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R90/6, 1974, right side cylinder and piston are trash. Looks like finding a cylinder and piston for that year is going to be difficult. I bought a cylinder off ebay, sold as a R90/6 cylinder, it might be, but likely a late 1975 cylinder or off another model. It was suggested to me that I might look at having the spigot on the new cylinder turned down to match the 97mm
of the correct cylinder that I have. The new one is 99mm on the spigot.
The other problem is the distance of the spigot on the 99mm spigot from the base to the end of the spigot is 22.30mm.
The correct cylinder I have is 24.15mm. The new cylinder will not go as far into the engine as the other one, I'm thinking that might cause a problem.
Physically the new cylinder is 1" shorter on the width, its not noticeable unless you have the correct cylinder next to it.
If i can use the new cylinder I would likely by the other side which is available.
The question - is the spigot depth on the new piston a problem?
Any advice or suggestion's, much appreciated!

Can't think opf a reason why a difference of less than 2mm would cause an issue, expecially as it's less rather than more. Many people have turned down later cylinders to fit early crankcases and I've never heard of a problem. You might want to compare the distance between the head gasket face and the base gasket face to be sure.

Rob