125 pistons

Could someone tell me what pistons the 125 racers are using? Thanks

RM125, YZ125 amongst others depends on barrel, some running water cooled specials others on BTW ally barrels.

You got a Race bike or building one?

Tony

I am putting together a 125 to race in the states. I am almost done with most of the work. I will be using an D7 barrel, since I am not allowed to run a water cooled.

So, does anyone have a suggestion for what piston I should use with a D7 barrel? Any help would be appreciated. Thanks

I could still use some help. Is no one willing to help? I know I am not racing in the UK but I thought that Bantam racers were always willing to help each other.
Thanks, Andy

Hi Andy you could try early tz piston which comes out at just under 54mm
that has a peg at the baack center so you can get away with a large exhaust port but it will be 2mm over std. but ok if your running 54mm stroke
hope that's some help
Robbie (52)

Robbie, thanks for the info. Sorry to read about your leg. I wish you the best.
Andy

Hi Andy,

Lets assume you are intending to run an air-cooled piston ported 125, using D7 barrel. I run the same barrel on my reed valved 125 Formula Bantam.

I think the problem may be that most pistons these days are desighned for reed valve induction, and thus if you are piston ported induction, you may have difficulty finding a pistn with a long enough inlet skirt.

However, if you get on to Wiseco, they may have something suitable. and being forged, seem the most robust. Based on my machine, I don't think you need worry about the ring peg position too much, as long as it is behind the two transfer ports.

Bear in mind that you need to look for an air-cooled original application , as the water-cooled pistons will have too shallower taper rate on the skirt. Unless of course you enjoy piston re-profiling...I don't!

Do your regulations allow you to sleeve the barrel? If so, this is highly recommended for reliability. I use a centrifugally spun cast iron liner in my barrel. Using a 56mm YZ125 piston circa 1985, I end up with a liner wall thickness of about 7mm. Nice and stable under adverse temperature stress. It also a proper surface for the piston to bear against, rather than the somewhat non-homogeneous nature of BSA cast iron.

Hope this helps.

Thank you for the info. My barrel has been sleeved. It was made to the specs of the 54x54 race engines but I can't use reed valves.

Alan I have used Suzuki RG250 pistons in my aircooled 58mm stroke 125. I think two pistons have lasted for some 8-9 seasons on and off moderate use. I respect Alan Browns wisdom and experience, but I used these with out reprofiling, 2.5 thou clearance in a 3mm liner in a 150 cast iron BSA cylinder. In my short stroke moter (also piston ported but water cooled) I used to use early aircooled RM125 Wiseco, but now use 56mm later wc RM125 to make use of 15mm piston pin in 16/20 RD250 rod. You could go out to 57 or even 59 or 60mm with various Wiseco KH400, RG150 or KR150 ; they are all case reed so have long enough skirts.

John, thank you for the information.
Andy