how to lap a piston

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how to lap a piston

Postby spacebug.049 » Mon Sep 07, 2009 5:01 pm

Does anyone know how to lap a piston and cylinder? I know nothing about this.
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Postby Jim Thomerson » Mon Sep 07, 2009 5:36 pm

One does not do this with ABC or ABN engines. With iron and steel engines, I take the head and backplate off, and put a prop on. I use Duco white rubbing compound which is water based. I put a little around the edge of the piston with the piston slightly above the exhaust port. I hold the prop and gently work the piston up and down a little. If the engine is as it should be to start, the piston won't go up very far at first. When the compound turns grey, I wash it out and put in new. I work the piston up to 1/8 or 1/16 TDC. I leave that last little bit to run in.

Then I clean the engine up, oil it up and run it like breaking it in. If I have any questions, I take the engine apart and look at the cylinder and piston. The top edge of the piston can be shiny. But, if there are any other shiny spots on the piston and cylinder, they are high spots. So I have to decide if I lap some more, or just continue break in and let the high spots wear down naturally.

Don't do this unless you can afford to ruin a piston and cylinder, because you may do so.
Jim Thomerson
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Postby loucrane » Sat Dec 12, 2009 10:43 pm


Jim's method is solid and simple enough to depend on.

There are a few other ways to consider, depending on the engine and the metals it is made of.

For iron piston in steel or iron sleeve engines, Larry Foster (L&J Fox reworks fame) recommends setting the max height lapped by putting a penny in the upper cylinder, lapping with the top flange resting against a block of wood (or whatever fits the flange, flat) and lapping with a fine compound (I use LAVA bar soap suds, worked between two glass plates until it turns grey) and with a 'handle' - say a piece of hardwood that fits the space under the piston crown, and a dummy wrist pin that fits the piston and the handle.

The lapping action is a combination of twisting the piston in the sleeve as it is moved deeper, then out of the bore. Rotating the sleeve after several such 'strokes,' helps preserve roundness.
This works very well with the Fox Stunt 35, and other engines of similar materials. The penny (or dime, if the bore is smaller than a penny) is to prevent cutting away the metal at the top of the stroke, which as Jim points out, is best 'fitted' by actual running wear.

More modern engines, as Jim suggests, are made of materials that are designed to attain best-fit at running loads and temperatures. Lapping could change the "cold" fit so much that this becomes impossible.

IOW, DON'T lap ABC, AAC or ABN engines' sleeves and pistons! Modern computer aided machining (CAM) reliably gives us engine fits that are close enough, in the box, that the manufacturers' recommendations for break-in actually work best. Their reputation depends on it working. Go with it.

These ABC, AAC and ABN engines are made to have a too-tight "pinch" through Top Dead Center - that's the hottest area when the engine runs. That's where the metals expand most. That's where the heating and loading have been calculated to provide best-fit - don't mess with it if you expect the engine to run as advertised or as others say it should! DO NOT force the engine to turn over, cold, through this "pinch" - you could destroy the hot-running fit designed into it!

There are other factors just as important as sleeve-to-piston fit! If the engine is not torqued-in on reassembly properly, it will suffer binds and wear that can be avoided rather simply.

The head should fit the main casting level and even all around. If the machining there looks rough, a few 'rotations' of the head against the sleeve with a grit paste as I described, and the sleeve against the main casting (ditto) can show important things. If there are severe 'high spots' that the paste grinding shows, the head-to-sleeve and/or the sleeve-to-main casting fit is unlikely to allow a good fit, free of unwanted warping.

If they look good after such a check, there can still be a few problem areas. The pieces may fit one to one, but still be skew to the total assembly. Proper re-torquing can help, here, and is often sufficient. If it isn't, some further lapping of the head, sleeve and casting may be in order.

To check, assemble the pieces loosely, and check how freely the piston moves to about 60° each way from Bottom Dead Center. Keep that 'feel' in mind. Run the head bolts in until they barely touch bottom, then back out a half turn.

For a 4-bolt head, use something like the political "cause ribbon" pattern: 2 o'clock, 8 o'clock, 4 o'clock, 10 o'clock, then across to 4 o'clock for the next rep. And so on, taking up about the same rotation of the screw, about 15° each pass. (It actually takes longer to describe than to do...)

For a 6-bolt head, call the front-most bolt 12 o'clock, or choose one if there are two parallel across the front, and use the sequence 12, 4, 8, then 2, 6, 10. Across from 10 o'clock to 4 o'clock and proceed with this sequence, which somewhat resembles a Star of David pattern...

IMPORTANT!: After each slight increment touching all the head screws, check that the 'feel' of the loosely assembled engine has not been compromised. If it has, back off a step and try it again.

You'll be surprised, as I always am, how much this frees up an engine, and I don't care how refined the CAM at the factory was!

Of course, there are other places where unnecessary wear may result...

Does the shaft turn freely when the rest of the pieces are not mounted? Is there binding (possibly from an out-of-true conrod?) IS the engine truly free-turning when loosely assembled? Most of the problems are easy to spot, and to concoct a solution for. If you do this, and it doesn't take Master Machinist skill to do what I'm talking about, you have compensated for the problem in mass production, of getting an optimum fit, and equally optimum assembly of a basically good engine. It will pay off in long and consistent service.
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