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Fitting a RC Nitro Engine


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ENGINE - THE FIT(revised)

Technotes (7/08)

The Engine Analysis Program

How to Measure the Seal of the Liner & Piston:(The Fit)


25 years ago, a very accomplished model boater (Ed Hughey) told me of a way he tested his engines to determine if they had The Perfect Fit. Since he was by far, the BEST MODELER at that time, I listened.



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 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. 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 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.

A little side note of interest: I helped build 3 engines for a close friend to try to qualify for the RC 1/8 Car World Championships held in Indy a LONG time ago. He made the A Main which is almost unheard of. The ONLY things that were done to this engine was the sequence indicated above and below.

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 a non-imbedding garnet 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 for 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. BE SURE NOT TO LAP THE PART OF THE HEAD BUTTON THAT STICKS INTO THE LINER - ONLY 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.


Now to the way to test the fit of the piston/liner, to see if you have a great fit and seal. I assemble the engine and apply a drop of “silicone platelets" to the piston surface on initial assembly. (You can get this at Boca Bearings Midas Touch Lubricant) After I turn the engine over several times to coat the piston and imbed the Teflon Platelets slightly, I put in some methanol to clean out most of the oil residue. I put in the Glo Plug and test the engine for seal by rocking the engine back and forth over top dead center by holding the flywheel. If you can't rock it back and forth over top dead center without loosing the seal and leaking down for at least 20 bumps, you don't have a good fit. After you find that you have an engine with the great fit, run the engine in your model getting the engine VERY WARM for a short period of time and then running rich for a short period, then running VERY WARM, etc. Several cycles like this will allow the metal of the piston and liner to find their set and you should have a GREAT ENGINE. I would suggest that you re-try the bump test AFTER you run in the engine to make sure that the seal lasts. This test method will be one of the best tips you will ever learn and will accurately predict the success of your engine build. This fit is “Far More Important" than any timing numbers or ANYTHING ELSE, and will be one of the primary indicators of great performance. This test takes into consideration the liner/piston roundness, the seal of the liner to the case, and the seal of the head to the liner.


When your piston to liner fit becomes less that perfect, it is EASY to refit the piston to the liner. Buy yourself a very good tubing cutter from Sears ( ) about $32.00. It will be a once in a lifetime purchase that you will use over and over. Don't ever ever ever ever use this cutter for anything else. The wheel that comes with the cutter will last almost indefinitely, but you might pick up a pack of spares. Keeping the cutter razor sharp is critical.

OK, how to use this tool.

Most pistons have an oil groove at the top of the piston. It is usually VERY NEAR the top of the piston. If the piston does not have an oil groove, it is easy to make one with the tube cutter. The cutter will EXTRUDE the oil groove making the piston tighter in the liner. It is CRITICAL to be very careful here not to get the piston too tight. You want to push the piston up into the liner and when it stops, look into the exhaust port and see the wrist pin in the MIDDLE of the exhaust port window. That is about how tight you want the piston to fit. You can re-fit the piston over and over this way and keep a great seal. The fit will go away quicker than if the piston was perfect to start with, but this will recover a loose fitting piston/liner.

I hope that you enjoyed this Technical Note. I would also like to offer you a Special Deal on The Engine Analysis Software available ONLY to you and those people who have signed up for our Mailing List on the Web Site. If you would like to have your copy of the Engine Analysis Software, use the order link at: and use the Discount Code: “MailList" and you will buy the software for $25 off of the regular $99.95 price. This is a limited time offer, so get the software the Pros use and advance your performance. ORDER NOW

I would also like to invite you to join the NEW EAP Discussion Forum to find answers on the most effective use of the software. To join this Forum, Sign Up at: This forum has not been made Public Except to my mailing list. It will build quickly as I put out the link to our Software Users. It is MANDATORY to sign up using your REAL NAME. It will be a closed list open only to people using the Engine Analysis Software. I will not use your e-mail address for ANY purpose other that this Forum!!!

PS: I also hope to write more of these Technical Papers, so invite your friends to join the Mailing List on the Engine Analysis Software Site.

Marty Davis Owner - Engine Analysis Software 7 time National Champion IMPBA Member of IMPBA HAll of Fame Multiple National Speed Records


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