I have a 1987 block that my machine shop says needs to be bored out to 100.5mm due to wear of the cylinder walls. Is this legal? I don't see anything in the rules about boring the cylinder walls.

Thanks,
Mark

If the rules doesn't specifically say you can, you cannot. Boring these Alusil blocks is a fairly fussy, and unique process, and tends to be more expensive than finding a better block. If it's a few small scratches, I wouldn't worry about it.

All of the spec cars out there are running the stock bore and stock pistons.
New pistons may not even be available. But if they are they have to be the stock size.
We have discussed in the past allowing a replacement piston to be made but so far that has not been needed.
The bores will get bigger towards the bottom of the stroke since all of these engines are usually high milage street motors. This is something all of us are dealing with and does not seem to be a problem.