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OK so just re-read this thread to get back up to speed, it's been a while.
The Pre-gen 250 has always had 12.5:1 SCR. Running 10.6:1 DCR.
The 300 because it was sold in South East Asia as well where fuel quality is generally lower they lengthened the stoke but kept the same bore & reduced the SCR to 10.5:1 so it will run on any fuel.
The engine bottom end is the same as the pre/new gen 250 except for the longer stroke.
The pre-gen with heavier pistons revved to 14K/rpm, the 300 to 13K/rpm due to the longer stroke but similar conrods. The 250 had 10gram heavier pistons & produced approx 8% more average inertia than the 300 with a peak 14% BDC inertia, so the 300 has some leeway to play with.
If you add a bigbore, the piston weight will have the biggest impact. Check the table I added. If the DCR does not change much then apart from adjusting fuelling to match the increased air volume to keep the AFR in a good range then everything should be fine.
The closer the new pistons are to the weight of the old ones the lower the additional stress level on the motor. YMMV.
Stock 300 SCR @10.6:1 gives 8.93:1 DCR, my 64mm 12.5:1 SCR gives 10.5:1 DCR which is right on the money for 95RON fuel.
Regarding the fuel, please note in Europe we use the RON measurement system, in US & US influenced areas they use the MON system. The numbers are NOT the same & the anti-knock levels are thus different. In Europe there are generally only two levels of unleaded fuel available 95RON is the basic unleaded & 97-98RON is the "Super" unleaded. There are a few sources of 99RON & or some street side higher octane rating fuel pumps, but these are generally only near race circuits, where the rules state many classes MUST run pump fuel, hence pumps near race circuits ! Plus you can buy octane booster to mix with Super for a higher octane rating again YMMV.
As to the info on DCR, I have run the numbers through the DCR calculator for many bikes over the years & regardless what SCR the maker claims the DCR's have all come out between 10.5-11:1 based on the valve timings. You will notice that any bikes with greater than 12:1 SCR will all have fairly large valve overlaps & you can't compress a gas in a cylinder until the valves close & as engines are designed to run on standard unleaded, then they can't have a DCR higher than 11-11.5:1 max without knocking.
Have fun running numbers ��
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