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Discussion Starter · #1 · (Edited)
If you build your own engine, it’s a good idea to spend some time making all pistons weight the same.

There are plenty of videos on YouTube showing how to do that in the worst possible way: they all show how to use a burr to grind the pistons in various ways to remove material.

The last thing you want is weaken the mechanical integrity of a piston by grinding away material at the wrong place.

Usually, forged pistons are machined to pretty close tolerances and a good piston set should be within a gram out of the box.

A good way to even out the weights is to shorten the skirts by a few tenths of a millimeter. This is not going to change anything to the structural integrity of the pistons, and the geometric considerations resulting from such shortening are immaterial: engines still run fine with the entire skirts chopped off, so skimming a few tenths isn’t going to do anything 😏

You need medium grit sandpaper (I used P220) a flat glass surface, a precision scale and some patience. A deburring tool is also useful: use the blade to chamfer the inside of the skirt after skimming it.

First, weight all pins and sort them from the lightest to the heaviest.

Second, weight all pistons (without circlips or rings) and sort them too.

Next, pair the lightest piston with the heaviest pin. The second lightest piston with the second heaviest pin etc. This way you even out some of the differences already. Use a Sharpie and number the pairs, as you just tied a piston to a particular pin forever.

Finally, weight your pistons and pins pairs, sort them from light to heavy and write down the number, and the difference from the lightest for each. This tells you how much you must remove to match the lightest.

Start from the piston where you must remove the most material. Hopefully it’s well below one gram. Lay the sandpaper flat on the flat glass surface after wetting it on both sides. Hold the piston straight up and skim the skirt evenly a tiny bit. The skirt is machined from the factory, so the piston stands - just keep it that way and slide it with light to moderate pressure down.

Clean/dry the skirt using a towel, weight to measure your progress. Rince and repeat until you get within 0.1g of the target weight, chamfer lightly, deburr lightly, weight again, skim again lightly if still needed, and zero on your target weight. Chamfer the outside of the skirt by holding the piston 45° sideways and rotating it against the abrasive surface with a very light pressure. A light touch is enough.

It’s not difficult but requires some patience.

Move to the next heaviest piston. There will be less work, and you are less likely to overshoot as you now have a feeling of what it takes.

These are JE Pistons pistons (kit number 337924) with special order skirt and crown coatings. 1mm first ring, 1.2mm second ring. Thin rings require special consideration for honing, I’ll write what I know in a separate post.
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