Piston question

I'm working on an F20 that had aluminum 3 7/8 overbore pistons that I am led to believe are M&W. A couple of the pistons were not what I call acceptable after I got them out, they were white corroded in very, very tight. No magic soaking elixor was ever going to penetrate that white stuff. After following some suggestions from the Farmall site I found a good used set of 3 7/8 aluminum pistons and sleeves that are claimed to be 8000 foot altitude. Today I finally had the time to compare the old pistons out of the tractor with the 'new' ones and they measure the same distance from the pin to the top of the piston and the ring lands are in identical locations.

The numbers on the pistons is what is puzzling me. Inside the piston skirts of the old pistons I found MWM or WMW cast in one side of the skirt and A1082 or A1092 cast into the other side, the 8 or 9 was fuzzy on all four pistons so I can't tell which one it is. The 'new' pistons have the same MWM inside the skirt but the number is clearly A1093.

Do any of you guys know anything about these numbers? Is there an M&W reference chart somewhere? The reason I ask is one of the 'new' pistons has a small chunk broken off of the skirt, I'm not too worried about it with this slow reving engine but I do have one original piston and sleeve from this engine that are good and could be used to replace the one with the chipped skirt. Both pistons are 3 7/8 flat top. Thanks. Jim

I would weigh all the pistons and compare weight. If dimensionally the same and weigh the same then you'll be ok.

Will do that tomorrow.

The piston weight does not matter between the old to new. the engine is balanced by having two pistons up and two down on a single plane crankshaft. They do need to be the same weight between the pistons you put in within a few grams. Sounds like you found a great set. Jim

Sounds like you are thinking of mixing one of the good old ones to avoid using the chipped skirt?

Those piston WMW are White Machine Works . Later Genuine Parts, then Gould Clevite. I have a Genuine Catalog. I will look Sun. to see if your numbers are in the book.

Yes I want to use one of the old ones. The old one looks so identical to the new ones I have marked it so I don't get it mixed up with the new ones.

Thanks Earl.

I went out and weighed them with a kitchen scale with a 25 pound limit so I couldn't get it down to grams but the needle stopped in the same place for all of the pistons, about 2 7/8 pounds each.

Sounds like they're all close. As Mr. Nicholson was saying, the absolute weight doesn't matter as long as they are all the same.
I would still try to seek out a scale that will read to the gram as there are 28 grams in an ounce. At tractor RPM it will probably run fine where you are at. Verifying they are within a gram will guarantee smoothest operation.

On the subject of must all pistons be same size/weight,a skirt broke on my 265 cid chevy and scratched the bore. The mechanic cleaned up bore and installed one over sized piston. I owned the car another 30k miles without a problem. I was 18 at the time so it saw high rpm many many times. That lead's me to belive the same could be done with the F-20 without worry.

The old book I have just showed the OE 3.750 cast iron piston & sleeve set . The newer book only showed the later 4 in set up. I don't have a older Clevite book. The newer Clevite book all the piston numbers start with A-_____ 4 digits. Of course all this is obsolete now. Sorry I couldn't ID your pistons.