Honda trx 250 r over-bores

Ted Hamilton

helmet painter and racer
Stock displacement is 246cc, max allowed is 255, so you're allowed to gain 9cc's in area, so working in reverse...
66 * 66 * 3.1415 = 13684.374.....add 900mm*2 = 14584.374..... divide by 3.1415 = 4642.487, take the square root = 68.136mm
68.136mm is the max allowed bore, Stock bore is 66mm.
 

Taft Racing

Member
Yeah Stock bore and stroke on the honda 250's are 72mm stroke and 66mm bore, how ever running a displacement calculator I found that a 67mm piston would put you at 253cc and the next size over bore would put you at 257cc which is illegal in UAS racing. I think this alone is a good reason to change the displacement rules regarding the 250cc 2 strokes. only being able to rebore a couple piston sizes isn't realistic. and at the 68mm piston you'd be 261cc still some 40cc less than a wankle.

http://www.mk5cortinaestate.co.uk/calculator2.php
 

arc100

Member
Stock displacement is 246cc, max allowed is 255, so you're allowed to gain 9cc's in area, so working in reverse...
66 * 66 * 3.1415 = 13684.374.....add 900mm*2 = 14584.374..... divide by 3.1415 = 4642.487, take the square root = 68.136mm
68.136mm is the max allowed bore, Stock bore is 66mm.
Dont mean to nit pick Ted, but 68mm bore will yield a 261cc capacity. Also most 250's after 2001 are 66.4 stock bore and 72 stroke.
 

arc100

Member
Yeah Stock bore and stroke on the honda 250's are 72mm stroke and 66mm bore, how ever running a displacement calculator I found that a 67mm piston would put you at 253cc and the next size over bore would put you at 257cc which is illegal in UAS racing. I think this alone is a good reason to change the displacement rules regarding the 250cc 2 strokes. only being able to rebore a couple piston sizes isn't realistic. and at the 68mm piston you'd be 261cc still some 40cc less than a wankle.

http://www.mk5cortinaestate.co.uk/calculator2.php
Actually Tim, thats a really good point to argue. I hope you also argue the case of 450mx competing with the 250mx 2 strokes in AMA. Also that the 125's couldnt even get a look in until they allowed the 144cc conversion.
 
Top