Is the only difference between the two types that increases the compression the length of the barrels? Are the pistons the same?

I ask as I have a few boxes of old pistons and barrels which I suspect are all 2cv ones but I was going to measure them incase any Dyane (High compression) ones slipped in then just clean and re-ring.

Also what is the difference in length? What would I need to take off a 2cv barrel to make it high compression?

cheers

Tim

The pistons are different. Compression ratio usually marked on top of piston IIRC. Ken may confirm.

Yep,
as knightley has already advised, the barrels are identical.
9.0:1 pistons have more of a 'hump' on the top, in addition to those identifying marks, which may sometimes have been polished to the point that the markings are no longer legible.

The compression ratio when using 8.5:1 pistons can be raised by machining the top spigot of the barrel, about 20thou / 0.5mm will give 'almost' 9.0:1, iirc.

ken.

hmmm... will browse the bits i have and see whats there. What are the marking like? (I expect I have seen them before but I have a memory like a drunk!)

I have bought NES high comp pistons with skimmed barrels in the past whenever I have re-built stuff but I am poor now

Tim,
once you clean any carbon of the piston crown, you should be able to find either 8.5 or 9.0 cast into the metal, along with the arrow indicating which direction the piston needs to be fitted.

Altenatively, just line them up and see if any of them have the crown higher than their neighbours...

ken