Lapping Compound

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Jul 8, 2007
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For a close but free moving piston to cylinder fit lapping is best.
If your piston is dead nuts round and straight, a home made hone can
be used to bring the cylinder to a perfect fit.

I use the piston to lap the cylinder bore to a perfect fit. I cut piston for a
close but movable fit to the bore then coat it with my favorite lapping
compound. (Plain Crest toothpaste). It's abrasive enough to lap the soft
metals but will wash out cleanly and completely with warm water.
I assemble the engine and spin it with a cordless drill chucked to the end
of the crankshaft until the fit is what I want it to be.

Is it the best way? Probably not, but it works for me every time.



I have tried them all, from adjustable laps to cylinder hones, but I always come back to getting the bore taper free and round then do as Rick does, make the piston for a slightly tight fit and lap the piston to the bore.
Again, as Rick suggests use a toothpaste product, I also use a brass polish that comes in a wadding form, I just rub the liquid from the wadding and use that, normal liquid or paste brass polish is ok to start with as is car rubbing compounds, or jewellers rouge, but these are for starting off only, just to get rid of the high spots, then the finest abrasive you can find to finish.
This is one of the most important parts in making an engine in my opinion. This is the arm aching, pain in the a** job of making a well running engine, and the better it is done the better the engine will run and it also makes the engine last a lot longer before replacement parts have to be made. It has just taken me a full day to lap in two bores and four pistons for a new engine that I am making, I think it is important to spend as much time as necessary to get the best fit that you can.
If I design an engine or modify from plans I always try to make the piston out of the softer material, so that it becomes the sacrificial part. It is usually easier to make a new piston that cylinder.
The way I usually check for a nice fit is, no lubrication, put the piston in the bore and gently suck and blow on the end of the bore, the piston should slide very smoothly up and down. Be careful about swallowing, because if you swallow a piston, in a couple of days by the time it gets thru your internal bores, 1 it will be painful when it comes out and 2 it will be even more painful when you find it won't fit the bore any more. So you will have to make a new one anyway.


Tin Falcon

Well-Known Member
Jul 9, 2007
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Acording to Rocky the cast iron gray Cat who is my resident lapping expert/consultant. There are lots of things that can beused as lapping compound.I tend to use metalpolish or automotive bubffing compound.I have also used some grit from an old rock tumbler kit that came from a yard sale. Just mix with a litlle fine oil . The same stuff can be used for machine turning/ jeweling. Just remember to plug threaded holes before the machine turning processDAMHIK.

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