R11S Technical Data - Piston diameter
 

Hi,

On page 11.9 of the repair manual it gives information on tolerences and wear limits.

Can someone give me a nudge as to what A, B and AB are refering to?

Cheers

Adrian


http://forums.pelicanparts.com/uploa...1563198574.jpg

If it were me I would only be concerened with A which is 6mm from the bottom edge of the skirt.

Could only guess what B and AB are refering to. Since they only give specifics for A go with that... mid-skirt is a common measuring area for piston diameter.

If I may suggest, the overall clearance is 0.005", assuming your pistons are pretty close to round still, focus on this, but when checking pistons, I mic the shirts to the crowns to see if the skirts have collapsed in, with these pistons it can happen, look at the oil ring groove and when the return runs across them, long slot, it's easy for them to collapse in, racing pistons often just drill holes here and even add stiffening ribs to the skirts to prevent bending in (collapsing), if the skirts are in a couple to several thousandths, replace but that's just my opinion...

Thanks for the replies.

Bad - I take it you are referring to the "Total wear clearance of piston and cylinder" values.

Further no one really knows where the 'B' level should be measured as they have negelected to point out where?

BMW made 2 sizes of cylinders and 3 size of pistons, based on machining tolerances. The “A” piston goes with the “A” cylinder and the “B” piston goes with the “B” cylinder. The “AB” piston will work with either A or B cylinders. I think AB pistons were only sold as parts. The letter A, B or AB should be stamped on the piston/cylinder, but I’ve seen pistons with no markings.

Steve

Interesting...

So the use of "A" as a measurment plane identifier and a size indentifier for cylinder matching is just meant to confuse the **** out of us.... Thx BMW.

Good info Steve. How did you find this out?

Correct...answer for A, B and AB above makes sense to me as there is always some tolerance variation in production and this sounds like a good way to close the gap without extra work or rejections that would go with just one spec...

Mmmm.

I've got A2 'embossed' (as in raised) on the inside skirt of both pistons.

To the left of that 'A2' I have 99L5 running vertically, to the right 'MAHLE' running vertically to the piston surface on both pistons.

Opposite skirt I have a 'stamped' (as in recessed) 51/8 on one piston and 50/8 on the other with 'F011' to the right on both pistons.

I guess A is close to A2...

Maybe I'll hasstle MAHLE....

Thoughts?

Adrian

I would not reference anything cast into the piston for a machined manufacturing tolerance, casting were before machining, which it appears matched final dimensions for the best fit across 3 groups of tolerances...I would mic the cylinders and the pistons across the same plane on both (not orthogonal to each other) to determine the clearance, only need the skirt sides in my opinion, the wrist pin hole sides do not typically wear much at all....