Working on a vintage sled with a blown engine? Take a look at the pistons to understand why.

So your vintage snowmobile engine just seized and you have decided to tackle the rebuild job yourself. Top end work on your snowmobile does not require much in the way of special tools as bottom end work does. [such as clutch pullers and flywheel pullers] Removing the engine from the snowmobile chassis is not necessary for top end work, but it is a lot more comfortable to work not leaned over your machine.

After you have decided whether or not to remove the snow machine engine, head and cylinder removal will be your first step. Before you do this you’ll need to take measures to collect the anti-freeze if you have a liquid cooled engine. It may be reused if you wish, so keep it clean.

After you have lifted the cylinders you may discover that a piston is the problem. Just cleaning the bore and dropping in a piston is not a big deal, but there is usually a reason for the demise of a piston other than another trophy for your shelf.

The two main causes of snowmobile engine piston failure are lean conditions and ignition timing. With today’s electronic ignitions, unless something comes loose, they do not cause the trouble of the older point ignition systems. Note that bad connections and wiring count as loose, not only mechanical pieces.

Lean conditions can be caused by numerous things such as carbs, fuel pumps, fuel lines, leaking crankshaft seals, ice, filters and debris. You should check out these things or you may suffer a repeat performance!

What to look for on the piston

Following are photographs of some of the pistons on my trophy shelf, along with information on what caused the damage.

If you look closely at this piston you’ll see the top of it has many divots or pock marks. These have been caused by the needle bearing coming apart and flying through the engine.

Notice the odd condition of what remains of the piston skirts and the dome: discoloration, not really burnt looking. This problem was traced to an anti- freeze leak into the combustion champber.

This piston demonstrates a classic example of ignition problems. Too far advanced ignition burns the center out of the piston and the melted metal flows toward the exhaust side.

This piston may resemble piston 1 in which the top end bearing failed, but this was a crank bearing. When rebuilding an engine it is always a good idea to tear the whole engine down to make sure you have identified all sources of the problem and to make sure there are no pieces left in the bottom of the engine.

This piston shows the results of running your engine really lean. Note how the piston was so hot the ring land droops from heat and metal has melted and run down the exhaust side. This was traced to a piece of debris in the fuel pump.

This piston is another example of the results of ignition problems. There is a hole in the top of the piston which was traced to a wire coming out of the engine. It had the insulation rubbed off and was shorting out.

This piston also has a hole in the center that looks like ignition timing but was traced to an over-revving engine. A sticky clutch caused the high revs in the engine.

The next series of pictures illustrate some of the things that you should also check once you have blown a piston.

This is a piston pin. Check for wear on the bearing area. Note that this one had a piston go out on top of it and the bearing has worn a groove where it goes.

If you look at the arrows on this photo you’ll see that the transfer port is broken. Both noted ports should be the same size.

If you look carefully at the sleeve of this cylinder you will see that a portion of it has broken off. It was found nowhere in the engine!

This image is of the head. Note that on the right side the damage from a broken ring has ruined the head. You will need to compare machining costs to repair the damage to replacement costs.

This final picture is of the pin bearing. Note the aluminum transfer on the needles [the white dots]. This piece must be replaced.

Good luck with that rebuild. This article originally appeared in SnowRider online Magazine in February 2001.