A forum for owners of Yamaha TTR250 trail and enduro bikes!

Darren · Veteran Member

Piston sizes

TTRfan · Super Guru

Same as Open Enduro Daz:

Piston Size (mm)

72.95 (standard)

The following oversizes are available:

73.45

73.95

74.45

74.95

The biggest oversize gives 263cc. Wouldn't want to go much bigger on the standard sleeve/liner as it would get a bit thin!

I advertise the Wossner pistons on TTR Extras (scroll down the page) but you might be able to source an equivalent a bit cheaper in Thailand wink

There was a guy used to tune TTRs in the UK (Phil Manning?) and, although his techniques were shrouded in mystery, I have it on good authority that he bored the carb. I have no idea how this is done so, as you have seen it, what do they actually do?

Brian

lewisfullard87 · Member

would you have to rejet the bike or do any other mods for a biger piston

TTRfan · Super Guru

Not in my opinion. The difference in capacity is slight.

Brian

davecambo · Senior Member

Brian what would be the difference in power/torque if you were to bore and add the biggest piston you list. would it be as noticable as adding an exhaust or something?

TTRfan · Super Guru

davecambo wrote:
Brian what would be the difference in power/torque if you were to bore and add the biggest piston you list. would it be as noticable as adding an exhaust or something?

I have bored the project bike engine out for the biggest piston but I can't comment on performance at this stage as I haven't run the engine yet.

However, logic says that 13 extra ceecees is over 5% increase in the basic 249cc. I am guessing that I will get a similar percentage increase in torque and bhp. Much the same increase as fitting a good after-market silencer.

The only real test would be to put the bike on a dyno.

Brian

davecambo · Senior Member

and if a layman was to do this, would it be as simple as purchasing the piston and rings, then taking off the top end and having the sleeve bored and honed by someone, then putting it back together?

TTRfan · Super Guru

davecambo wrote:
and if a layman was to do this, would it be as simple as purchasing the piston and rings, then taking off the top end and having the sleeve bored and honed by someone, then putting it back together?

Yep - although taking the top end off isn't all that simple - quite a bit to it - but not beyond a reasonably confident spanner twiddler - see http://www.ttr250.com/Removing_Cylinder_Head/TTR_head_removal.htm

Whilst the head is off, and the barrel is away being bored, it makes sense to drop the valves and check them before grinding them back in, fitting new valve stem oil seals and adjusting the valve clearances wink

Brian