Bruno,

All MC21 and MC28 cylinders/heads are interchangeable, but there are differences between the R and the SE/SP models:

The R model heads are low compression.
The SE/SP heads are high compression.
The SE/SP barrels are taller than the R barrels.

The piston rings are the same across the board for all models now. Just make sure they are gapped correctly.

If you use SE/SP barrels and heads on the R bottom end, you will be fine, you will simply have stock compression.

If you use SE/SP heads on R barrels, you will increase the compression, consequently R heads on SE/SP barrels will give a low reading.

The ideal way to go therefore, is R barrels with SE/SP heads, if your class doesn't allow modification of components. Even if it does, this is the best starting combination before you begin the port work.

Andy.
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