TM125 Piston

[Mick22]

I'm trying to help a guy get his TM125 back on the track and need to sort a piston out for him.

I know bugger all about TM's so any help appreciated

He had it rebuilt and at the time was told TM pistons were no longer available so the guy that rebuilt it used a Wiseco Honda piston that was similar but had to  cut down the skirt. He raced it and burnt a hole in the piston, so the wiseco number is no longer visable

So are TM pistons easily available?

Any ideas which Honda piston it could be?

Thanks, Mick

[cyclegod]

Use TS125 pistons and cut 5mm from the skirt or scour eBay for a TM piston, what size is he after?

[Graeme M]

I've used both TS125 and CR125 pistons quite happily. 73-75 CR pistons (easily found, I have quite a few now), and I think TS125 up to mid/late 70s sometime. Both need their skirts trimmed to work properly. You can find TM pistons on the odd occasion, but needs careful searching. Don't forget to try searching on 1975 RM125 as well, as they are the same piston.

[cyclegod]

Don't forget to try searching on 1975 RM125 as well, as they are the same piston.

No so Graeme, the RM125S has a piston that runs ultra thin rings and is different to the RM125M which is similar to the TM piston.

[Graeme M]

Ah yes, good point, the S is different. I have an M and the TM and RM-M pistons are very similar. I have never seen an S piston. I have seen a few differing opinions about this piston trimming thing but I have used CR pistons in my RM for a few years now and my TM125 before that and had no problems. I am not especially careful about how much is trimmed though - 5mm seems to be the accepted figure. It is possible that is the reason my TM was not very quick and my RM feels not much faster. Any thoughts on that?

[TT5 Matt]

the rm125s piston/rings would be the go as the super thin rings drag less power [less pumping losses] but need replacing more often

[cyclegod]

the rm125s piston/rings would be the go as the super thin rings drag less power [less pumping losses] but need replacing more often

Every 3 racing hours 

[Lozza]

I would doubt 3 hours being the service life of a ring, 0.7mm Honda RS 125/250 and TZ125/250 rings last 500km. An RM-S would unlikely to be as highly strung as one of these.The 0.7mm ring isn't used for 'less drag' or pumping losses(which something else altogether) it's because they have a habit of trapping rings on the exhaust side of the piston. The 0.7mm (down from 0.9mm) stopped but not eliminated the problem.

Wossner CR125 single ring 

[DR]

6 hours of race time for the thin rings but they're easier and cheaper than the regular rings as you can simply use the 3 piece 4 stroke oil control rings so long as the bore size is close. Butt ends means you can file back the ring gap if needed. 6 hours of 'race' time is actually quite a bit of riding

[ty4]

checked the wiseco piston i have, the number is 364  p8 but is early early red box style
box has tm125  rm125m  rm125s  written on it  80 thou up which i think is 2mm
hope that helps
tony

[Mick22]

Thanks guys, Cr125 piston on the way