Andy makes a good point here. That is fitting SP heads to an R motor.

It could get a bit close to the edge if you ain't running top grade fuel. HRC actually recommend avgas for this very application, so if you're pouring low quality juice in then don't expect it to last long.

To be honest, this conversion is probably only best for the hardcore racer who wants the last drop of performance from the machine, and has no problems spending the cash on good fuel and good oil.

But then again, if you aren't a hardcore racer or a serious back lane scratcher why the hell did you buy an NSR?