Piston size

[Myowncreation]

I purchased a new set of 75mm jugs. This might be a stuipid question but the old pistons look good but are 74.90 at the skirt. Should I use them or get new pistons? I think I know the awnser but this is my first engine rebuild so any advice would be awesome.

 

[TwoManyXS1Bs]

Wow, the threads/posts on this subject number in the hundreds. A good bit of forum searches should make your head explode. Start with the tech section and the 'similar threads' noted below. You will find answers to questions you never thought to ask...

 

[XSLeo]

The bore size is never exactly 75 mm. The pistons are all 74.xxx. The bores are always 75.xxx. If you look on top of the pistons you will find a three digit number. This is the xxx in the 74.xxx.
If you look at the sleeves where they stick out the bottom of the cylinders you will find a three digit number. This is the xxx in the 75.xxx.
Now subtract the 74.xxx number from the 75.xxx number and this is the clearance between the piston and cylinders.
It will be between .050 and .055 mm
Lets say your pistons have a 956 on them, your cylinders are 008. The 956 is actually 74.956. the 008 is 75.008. So 75.008 - 74.956 = .052. Right in the middle of the .050 to .055 spec.
You need to measure your cylinders and find exactly the bore size. Then subtract the piston size. This will give you the clearance. Is your clearance between .050 and .055 mm? If you can't do the measurements take them to a machine shop and let them measure the pistons and bores to verify the clearance. If in spec use them If not get the next oversize and get your cylinders bored to match the new pistons.
All this info is in your repair manual. You do have a repair manual don't you?
Leo