- Clogged/Damaged Air or Fuel Filter
- Faulty Solenoid or Starter
- Seized-up Engine Blades
Check Fix Your Lawn Mower That Struggles to Turn Over:
Check these top six things before approaching a professional and consulting about your riding lawn mower struggles to turn over.
Checklist of the required tools and materials:
| Tools | Materials |
| Standard Mechanical toolkit | New Spark plug leads |
| Wrench | |
| Spark plug Gapping tool | Solenoid replacement kit |
| Wire brush | New Starter kit |
| Spark plug cleaner | New Battery Connectors |
| Spark plug wire tester/Multimeter | |
| Protective Gloves |
Bad Fuel or Fuel Flow
The first thing that one should check is the fuel condition of your mower.
If the fuel tank is empty or does not have enough gas, your mower will likely fail or struggle to start over.
You will witness a similar issue if you have not changed the fuel for a while, and it has been sitting for over 30 days.
A Similar issue can arise if fuel could not reach the engine or motor due to any interruption in the fuel flow system.
An unexpected increase in fuel consumption due to leaks in the tank can also lead you to encounter such an issue.
How to Fix
Diagnose whether you have old or bad gas in the fuel tank and replace it with an adequate amount of fresh gasoline.
Look for a leak presence in your fuel tank and fix it accordingly.
If fuel fails to reach your mower engine, tap on the carburetor to clear the flow line.
Inspect whether the carburetor is clogged or severely damaged, and clean or repair the carburetor as needed.
Defective Spark Plug Lead
A broken/damaged spark plug can drive your mower to struggle. Even loose, disconnected, or worn-out spark plug leads can also cause your mower to stumble.
How to Fix
Disconnect the spark plug lead and check the condition.
Clean the surrounding area if needed. Then, locate and remove the spark plug.
Now inspect the spark plug and if it looks darker and dirty, use a wire brush or a spark plug cleaner to clean it.
Note: Never use a shot blaster or abrasive to clean your spark plug.
Be careful not to damage the porcelain insulator around the electrode during cleaning.
Next, use a spark plug gapping tool to check the spark plug gap and if it needs a bit of adjustment, do that.
However, if the condition is too severe, replace the spark plug.
Also, check the spark plug lead with a spark plug tester or multimeter, and if required, replace the spark plug lead.
Battery Connection Error
A bad/dead battery, corroded battery links, or connections can also lead the mower to experience similar issues.
A similar problem will gradually occur if the battery cables or connector become loose from their connection.
How to Fix
Check whether you have left the key in the “on” position because a fully charged mower battery can discharge within 25 hours if you leave the key in the On (S) position.
Make sure the engagement lever is disengaged. Ensure that the PTO switch is in the off position.
If the lawn mower is still struggling, thoroughly inspect the battery and take necessary action.
If you notice loose or disconnected cable connections, securely tighten them. If the cables have worn out, replace them.
Clogged/Damaged Air Fuel Filter
Over time the air and fuel filters can get dirty or clogged due to abuse and excess built-up debris.
Damaged filters can prevent the mower engine from functioning properly. As a consequence, you will eventually witness turning over failure.
How to Fix
The air filter is the plastic or metal part residing near the mower’s engine compartment, but based on model design and brand, air filter location on a riding mower can vary.
So, verify the accurate location with the help of your mower’s user manual.
Based on the severity of the air filter, either clean or replace the air filters.
Similarly, remove the fuel filter to inspect its current state. If the fuel filter looks extremely clogged, or you see stretched/torn holes in the filter, replace the fuel filter.
Note: Fuel filters should be serviced and replaced annually to prevent such commotions.
Faulty Solenoid Starter
Even a defective solenoid and faulty starter can gradually lead you to encounter a similar problem.
Especially when you are sure there’s no issue with the fuel, filters, spark, and battery.
How to Fix
First, take a multimeter to perform a Volt DC test and a Continuity test of the solenoid to find out its current condition.
While testing the existing solenoid, if you can’t hear a click or notice that the solenoid ground looks bad, replace the solenoid.
