Lapping Piston Rings


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This short document will explain how to "lap" a piston ring (in this case for a Yamaha).

To completely lap a ring is a multi-step process.

Please Note: this assumes you have already made certain that the ring "light checks" correctly, has the correct tension on the cylinder wall, has the correct end gap, and is correctly deburred on the ends. Do that FIRST!

First, the top and bottom of the ring need to be lapped FLAT.

To do this correctly, you need a holding fixture for the ring that is the same size as the bore. It doesn't need to look exactly like the drawing (I didn't spend much time on the drawing, sorry). And 15 years after I originally wrote this article, here is a 3D view of what's required.

If your fixture is very much bigger or smaller than the bore size, the ring will become "coned" or "concaved" when held in the fixture, meaning the ring will no longer be flat when it is in the bore of the engine. I would recommend having a few holding fixtures in no more than .010" increments. (ie: one around 2.054" to cover from standard bore to 52.25mm, another about .010" larger for lapping rings from 52.25mm to 52.5mm, and so on). I personally have holding fixtures in .004" increments. Please note that the ring only needs to stick out of the fixture .005" or so. Measure the thickness of the rings you'll be lapping, and make the counterbore just .005" shallower than that.

Squeeze your ring into the fixure, (make sure it's all the way down in the counterbore you've cut in to the fixure... set the fixture down on your surface plate with the ring down, and push hard to make sure the ring is completely seated). Lap both sides of the ring on some solvent soaked 400 grit silicon carbide sandpaper on a surface plate. Don't push down as you're lapping. Just let the weight of the fixture do the work. Sandpaper is flexible, if you push down as you're lapping, you will round the corners of the ring, and it won't be as flat as it should be. To remove the ring, just flip the fixture over (ring up) and give it a sharp tap onto a flat surface (the floor?) and the ring will pop right out. Lap ONLY until the ring is "clean", no need to remove any extra material.

Second, the OD (outside diameter) of the ring needs to be lapped to the bore size of the motor.

Doing this requires a few items:

First, you'll need a sleeve that has been honed to the same size as your cylinder. I personally use a "Sunnen Trueing Sleeve, part #ST-2000", that I bore and hone to the correct size. (Sunnen trueing sleeves are mild-steel. Cast iron would probably be better, but these sleeves are relatively inexpensive, and easy to bore and hone to the correct size). Once again, I would recommend having a sleeve that is very close to the bore size (within a couple thousandths or less). When I was actively racing, I kept a separate sleeve for every engine I had; when I honed the cylinder, I honed the sleeve to exactly the same size. Each cylinder I owned (of different size) had a corresponding sleeve. I was [quite] probably going "overboard" on this, but that's the way I did it.

Second, you'll need some sort of a handle to attach to a piston for stroking in the sleeve. I use a piece of 1" diameter aluminum that I milled on one end (in thickness) to fit between the wristpin bosses on a piston. I then cross drilled (and reamed) a hole for the wristpin. My handle is about 10" long.

Lastly, you'll need some diamond lapping compound (since the ring is hard-chromed on the outside, regular lapping compound is inadequate, and too coarse). I personally use "9 micron" diamond lapping compound. (1 micron is 1 millionth of a meter, or approximately 40 millionths of an inch, so 9 micron grit size is about 360 millionths, or 3.6 "tenths", in size). You may want to experiment with the grit size, but be aware: diamond compound is not cheap. I did quite a bit of experimenting, and settled on this grit size.

Once you have the above, the procedure is to first lap the top and bottom of the ring as outlined above in step #1.

Then, clean your trueing sleeve thoroughly after honing it (I use something like "409" and hot water). Then, just wipe a very small amount of compound on the inside of the trueing sleeve... just a small amount will do it. If you get just a bit of compound on the tip of your finger.... you can just wipe a "ring" of it all the way around the inside of the sleeve in one spot.... that's all you'll need.

Now, what I do is use the old piston (the one that came out of the engine, provided you have not gone to the next piston size), and place the new (clean, lapped top and bottom) ring on the piston. (BTW: pay attention to the direction that the ring is on the piston as far as top and bottom, and make sure you install it in the engine the same way). With your handle attached to the piston, slide the piston and ring into your sleeve, and begin stroking the piston from one end to the other. No need to rotate the piston, move it straight back and forth, as it would in an engine. After 10 or 20 strokes, pull the piston out, remove and clean the ring, and inspect it closely with a magnifying glass.

What you are looking for is a dull, satin, consistent finish. A bit of practice with some old and new rings, and you'll develop an eye for what you are looking for. There should be no inconsistencies in the "finish" of the ring on the outside if it's lapped completely.

This outside lapping process will not only remove the chrome "flashing" that tends to build on the edges of the ring, it will also fit the ring perfectly to your engine's bore (assuming the cylinder has been honed correctly, but that's another article). Not only will there by virtually no break-in time required, but your engine will not have the usual small scratches on the bore after initial running. Most of those happen from micro-small chrome particles breaking off the new ring.