A stamped pistons in B stamped bore
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I have just set about stripping the head off my donar engine and found the piston crowns to be stamped with a B. The pistons I was planning on putting in are stamped A.

I'm guessing B stamped pistons are larger then A stamped pistons and will throw piston to skirt clearences out.

If this is the case I'm screwed!

Edited by minimole23 on 25th Mar, 2008.

I don't know what the A & B means...

Measure it and see. This is the only way.

if im right in thinking (i may be wrong) the a b thing is the grade of piston not the size

a and B are different pistons in the sens that they are not exactly the same bore so A bore pistons are going into A bored bore and B bore pistons are going into B bored blocks,
you can have an A block honed to B specs, then reverse isn't possible

Edited by Sprocket on 23rd Mar, 2008.

Oh, and there is nothing wrong with a little more piston clearance on a high performance engine

Thanks, I will just get on a tear it apart.

The mains are the same history. Just had an A+ ABA block line honed, where the centre main bearings had wiggled like crazy. B= Bad.

ok, to give me an idea if they are likely to be fine, so I dont rip a good engine apart for no reason i tried the pistons in the bore from the top down.

The clearences were if anything a little tight at 90 degrees to the gudgoen pin and a little slack at the end of the gudgeon pin. I can only assume that it is the clearences at 90 degrees to the gudgeon pin that are important.



Also One of the pistons has a slight ding in the skirt next to the gudgeon pin. It is only slight but Will this be ok to use?



Thanks

Piston Clearance is measured at the bottm of the skirt and across the pins ( not along)

Thats because there will be a larger clearance at the pin boss because there is more mass there and they expand more, its at the bottom of the skirt too because the piston has a larger clearance at the top due to there being more mass there also

If I recall, for a high performance engine, clearancies are around 2.5 to 3 thou, but if its a reguar road engine expecting to see decent mileage, then set for the upper limit on the factory clearancies.

If the clearancies are bigger than 3 thou, i'd be looking at either new liners or oversize pistons. dont forget pistons wear as does the bore. to much slop and the engine will be noisey when cold, loose an efficient ring seal, suffer blowby, and increased oil consumption.

I just hat a look at mine. They're AYA..Y=Junk :)

Y = yellow and is used if the Green is too tight. Red is used if its too loose.

I have a block thats YYY lol

Well, I took the block to my machine man today, and mesured the pistons. They will indeed fit the block after a hone which is good.

I have a YYY stamped block.

Thought id add here rather than start new thread.. i have 2 engines.. one with an A stamped on the face of the block.. and one with a B stamped on the face of the block.. this one also has B stamped on the pistons.. from what i gather the tolerance thing is b block means b pistons.. etc but what is the difference between a and b?

The difference is minimal I would say, but relevant for a production engine.

We just honed the B block out, ran 5 thou piston to bore clearance and it ran like a dream. Now sat under the bed for a rainy day as I replaced it with a 1380 engine.