How to Install a Piston on a Trimmer

The main malfunctions of the Stihl ms 180 chainsaw are associated with the piston group. Consider the main reasons why the piston must be replaced. Let us give a description of the piston and the algorithm for its replacement.

How to Install a Piston on a Trimmer

When to replace a piston

The reasons why the piston needs to be replaced, as well as the completely piston one, can be quite a lot, but all of them can be divided into natural and resulting from improper operation. Natural, this is the wear and tear formed due to the long-term operation of the Stihl 180 chainsaw. Many more artificial ones, for example:

  • Engine overheating and the appearance of thermal seizures;
  • Getting into the cylinder of a foreign body and the appearance of mechanical scoring;
  • Broken piston ring as a result of exceeding the maximum engine speed;
  • Breakdown of the piston skirt due to excess speed;
  • The use of low-quality fuel, as a result of which the piston group of the chainsaw wears out from the inlet channel in the cylinder;
  • Operation of a Stihl chainsaw with a contaminated fuel filter, resulting in a reduction in the percentage of oil in the fuel mixture;

It is necessary to diagnose the CPG if the Stihl ms 180 has sharply lost power or will not start, while the tool starter does not crank. If the starter turns, but the saw does not start, while there is a spark and fuel flows into the cylinder correctly, it is necessary to measure the compression in the engine.

9.5 – 11 Atm. It is considered a normal indicator at which the Stihl chainsaw starts up regularly and gives out maximum power, if the compression is lower, there is a chance that there are scuffing on the piston-cylinder group or its wear is quite large. In any case, with reduced compression, for further diagnosis, it is necessary to completely disassemble the saw.

To determine the presence of seizure on the piston, as well as the breakdown of the piston ring, you can remove the muffler and conduct a visual inspection of the visible part of the piston, while the rings can be checked for elasticity by pressing on them with a small screwdriver. There will be slight walking on the working rings, the defective or pinched piston ring will not go.

Description of the piston group Stihl 180 chainsaw

On a Stihl 180 chainsaw, the piston group has a diameter of 38 mm. The dimension of the CPG of the chainsaw is determined by the diameter of the piston, the cylinder is slightly larger. As standard, the MS 180 CPG comes complete with a piston pin, two piston rings, and two retainer rings.

A distinctive feature of the CPG on the Stihl MS 180 is that it has two compression rings, while almost all of its main competitors have only one. According to the manufacturer, the installation of a pair of rings makes it possible to better stabilize the piston in operation, enhance its cooling and increase compression in the engine, thereby improving start-up.

How to change a piston on a Stihl 180 chainsaw

If you need to replace the old piston and install a new one, you need to know how to do it correctly and follow a certain algorithm of actions. It is also necessary to have a small slotted screwdriver, a special Stihl combination wrench, a power screwdriver, a hammer, a drift of soft material, a compressor and a stop for the Stihl piston.

Starting work on replacing the piston or piston group assembly, the chainsaw must be cleaned of dirt as much as possible. Further, the work is carried out according to the following algorithm:

  1. The top cover is removed from the saw and the spark plug is twisted.
  2. The side cover is removed.
  3. Using a power screwdriver, the stoppers are removed from the shock absorbers of the chainsaw, after which its handle is disconnected from the body.
  4. Next, you need to remove the drive sprocket and starter.
  5. Install the piston stopper in the spark plug hole and loosen the flywheel mounting nut and clutch.
  6. Remove the flywheel, clutch, and oil pump worm gear mechanism located behind the clutch.
  7. Loosen the two screws securing the carburetor and air filter. Remove the filter, engine control lever and Stihl carburetor.
  8. Remove ignition coil and silencer.

After all these operations are carried out, it is necessary to once again clean the saw from dirt, then turn it upside down and unscrew the four screws securing the engine to the chainsaw body. Unscrewing the screws, the engine is separated from the housing and cleaned again.

On the removed engine, the pan is untwisted and the pallet is removed, after which the piston assembly with the crankshaft is removed from the engine.

Then, using a small screwdriver, the piston pin retaining rings are removed and the crankshaft is disconnected from the piston.

In the case when it is necessary to replace only the piston, you can install it on the crankshaft, and then mount the lock rings of the piston pin, while arranging them so that the break point of the lock ring is directed downward. This is necessary in order to exclude the possibility of spontaneous popping of the retaining ring during engine operation (according to the manufacturer’s instructions Stihl). After that, compression rings are installed, the cylinder is lubricated with engine oil and the engine is assembled.

Video: How to Install a Piston on a Trimmer

The crankcase is mounted on the engine, sealant is used as a gasket, after which four mounting screws are mounted and tightened crosswise. Next, the engine is mounted on the body of the chainsaw. Subsequent assembly is performed using the reverse disassembly algorithm.

Why Stihl chainsaw does not start after replacing a piston

If the saw does not start after replacing the piston, it is necessary to check the fuel supply to the chamber, as well as to diagnose the ignition system. Very often, when installing the control lever on a chainsaw, masters forget to connect the wires coming from the ignition coil, as a result of which the spark disappears and the saw does not start.

Also, the problem may be hidden in the pinched fuel supply hose or if it is not connected at all. Of course, there can be many reasons, but it is worth remembering that if all work on replacing the piston is carried out correctly, the tool should start and work normally without any problems.


