SP barrel, SP head normal compression
R barrel, R head normal compression (both the same in fact)
R barrel, SP head higher compression.
SP barrel, R head lower compression.

R barrels are marked H and 86.3mm tall. R Heads are also marked H
SP barrels are marked L and 86.5mm tall. SP heads are also marked L

You can get .4 and .6 HRC base gaskets as well as the std Honda .5 ones.

To illustrate Steve's point I have a L barrel here (should be 86.5mm tall) and it measures way under 86mm.
It also has a cracking score mark down it where it seized at around 100mph!
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