Author Topic: RM125S Piston rings without notch for pin, eh?!  (Read 1251 times)

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Offline John Orchard

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RM125S Piston rings without notch for pin, eh?!
« on: July 15, 2023, 09:06:13 pm »
I have some NOS pistons & rings to suit '75 Suz RM125S, the rings (the .60mm thick ones) do not have a notch where they end at the pins, with the standard size rings fitted into a new 56mm bore, there is a 1.4mm end-gap. I think that when fitted to the piston, and slipped into the cylinder, a minimum of 1.2mm gap is required for the pin thickness, so the 1.4 would allow them to not jam hard against the pin when hot.

What are your thoughts of running such a wide end-gap? I was thinking of running .25 over rings, and grinding an angle at the ring end, to minimise the end-gap, but I worry that a .6mm think ring does not leave much metal to handle the heat?

Has this matter ever arisen for RM125S owners before?
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Re: RM125S Piston rings without notch for pin, eh?!
« Reply #1 on: July 16, 2023, 05:06:16 pm »
Not sure I've seen that on RM125S before but it is common on most Ossa's. The ring peg in the piston is flush with the outer diameter of the piston and the ring gap is around 2mm. First time I sore it I was convinced the rings were stuffed with such a big gap but with the new rings in the bore it looked the same so I looked a bit closer.
Just dug out a new RM piston that has been sitting here for many years and yes it has very narrow ring grooves and a pin the comes very close to the top so that's normal as it seems. Not enough room to file the rings to fit over it unless you take the peg down a bit first but all a bit tricky to do. Just run with it as is, it seemed to work back then.

Offline John Orchard

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Re: RM125S Piston rings without notch for pin, eh?!
« Reply #2 on: July 16, 2023, 08:36:07 pm »
Johnny O - Tahition_Red factory rider.

Offline pokey

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Re: RM125S Piston rings without notch for pin, eh?!
« Reply #3 on: July 17, 2023, 07:23:51 pm »
This service bulletin for 1975 I think you may find handy. http://www.ozebook.com/compendium/techbuls/SerBulletin%20Specifications%201-5.pdf

Offline John Orchard

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Re: RM125S / RM250A Piston rings without notch for pin, eh?!
« Reply #4 on: July 18, 2023, 11:22:49 am »
This service bulletin for 1975 I think you may find handy. http://www.ozebook.com/compendium/techbuls/SerBulletin%20Specifications%201-5.pdf


That is interesting, thanks. I notice that the RM250A (70mm bore) has the same situation re ring end-gap.
« Last Edit: July 18, 2023, 02:55:09 pm by John Orchard »
Johnny O - Tahition_Red factory rider.