does any body know if a 1985 atc 200 cylinder will fit a 1990 trx 200?

 

No. It is a completely different design.

The cylinders you need to be looking for are.......

'86-'87 ATC200X
'86-'88 TRX200SX
'86-'87 TR200 FatCat [Motorcycle]
And another model as yours....'90-'97 TRX200

 

i've read somewhere that the internals of a atc 200s are the same as the atc 200x. is this true??

 

The cam is different and the cylinder has larger cooling fins, but everything else in the topend of the ATC200S is identical to the 1983-1985 ATC200X.

The 1984 TRX200 [only year made....ompletely different from later models] will also interchange with the '83-'85 X top end.

However, the S and X bottom ends are in no way similar other than engine mounts. They are the same.