The Perfect Fit of a Nitro RC Race Engine:
You have to make the piston as round as the liner. There are 2 effective ways to make the piston
round.
1. Lapping the piston in the liner with 1200 grit non-imbedding garnet. You must be careful to do
only do a slight amount of lapping so you don't eliminate the proper tightness and fit of the
liner/piston.
2. The other BEST WAY to make the piston round, is to use a Helical Lap. There are 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 liner to the Crankcase fit is also very important. 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.
NEVER LAP THE PART OF THE HEAD BUTTON THAT STICKS INTO THE LINER - ONLY LAP THE PART THAT STRIKES
THE TOP FLANGE OF THE LINER.
I also add 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. 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 assume 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.
DO NOTING ELSE BUT THIS AND YOU WILL HAVE A GREAT PERFORMING NITRO MODEL ENGINE !!!!
Marten (Marty) Davis Creator of Engine-Analysis-Software.com 7 time National Champion IMPBA Member
of IMPBA HAll of Fame Numerous National Speed Records
