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Fitting the Piston of an RC Engine

Aug 14, 2008
The Perfect Fit of a Nitro RC Race Engine:

You have to make the piston as round as the liner. Here are 2 effective ways to make the piston round.

1. Lapping the piston to the liner with 1200 grit non-imbedding garnet. Be careful and only do a slight amount of lapping so you don't eliminate the proper tightness and fit of the liner/piston.

2. Here is the BEST WAY to make the piston round. Use a Helical Lap. Only a few people I know who have this capability. If you are interested in having your liner and piston fit, write to me at fit@rcboat.com and it will be automatically re-directed to Norm Doerr who has all of the helical laps that I used to own. He can also sell you a small quantity of the Non-Imbedding Garnet Lapping Compound.

NOTE: NO AMOUNT of running in of an engine will assure you of a perfect liner/piston fit. It possibly might happen, but doesn't over 80%+ (I guessed at the percentage, it might be over 95%+) of the time. You might get a good engine by running it in, but you will get a SUPERB DOMINATING ENGINE by following these guidelines.

I helped build 3 engines for a close friend who wanted to try to qualify for the RC 1/8 Car World Championships held in Indy a LONG time ago. He made the A Main. The Engine Analysis Software was the basis for this engine build.

The fit of the liner to the Crankcase is also very important to the perfect fit. The liner should be placed into the crankcase so that the fit is not forced but a snug slip fit. If the liner does not slip in and out of the case smoothly, you should LIGHTLY lap the liner to the case. The underside lip on the liner should also be lapped to the top of the case so that there is no distortion when you tighten down the head.

I use non-imbedding garnet as a lapping compound which is 1200 grit (available from the above e-mail address). You should have a little bit of this compound to use for MANY applications in our hobby.

The fit of the head button into the liner is also very important. On some engines the machining cutter leaves a radius where the head button fits into the liner so as to spread the top of the liner when tightened.

DON'T LAP THE PART OF THE HEAD BUTTON THAT STICKS INTO THE LINER - ONLY LAP THE PART THAT STRIKES THE TOP FLANGE OF THE LINER.

A VERY SMALL AMOUNT of Dow Corning Silicone #738 RTV around the liner and under the lip of the liner when placed into the engine crankcase will assure a great crankcase seal. This material is white and does not harden. It semi-hardens and seals. I apply a VERY VERY SMALL amount to the underside of the liner top flange to contact the case and form a seal of the liner to the case. This will not allow any crankcase pressure to escape from under this flange.

I am making the assumption that I don't have to tell you to use a toothbrush, lots of water, and dishwashing detergent to clean off the Garnet Lapping Compound and any dirt and residue from all parts, BEFORE assembling the engine.

THIS TIP IS THE MOST IMPORTANT PART OF BUILDING ANY NITRO PERFORMANCE MODEL ENGINE !!!!

Marten (Marty) Davis Owner - Engine-Analysis-Software.com 7 time National Champion IMPBA Member of IMPBA HAll of Fame Many National Speed Records
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