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G'day!!! How do I get myself into these dramas!!!?
I recently bought some standard size, good condition, 90S pistons off AUS Ebay and presumed that they were the same size, weight, etc. Wrong!!! The pistons are a 89.970mm (3.5421") 'B' bore code and a 89.980mm (3.5425") 'C" bore code. Both have different weight designations (+ and -). I have two new standard bore barrels, 'C' bore code 90.020mm (3.5441"). Can I? should I? use these pistons in these barrels? The standard piston clearance for the /6 is 0.035-0.045mm (0.0014-0.0018") and the maximum permissible piston/cylinder clearance: 0.120mm (0.0047") which I figure gives me a bit to play with on the 'sloppy ' 'B' bore. Am I stressing too much!!! ?
The pistons came with new rings which more than offsets the cost of the pistons if I can't use one. If I was to weigh each piston ( haven't yet) and determine the difference, is judicious filing of the weightier of the two (from the inside somewhere!!!) accepted equalizing practice??? Any thoughts, comments please!!.............. hools.

Filing/grinding/machining weight off the wrist pin boss to match the other piston is the way it is done. I'd use the B piston in the C cylinder and forget it. The B piston is only .0004" smaller than the C piston.

George Ryals wrote:Filing/grinding/machining weight off the wrist pin boss to match the other piston is the way it is done. I'd use the B piston in the C cylinder and forget it. The B piston is only .0004" smaller than the C piston.

Accurately measure each cylinder and each piston –
Measuring & matching may result in 2 assemblies that will be within operational tolerance.

If you're not too particular, you can probably get away with using them. Many manufacturers don't go to the bother of 'A', 'B' and 'C' grading pistons at all. I would say the weight variation is more important (if a smooth engine is what you desire).

Rob

Rob Frankham wrote:If you're not too particular, you can probably get away with using them. Many manufacturers don't go to the bother of 'A', 'B' and 'C' grading pistons at all. I would say the weight variation is more important (if a smooth engine is what you desire).

Rob

Because, as I understand, flat motors tend to be more prone to piston slap, BMW's more careful matching of clearances was likely to help control that issue. Certainly running one piston size "too small" in the letter grading would be a very slight change in clearance. Going one size too big could possibly be a lot bigger deal.

I've found more deviation in clearances on aftermarket VW pistons (from the same set) than that. The main thing to check is the ring gaps.
Be careful removing weight from pistons. There are only a few spots you can safely remove material without weakening the piston. You may just be able to put the heavier con rod on the side with the lighter piston. You are going to change your big end bearings while you're in there, right?

If you will be "lucky"
You may have a letter coded cylinder at one end of it's tolerance limit
and a piston at the other end of it's limit.
I have seen some "mismatches" that will fit together better than stock issue ...

Bottom line:

Don't speculate – MEASURE.

Thanks Vanzen!!! Yes I'd hope that I'm lucky but yes, I will measure and make a final decision on the smaller piston before proceeding. Thanks guys for your insightful and postive vibe..... I was unsure in unfamiliar territory. Mattfish, I'll be changing bigend bearings whilst I'm in the vacinity!!!! and weight matching the pistons and rods didn't occur to me, I might get out of it without having to remove any excess.... cheers, hools!!!

Major Softie wrote:

Rob Frankham wrote:If you're not too particular, you can probably get away with using them. Many manufacturers don't go to the bother of 'A', 'B' and 'C' grading pistons at all. I would say the weight variation is more important (if a smooth engine is what you desire).

Rob

Because, as I understand, flat motors tend to be more prone to piston slap, BMW's more careful matching of clearances was likely to help control that issue. Certainly running one piston size "too small" in the letter grading would be a very slight change in clearance. Going one size too big could possibly be a lot bigger deal.

I can't go along with that because BMW do (or at least did) it for all their engines, car or bike, flat or vertical. I guess its just one of them BMW thangs...

Rob

Yesterday, I wrote up a response to this thread before there were other responses. I clicked a wrong button and it was lost. Part of the response regarded coatings applied to reduce friction. They are applied to the piston skirt, usually by a powder coater specializing in automotive work. I think they may add the tiniest amount of dimension. It sounds like it may be worth looking into in this case.

Years ago, I had heard of knurling piston skirts to grow them dimensionally. I do that on the headset fork crown race area of bicycles to make loose crown races fit tightly. The knurling of piston skirts is where I got the idea. It works great.

Chuey