xc300 parts combo?

[Jakobi]

Well yes inded if you had the S3 head you could simply swap inserts. However, if you wanted optimal fuelling doing it this way you'd have custom inserts machined up for both sizes too.

On my 300 with the ports timed for top end, and the highest compression insert (read tightest squish), the clearance still measured in at 1.9mm down from 2.6mm stock. You want to get around 1-1.2mm and that wouldn't have been achievable without either having a complete custom insert machined or milling the mating surface of the outer head as well as the insert which would make all the other inserts useless unless also machined to match the new machined head.

In the end I compromised on the 250 and set the port timing for more mid range which gave me the 1.2-1.3mm squish I was after. It worked out ok being a race ported cylinder that favoured top end anyway. On a 300 if you start dropping it too far you'll lose out up top.