RM125M piston specs.

[John Orchard]

Just wondering if anyone could tell me a couple of measurements of an RM125M ('75) piston please?

1) Measurement from top edge of crown down to the bottom of the skirt on the inlet side.

2) Measurement from top edge of the crown down to the top of the pin hole.

Thanks :-)

[Hoony]

what hybrid you building O man ?

[sleepy]

The compression height(pin center to top outer edge) is the same as TS125 & TM125 which is 29mm
The skirt length I'll have to dig out an old piston and measure it but it is a couple of mm shorter than a TM from memory.
If your looking for a piston for an RM125M there is a Wossner for a TS125 that should work but it needs to be shortened.
Cr125 74/75 is pretty close but the dome is higher and needs the head re-shaped.

[John Orchard]

The compression height(pin center to top outer edge) is the same as TS125 & TM125 which is 29mm
The skirt length I'll have to dig out an old piston and measure it but it is a couple of mm shorter than a TM from memory.
If your looking for a piston for an RM125M there is a Wossner for a TS125 that should work but it needs to be shortened.
Cr125 74/75 is pretty close but the dome is higher and needs the head re-shaped.

Thanks Sleeper, actually I have a rare '77 TS125B, I am porting it to M specs, blocking-off the reed sub-port so to keep good intake port inertia. I might just need to trim-off some intake skirt off the stock piston? I might consider the CR piston but will need all the measurements before I commit to that, anyone got an old one they can throw my way?

[John Orchard]

what hybrid you building O man ?

Just hav'n fun in the shed ...... tinkering :-)

[sleepy]

The compression height(pin center to top outer edge) is the same as TS125 & TM125 which is 29mm
The skirt length I'll have to dig out an old piston and measure it but it is a couple of mm shorter than a TM from memory.
If your looking for a piston for an RM125M there is a Wossner for a TS125 that should work but it needs to be shortened.
Cr125 74/75 is pretty close but the dome is higher and needs the head re-shaped.

Thanks Sleeper, actually I have a rare '77 TS125B, I am porting it to M specs, blocking-off the reed sub-port so to keep good intake port inertia. I might just need to trim-off some intake skirt off the stock piston? I might consider the CR piston but will need all the measurements before I commit to that, anyone got an old one they can throw my way?

That reed is what made the RM125 A so much better to ride than the M or S. I recall racing my S back in the day and being killed by the A models out of corners. The S was probably as fast once up on the pipe but the A would get bike lengths out of every tight corner.
I'll try and find the M piston today and give you the dimensions of it and a CR125 piston.

[sleepy]

As close as I can measure the skirt length on the intake side for the "M" is 56mm, "TM" is 58mm, CR125 is 60mm and a TS125 is 62mm. All are 29mm deck height and the domes are, Suzuki's 4mm high with the Honda 5mm. The ring pegs on the CR are 1mm further apart than a TS125 piston. The Ts piston I believe from around 73.

[John Orchard]

That reed is what made the RM125 A so much better to ride than the M or S. I recall racing my S back in the day and being killed by the A models out of corners. The S was probably as fast once up on the pipe but the A would get bike lengths out of every tight corner.
I'll try and find the M piston today and give you the dimensions of it and a CR125 piston.

[/quote]

I hear ya, back in the day I raced a TM125, HOT-up kit TS125 and an RM125A, I agree the 125A had good response from midrange BUT the way I see it; to get good midrange the piston-port (PP) timing had to be 'mild', that tiny reed aint gunna flow much at high rpm, I realize that the reed starts flowing a little earlier than the PP, and flows a small amount after the PP signs-off.

My curiosity is eating me up, I'm keen to see what top-end I can get out of a wild PP set-up. Most of my rides are big-bores, I reckon I could keep a 125 screaming? I can always swap back to the PP/reed set-up, may even get an RM125A barrel for the TS (same base gasket) to try?

