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.

Is the R21 piston pin at the same height, seen that XZ Vii

As the rod is the same and all the ports do come at the expected timings then I dare to say that the piston pin has the same height but I can not tell about the XZ because the XZ has a different bore/stroke so a different design than the the current 210x (based) models

Speaking of mix matching parts sets and cases, I had surprisingly good results with a Shimo 2 case, crank, carb, button, with 9mm ultra short venturi with the Blue head r21 piston and sleeve with the Ielasi 2185/V3 manifold. I hate to say it but it makes more power and made more snap and pulled farther than my OS Gt2, I was in disbelief and swapped engines 2x to back up what I was feeling. Down side is it drank soooo much fuel it was shocking. It drank as if I was running a Roma,no bs. If I was running the 7mm then the playing field would have been about equal and the Gt2 having a more direct feel and the fuel consumption would have favored the Gt2.
So after that little test I swapped in my spare (broken in) 2024 One part set in the Shimo case and other components and WOW, again it drank fuel but at least at a believable rate . It made a tick lest of the tip in on the trigger but the spool up was a lot more aggressive and pulled longer than the r21 Hybrid, possibly my fastest Gt engine that isn't a Big bore.(Memphisto/Mito 35 GT5) In terms of outright performance these both make my Gt2 look average ..not to mention this Gt2 is an ASM version modified by Muran, hate to say my Hybrid engines with zero modifications run away or will run down a VERY expensive engine that has been modified . Both ran a Rp6 .If any ones has these engines and are bored and want to see what I'm describing ..your welcome ,oh the Muran ASM Gt2 had a Max power ported back plate to boot.

To get more insight of what you are doing with changing parts you must measure the created timings because there are the performance differences made.

Is that using the same crankshaft?

Need to rebuild a Euro 1. Any advantage to using Euro 2 P/S/R SET? Assuming they are interchangeable. I know the cranks are different but stroke the same.

That will work.