Seized-up Engine Blades
After confirming that all the major parts are in perfect working state, including the battery and starter, if you still notice a struggle during turning over, it means the engine blades are badly seized up.
– How to Unseize Engine Blades
Get access and manually push on it until you are sure that the motor is free and bound up as required.
Remember not to push too firmly or hard. Otherwise, you might break the engine blades since those are plastic.
How to Turn over an Old Lawn Mower
Here is a simple and quick step-by-step tutorial to make your old riding lawn mower run like brand new:
Maintaining a beautiful lawn can be a daunting task, especially if you lack the appropriate know-how and tools to handle the challenges that may crop up. Fortunately, LawnAsk is here to offer you an all-encompassing resource that covers everything you need to know about lawn care.
Recent Posts
Learn how to identify a seized engine and steps to repair a small seized engine from Gary Grinnell’s guide.
Much as we like working on our old engines, there’s a limit to what many of us can handle, and a seized engine is pretty much that limit. But don’t throw in the towel if your engine is stuck, because a little insight and the right approach can often get that old iron spinning once again.
Solved! The Mysterious Case of the Honda Lawnmower That Wouldn’t Pull Backwards
Confirming a seized engine
Engines seize for any number of reasons; rust (usually from sitting too long), excessive heat (from running the engine without coolant or oil), or for some mechanical problem, such as a foreign object in the cylinder or a crankshaft bearing failure.
If you think your engine is seized, you need to confirm that before doing anything else. The first thing to do is remove the spark plug (s) and try rotating the engine. On the small engines most of us are working on you can bet that if you can’t rotate it by hand, it’s probably seized. On larger engines you’ll want to use a breaker bar to see if it will turn.
Assuming everything is okay with the crankshaft, the easiest and cheapest thing to try is penetrating oil. This works surprisingly well, particularly with engines stuck from years of sitting out in the rain, where water has rusted the piston rings to the cylinder.
Everybody has their favorite penetrating oil, but I like P’Blaster. Start by shooting oil down into the cylinder through the spark plug hole, and don’t be shy – use a lot of the stuff. Wear goggles in case the penetrating oil sprays back and hits you in the eye. If you have the patience for it, spray some in every day for about two weeks, followed by gently tapping on the cylinder wall with a hammer handle. This sets up vibrations that help the oil penetrate between the rings and the cylinder wall. After soaking the engine for a while try rotating the crankshaft. If you’ve been patient, and if the engine wasn’t too badly rusted, it will usually break free and rotate. If it doesn’t, hit it with more penetrating oil, and make sure you use enough. I’ve found that about half a can per cylinder is right. If it still doesn’t rotate after four weeks of daily treatment, it’s time to resort to more serious measures.
Pulling the cylinder apart
If penetrating oil doesn’t work it’s time to remove the cylinder head and, most likely, the crankshaft. On smaller stationary engines this is pretty straightforward, but on larger tractor engines you’ll be getting into quite a bit of work. On a multi-cylinder engine it’s often possible to discern which cylinder is stuck, sometimes just by looking into each cylinder. If you’re lucky and you can figure it out, remove the end cap on the connecting rod of the offending cylinder and then rotate the crankshaft so the crankshaft throw for that cylinder is out of the way. With the cylinder head off take a mallet and try and pound the piston out, placing a block of wood on top of the piston so you don’t bang up the piston with the mallet. Make sure the wooden block is clean – you don’t want to mar the top of the piston with sand or small rocks. Make sure the piston can slide out the bottom of the cylinder without smashing the crankshaft.you don’t want to make things worse than they already are. Most of the time this is the extent of removing a stuck piston, but if it’s still stuck it’s time to bite the bullet and head to a machine shop to have the piston pressed out or, even worse, bored out.