Thanks for the measurements :-)

[sleepy]

My first real MX bike that I raced in juniors was a 73 TM125 that I got second hand. With the help of a local engine reconditioner we machined the carb out to 28mm and cleaned up the ports with a tiny rat tail file. I was able to keep up with stoke CR125's but there were a couple of kids with rich parents that got all the hot DG stuff for their kids and they just pulled away in a straight line. I raced it like that for a bit then modified the the back suspension by moving the shocks forward and using my high school lathe made longer damper rods for the forks to give an extra inch of travel. Definitely handled better but still not fast enough. When I was almost 16 one of the local club guy's lent me his white tanked YZ125 mono for a race meeting as he thought I'd buy it and it was way better than the TM and I almost handed over the money but my local bike shop owner John Walmsley talked me into an RM125S and it was so much better again. Think my first C grade race I won by the length of the straight. I didn't get to race much on the S as I was only 16 and relied on family members getting me to races but I still had the S when the RM125A came out but went from running near the front to at best mid field. Good thing happened when I started to race Vintage I was able to track down my original TM125 and bought it back which I still have. Enough raving. 
Hope the piston dimensions help. 

[John Orchard]

My first real MX bike that I raced in juniors was a 73 TM125 that I got second hand. With the help of a local engine reconditioner we machined the carb out to 28mm and cleaned up the ports with a tiny rat tail file. I was able to keep up with stoke CR125's but there were a couple of kids with rich parents that got all the hot DG stuff for their kids and they just pulled away in a straight line. I raced it like for a bit then modified the the back suspension by moving the shocks forward and using my high school lathe made longer damper rods for the forks to give an extra inch of travel. Definitely handled better but still not fast enough. When I was almost 16 one of the local club guy's lent me his white tanked YZ125 mono for a race meeting as he thought I'd buy it and it was way better than the TM and I almost handed over the money but my local bike shop owner John Walmsley talked me into an RM125S and it was so much better again. Think my first C grade race I won by the length of the straight. I didn't get to race much on the S as I was only 16 and relied on family members getting me to races but I still had the S when the RM125A came out but went from running near the front to at best mid field. Good thing happened when I started to race Vintage I was able to track down my original TM125 and bought it back which I still have. Enough raving. 
Hope the piston dimensions help.

Wow that is cool, thanks for that story, I love hearing them, tell me more :-)

My TM125L was my first new bike (Dad went halves with me), after a Tiny Italjet 50, then Gemini Boss 80. I struggled racing the TM against the CR's, mine was stock save a modified pipe & muffler, my parents lived next door to an unused sand quarry at the time, I wore the bike out, at 15 I realised I could undercut gear engaging dogs, as I could not afford new gears. Later in the TM's life I got a set of Skunkworks LTR linkages, they helped but I was running against RM S models by then. I went straight to an RM125A then, Steve Cramer won it as a prize up in QLD at the time, got it cheap.

[skypig]

I’m not interested in early RM piston dimensions, but this thread is great!!

[John Orchard]

Man I love building these old Suzuki's, so much interchangeability, a complete set of RM80 steel & fibre clutch plates $43 delivered !

[sleepy]

You can be lucky some of the time. Some models though look similar but when you get into them they seem like they have come from a different brand. I could never understand how a PE400 motor could be so different to an RM400, they must have had different design teams and had competitions as to who could do the most different design.
I do remember breaking the cable lug on my TM125 rear brake plate back in the day and tried to convince my neighbor to swap his TC125 brake plate for mine as the TC used a rod pull and didn't need the cable anchor. He wouldn't do it though, don't know why as it made perfect sense to me. Think I cobbled up something with fencing wire and drilled holes as getting alloy welded back then was almost impossible.   

[pokey]

Suzuki  got their monies worth out of plenty of components. That clutch has been around and still around today only the amount of plates vary tween small bores. Frame sections, gears seals conrods and pistons all reused, even many of the hop up kit parts were found as every day parts on other models. Then there are the oddballs like the TC185 that looked like the TS185 but only the conrod and piston in the engine is the same and all other castings are unique and the electrics borrowed from the GT185.  I believe the First RM125's used the same 125 family piston, though like the bores were hand finished. Here is an old thread with a plethora of info that likely you may have already found but if not it may shed some light. http://forum.ozvmx.com/index.php?topic=37151.0