On the video you can see a detailed video instruction, in which the master showed in all details how to properly disassemble the saw and install a new piston on it. Also in the video you can see how to properly clean the saw during the repair process.


We examined the main causes and malfunctions of the Stihl ms 180 chainsaw for which the replacement of the CPG is necessary. But, as a rule, in each case, the reasons may intersect, and for proper repair, an integrated approach to solving the problem is needed. Putting a new piston on the saw does not always mean solving the problem. It is imperative to eliminate the causes of the malfunction and the tool will last for a long time without problems and the need for repairs.

On gas trimmers, both two-stroke internal combustion engines (ICE) and four-stroke are used. But the ignition system in them is no different from each other, so the repair and adjustment of this fuel ignition system will be the same for different types of ICE.

How the trim system works

The ignition system of a two-stroke ICE consists of 2 elements: a flywheel and a coil (magneto). The image below shows how they look.

An impeller is located on the flywheel, which acts as a fan to cool the engine. Also on one side of the flywheel are permanent magnets, shown in the figure by an arrow.

When the flywheel rotates, an electric current is generated in the coil, which serves to produce a spark on the spark plug. A spark appears when the magnets are directly below the magneto.

Why do I need ignition adjustment

In some cases, when the engine does not start, ignition adjustment will be required. Of course, first you need to check the serviceability of the candle and high-voltage cable with a cap.

Ignition control on a two-stroke engine is necessary in order to ensure timely ignition of fuel in the engine combustion chamber. Below is a diagram from which you can understand the principle of the internal combustion engine.

A spark in the candle should appear at the moment of compression of the fuel by the piston, when it does not reach the top dead center (TDC) just a little. When the piston passes TDC, the fuel mixture ignites, as a result of which the piston goes down under the action of the explosion energy.

Therefore, if for some reason (mainly due to the displacement of the flywheel on the shaft relative to its initial position), the fuel ignites before the piston passes TDC, then it goes back and the crankshaft scrolls in the opposite direction. This movement can damage the starter assembly, cut off the starter cord, etc. In this case occurs early fuel ignition.

May also be late ignition on the trimmer: the piston after compression of the fuel, which did not ignite, goes down, and at this moment a spark appears. In this case, the engine will either not start, or it will significantly lose power and will not gain momentum well.

But such problems with early or late ignition on two-stroke ICEs are quite rare, since the flywheel with magnets is already installed in the correct position, which is perfectly synchronized with the movement of the piston. In addition, the flywheel is fixed to the shaft with a dowel and nut. Therefore, incorrect installation of the part is excluded.

How to check the health of magneto

Magneto is the main part in the engine ignition system and is a coil with primary and secondary windings located around the core. Often, due to a magneto malfunction, a spark disappears on the spark plug, and ignition of the fuel does not occur. To check the coil, you will need to use a tester, and perform the following steps.

  1. On the tester, going into resistance measurement mode, you need to set 200 Ohms. In this way, the primary winding will be checked.
  2. Connect one tester electrode to the “mass” of the coil, and the second to the contact coming out of the magneto. Normally, the resistance should be between 0.4 and 2 ohms.
  3. To check the secondary winding, you will need to set the switch of the device to 20 com. Further, one probe of the device is inserted into the cap of the candle, and the second is connected to the “mass”. Normally, resistance indicators should be in the range from 4-6 kom and above (depending on the model).

If you don’t have a tester, then you can use the following method, with great care so that the electrode does not touch the cylinder, otherwise the coil will burn out.

  1. Remove the cap from the candle. You do not need to twist the candle to maintain compression.
  2. Insert a nail without a cap into the cap. The nail should be of such a diameter that it holds well in the cap.
  3. Tie the cap with the nail to the cylinder using a dielectric so that the gap between the electrode and the cylinder body is between 5.5 and 7 mm (but by no means touches the “mass”).
  4. Using a starter, simulate the engine starting several times. The ignition button must be on.
  5. Observe whether the spark appears and what color it has.

Normally, the spark should be strong, white or have a bluish tint. If the spark is weak, and its color is orange or yellow – this indicates a faulty magneto.

It should also be borne in mind that sometimes a cold magneto can produce a good spark. But after heating due to internal malfunctions, it ceases to produce current. As soon as the coil cools down, the trimmer starts again and works for a while until the magneto is again heated. Such a coil must be replaced.

Ignition Algorithm

Ignition on the trimmer is fairly easy to adjust.

    Remove the casing from the engine by unscrewing all the bolts holding it. For example, on the Patriot (Patriot) PT 2540 lawn mower, to remove the casing, you will need to unscrew the air filter housing and remove it from the latches with a flat screwdriver. After that, the plastic case can be easily removed.

  • Using a screwdriver, unscrew the 2 bolts (it is not necessary to completely unscrew) holding the magneto.
  • Turn the handwheel so that the magnets are at the top.
  • Further, to correctly set the best clearance between the flywheel and the coil, you can use the usual business card or pocket calendar. If you don’t have either one at hand, then to set the ignition, you can use a sheet of A4 paper, folding it 2 times (in the end, you will get a thickness of 4 sheets). This thickness will be enough to get the optimal clearance and adjust the ignition. Insert a business card or paper between the spool and magnets.
  • A magnetic field will pull the coil to the flywheel. Holding magneto with your fingers in this position, tighten the bolts holding it.
  • Remove the business card. After that, the ignition on the trimmer will be adjusted.