Montesa Rings Dilemma

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I thought this deserved a new post subject. I am trying to source some new standard 72.5mm rings for my Cota 247 T. I bought a set off eBay but they are twice the thickness of the originals ie: 1.5mm rather than 0.75mm.

So far I have tried Don Newell , Bultaco Australia, In Motion UK, Kevin Breedon UK, South West Montesa USA - with no luck so far.

Can anyone suggest another supplier.

This may me be silly idea but would it be possible to have the 1.5 mm thick rings that I have ground down to .75mm and if so where would I go to have that done?

Thanks in advance

Andrew

[Stork955]

Gday, its easier to open the ring grooves up in the piston in most cases, however the pegs may give some trouble. Finding the correct set will be easiest. Google a wiseco catalogue and see if you can match some rings from the dimensions in there.

Cheers,

Stork

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Stork - the Wiseco 2 stroke rings 72.5 mm rings are all 1mm thick - they have an XA designation which is 0.80 - I can just fit a 0.80 feeler guage in the groove. This ring is a 4 stroke ring - would that make a difference.


I found francetrialclassic.com and they have the correct size listed but not certain as to thickness - awaiting confirmation - 39 Euros plus post - I could purchase a new piston and ring set but at 195 Euro's I'd rather hold out for a little while.

Andrew

[Stork955]

Gday Andrew, yes -4 stroke rings are different in that they dont have a groove for the locating peg. Often they are a different profile as well for oil control reasons. Obviously on a 2 stroke that is not an issue. Hopefully the french set are the correct size for you. When they arrive you can do a couple of quick checks to make sure they are OK.
1. Check the fit in the groove - they should slide in nicely and not have much "up and down" play (dont put the ring on the piston, just try this with the ring beside the piston and gently tilt if that makes sense)
2. Check the back clearance -push the ring down to the bottom of the groove and make sure it sits below the surface of the piston. This makes sure the ring is not too wide for the groove. (check as above)
3. Check ring end gap by setting the ring into the bore using the crown of the piston. Watch out for the ports. Around 4 thou per inch of bore is good (0.1 mm per 25 mm of bore size, so around 0.4 mm for your bore size). It doesnt have to be perfect, but too little clearance is no good. You can gently file the edge of the ring to adjust, make sure you keep the profile of the locating point if you need to file.

Cheers,

Stork

[paul b]

I recommend you try Serco in Brisbane. They are experts on bikes. You can post them the barrel and piston.
Serco stands for small engine reconditioning company.
I am always happy when I deal with them.

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Paul - yes I have been speaking to Serco they are the ones I checked for a Wiseco option

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Does anyone know if a Bultaco Tarabusi piston kit will fit my Cota?

[TriCub]

I have had a look at a Montesa Capra piston that is here for a cylinder boring job and it is closer to an Ossa piston than a Bultaco. The deck height for the Bultaco piston is 24mm and the Montesa and Ossa are 35mm(that's the center of pin to top edge)
If the ring groves are the same as the Capra Tarabusi piston you can use the .8mm 4 stroke rings from Wiseco and just file the end gap to suit. Your piston will have some carbon in the groves making them measure a bit smaller than they are. The ring gap for the Tarabusi pistons is 1.9 to 2.15mm from memory. This may seem wide but the ring pegs go all the way to the outside of the piston with the ring ends either side. By the look of your piston from the other post there hasn't been to much ring leakage due to the lack of carbon build up down the side of the piston. What is your current ring end gap?

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TriCub

My current ring gap is 2.20mm - are you saying my rings are ok? I don't know whether to be happy or sad

Thanks

Andrew

[TriCub]

I've seen Ossa's with a bigger gap than that and they ran ok. The ring pegs are about 1.5mm in diameter so your real gap would be .7mm which is about the wear limit but if there is any wear in the bore the ring gap would be large even with new rings. I would need to measure things myself to give you the complete story but I would stick it back together if it was my bike.