I am rebuilding a OSMAX R21-MAX which is the cheap 5 port engine with the blue head. For this I am using a 7 port OSMAX R21-Eurospec set because the price difference is not big. Last year I have heard from someone who did the same with a Hasi set and could not get the engine right. After some checks it seemed het had 0.5mm more head clearance.

Meanwhile I have seen several OS based engines and yes, there are differences, the 5 port has a 2.5mm upper edge on the sleeve and the 7 and 9 ports have a 3mm edge.



On the left the eurospec and on the right the 5 port R21-MAX

When putting the wrong sleeve into the crankcase the ports are on the right height so the differences are with the combustion chamber. The 5 port has a complete flat surface arround the chamber while the 7 and 9 port engines have a 0.5 mm deeper surface on the spot of the head shims to correct the 0.5mm thicker edge on the sleeve.
I can not tell if all 7 and 9 port engines have the 3mm edge by standard, it may differ between the many other OS based brands.

It is something simple to take care about when you want to place a different P/S set in your OS crankcase. In my case the easiest and cheapest way was to trim down the edge on my lathe.