KT100 OverBore


New member
looking to see if i can get a piston to increase the CC's on our KT100...? Can they be bored to a 125cc ? we race at a track that basicaly has no rules, and we are heavier then everyone else, so we need to make some more steam.....thanks in advance for your help.....JD
The biggest piston you can buy for a Yamaha is a 55mm. Stock stroke is 46mm, 55mmx46mm=109.2cc. If you buy a stroker crank they are a couple of different options. Buller sells a 52mm, or a TT25/27 crank will drop in the cases just needing clearance for the rod and its 53.7mm, or you can send your stock crank to Falicon and have it stroked to what ever they can get out of it safely which is probably going to be 52 or 53mm. The problem there is you will probably then need to re-sleeve to change the port timing and can be quite costly for an engine that will only be for your local track, 55mmx52mm=123.5cc, 55mmx53.7mm=127.5. I would suggest a reed or rotary 100cc, run it stock and you will be in the 28-32hp. Open it up to a 115cc you will be in the low 40's and have a lot less invested than a full blown Yami.jmo
Leave it stock and go buy another stock KT to match. Set them up as twins and go have some fun. Twin KT's are a blast! You will not be disappointed in the amount of power for the amount of cash you would have to spend compared to what you would have in a full blown open KT.

Brian #89
I have a 98mm iame rod for sale. if stroking for stock appearing, this rod will eliminate the spacer plate..