Assessing cylinder and piston damage
Depending on what caused the engine to seize, you may have damage to both the piston and cylinder. If you had to beat the piston out with a mallet there is a good chance the rings will be damaged. Piston rings are fairly brittle and will shatter if subjected to too much force. If the rings aren’t busted make sure they rotate freely in the piston grooves. Rings often get glued to the piston grooves by varnish or carbon deposits, keeping the rings from expanding against the cylinder wall and doing their job. Stuck rings can usually be freed with penetrating oil, but if they are broken you’ll have to replace them.
If you have to replace the rings, make note of where the ring end gaps are positioned, and if the piston uses different types of rings in each groove make sure to mark their type and position. Clean the piston ring grooves with a piece of an old piston ring, making sure to clean out any dirt or varnish from the ring grooves so the new rings won’t stick.
Take a good look at the piston itself. Clean it thoroughly and examine it under a light. Is it scored, torn, cracked or broken? Measure the piston at different points and compare the numbers with the specifications in a manual, if you have one. Clean the cylinder wall, oil the piston and try reinserting it in the engine. It should slide the length of the cylinder freely. Rotate the piston as you move it up and down – if it jams at any time you have a problem. Pistons can get distorted when they seize, and how you deal with this depends on how you are going to use the engine and how much money you want to spend.
The most remedial repair is to simply file off any metal that keeps the piston from working freely. Sometimes you can set the piston up and turn it on a lathe. The best bet, and the most expensive, is to simply replace the piston.
That said, be sure to measure and inspect the cylinder for damage before running out and getting a new piston. Clean the cylinder wall and shine a bright light down the cylinder – is there any visible damage to the cylinder wall? Minor scratches can be removed with a cylinder hone, but deep scores will require the services of a machine shop. Depending on how much metal has to be removed to return the cylinder to working order, you may need to put in an oversized piston or resleeve the cylinder. The sleeve is a new cylinder pressed into the hole where the old cylinder was.
You can measure the size of the cylinder with a telescoping bore gauge and a dial caliper or a micrometer. Be sure to take measurements at several locations in the cylinder (generally at the top, middle and bottom) and at 90 degree and 45 degree angles to each other. This will help you determine if the cylinder is out of round. A really worn engine will have an oversized bore, and it’s always good to check the manual for the acceptable dimensions for your engine. I realize that if you have an antique engine there may not be a manual, but often you can find a manual for a similar engine of similar vintage and work from there. Working tolerances, by and large, are fairly standardized.
Checking pin and rod bearings
This is also a good time to check the wrist pin and connecting rod bearings, as loose wrist pins or worn bearings will cause a rapping or knocking sound and will accelerate engine wear. Always replace worn or damaged bearings and wrist pins, and check the connecting rod for any damage, as well. Simply lay the rod on a straight edge or the top of a table and check to see if it’s warped. And check for cracks – rods can break, so look carefully.
Before installing the rings, make sure you know which way the bevels (if there are any) are orientated, and don’t forget that the gaps at the end of the rings need to be properly positioned. If the gaps are lined up in a row top to bottom you’ll lose compression as combustion gases escape through the gaps. Generally speaking, you want to space the gaps at 45 degrees to each other, starting at the top, trying to avoid any overlap.
New piston rings are surprisingly sharp, so wear gloves to protect your fingers. The rings have to be expanded to fit over the piston and into the groove, and because piston rings are brittle you need to open them slowly and carefully to keep them from breaking. A piston ring expander is the best option, but you can install piston rings by hand with just a little extra care. If you have any old, unbroken rings, practice with one of those first so you can get a feel for it.
Once the rings are installed, coat the piston and the cylinder with clean engine oil. Double check that there isn’t any dirt or grit in the cylinder befores installing the piston. Use a ring compressor (a large hose clamp will do just as well) to compress the rings into their grooves before inserting the piston into the cylinder. Don’t force a piston into the cylinder with the rings sticking out of the grooves. This will break the rings. Gently shove the piston into place, and then install the connecting rod bearings and end cap, making sure you didn’t get any dirt on the bearings. With everything back together try rotating the engine – it should rotate freely, and it will feel so good you won’t stop until your arm falls off.
Contact engine enthusiast Gary W. Grinnell at 9 Laurel Pak, Northampton, MA 01060-1196.