Actually 123.6722256 cc ....... LOL . Equal bore x stroke (aka square) motors rev harder and don't get as hot as longer strokers . They've got the best balance between torque and HP ......... Long stroke motors reach critical mass of the reciprocating parts much earlier meaning they hit a mechanical rev limiter and they break parts and wear out much quicker if revved constantly.........
Note how Honda went to a Bore of 66 mm and a Stroke of only 43.7 mm when they wanted a Hi Powered 4 stroke race engine for the CRF150R !!!!!!!! They achieved around 22HP stock and there's NO way on the planet that they would have done it with a longer stroke . Most of the gains are from cutting down on piston speed and reducing rod angularity which equates to a drastic cut in friction from piston thrust on the cylinder wall . That allows them to use a lightweight low drag piston with virtually NO skirt .
In a lot of cases you can gain a lot of power simply by cutting out frictional and drag losses which are similar to the brakes on a vehicle dragging and holding it back ........
I have modified XR75's since 1974 and have found that a 50 mm bore a 41.5 stroke goes the hardest ........ I've bored them to 54 mm (Powroll 96 cc kit) but the rev limit dropped drastically . The 54 mm piston is a lot heavier and the extra cc's of the bore can only breath to the max limit of the head .
Incidentally I have a genuine Yoshimura close ratio 5 speed tranny that has super thin gears less than 7 mm wide . That tranny alone increases the rear wheel power by heaps due to far less oil drag on the thinner gears .
Taylors mowers (GT Motors Aust) used to have pamphlets stating that you could order Jialing 125's with special thinner geared racing trannies ...... but the added price was sky high .
