Minimoto Garage - Top-End Gasket Replacement

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Tech Tips from Racers for Racers
Step by Step
Top-End Gasket Replacement
(Note: The pictures shown below are of a Polini 6.2hp water-cooled engine, but the basic steps are the same on all minimoto engines.)
Step 1.
You will need to drain your coolant and remove your engine before doing the following steps. Please visit the following tips to learn how:
Step by Step: Radiator Draining
Step by Step: Engine Removal

You might also want to remove the spark plug for this tip.


Step 2.
Remove the head bolts, you'll need a 10mm socket or wrench.


Step 3.
Remove the head.


Step 4.
Remove the cylinder. And remove the base gasket off the bottom of the cylinder.


Step 5.
This is what the engine looks like with the head and cylinder removed.


Step 6.
Here is a picture of the top-end gaskets. You should have three base gaskets and two different sized o-rings.


Step 7.
Pick one of the base gaskets and put it on the bottom-end of the engine, as shown.


Step 8.
Replace the cylinder.


Step 9.
You'll need a piece of solder wire to measure the squish clearance. I used Radio Shack's 0.032" for this tip.


Step 10.
Place a piece of solder wire just smaller than the cylinder's bore across the top of the piston as shown. Use a dab of grease, if needed, to keep the solder in place.


Step 11.
Replace the head.


Step 12.
After you have put the cylinder and head back on, finger tighten the head bolts then grandually and evenly torque them to 0.8kgm (5.78ft/lbs or 69.36in/lbs).


Step 13.
Using the pull starter, move the piston through Top Dead Center (TDC) a few times.


Step 14.
Remove the head bolts.


Step 15.
Remove the head.


Step 16.
Measure the ends of the solder wire with a caliper. With a 40cc engine, you want it to be as close to 0.50mm as possible. With a 50cc engine, you want it to be as close to 0.45mm as possible. If you did not get it this time, then repeat steps 7-16 until you have the correct squish clearance.

You're still not done if you have the correct squish clearance. Read on.

Step 17.
Remove the cylinder.


Step 18.
After disassembling the top-end, use a little grease on the base gasket to create a vapor seal. Then put the gasket back on the engine.


Step 19.
Replace the cylinder.


Step 20.
Now you need to remove the two old o-rings, I found using a pair of snap-ring pliers works best.


And o-ring number 2.



Step 21.
Now put the new o-rings on the top of the cylinder.


Step 22.
Replace the head.


Step 23.
After you have put the head back on, finger tighten the head bolts then grandually and evenly torque them to 0.8kgm (5.78ft/lbs or 69.36in/lbs).


Now you're all done and ready to put the engine back into the chassis.

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