ENGINE - “THE FIT”
Technotes (2/95)
How to Measure the Seal of the Liner & Piston:(“The Fit”)
20 years ago, a very accomplished model boater told me of a way he tested his engines to determine "The Fit".
THE PISTON:
You have to make the piston as round as the liner. There are 2 ways to make the piston round. First by lapping the piston to the liner with the previously mentioned 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. The other way to make the piston round, is to use a helical lap. The only people I know who have this are John Ackerman & Steve O’Donnell. Ackerman will do this for you. jfacustom@earthlink.net
The fit of the liner into the Crankcase is also very important to this 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 lap the liner to the case. The lip on the top of 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 probably about 1200 grit. (Available from the Helical Lap Company)
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. I also add a very small amount of a Dow Corning Silicone #738 RTV. This material is white and does not harden. It semihardens and seals. I apply a very small bead to the underside of the top flange of the liner 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.
THE TEST:
Now to the way to test all these items, to see if you have a great fit and seal.
I assemble the engine and apply some “silicone platelets” to the metal surfaces on initial assembly. (You can get this as Boca Bearings Magic Engine Oil) After I turn the engine over several times to coat everything 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 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 boat 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 you will ever get and will accurately predict success of your engine building.
This fit is “Far More Important” than any timing numbers, 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.
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