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315 Pistons

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On 26/11/2017 at 11:27 PM, dwayne said:

Hi all,

I got the new piston, it has only two rings with it, there are no identifying marks or makers name on it. Wrapped in a soft grease proof paper in a plain box.

I have sent it and the cylinder to Langcourts, i am waiting for them to get back to me with a price for fixing the cylinder and matching it to the piston. Fingers crossed.:unsure:

Hey Dwayne,

What piston did you get and where from and did it work ok

Edited by Andyb1987
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Original piston kits still available:

http://www.splatshop.co.uk/montesa-cota-315r-piston.html

We had a couple of batches of the aftermarket piston rings and sent them back as the quality was terrible.  The only nice option we have found at the moment is not easy:

Dykes (Top Ring), just need gapping:
http://www.splatshop.co.uk/72-25-dykes-piston-ring-top.html

Flat Piston Rings - needs machining:
http://www.splatshop.co.uk/72-25mm-x-1-2mm-flat-piston-ring.html

On the flat piston rings there are a few issues that need to be sorted:
1. The internal diameter is about 0.5mm too small so needs machining out.
2. The pin cutouts are slightly too small so need filing out to 2mm wide and 1.8mm deep so it leaves 1mm to the outer edge of the ring.
3. The end gap is a little tight, so it will need checking and gapping to the right spec.

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Hi, No markings on the package at all, its working fine, it only has two piston rings just like a cr 250 piston. I sent the cylinder and the piston to langcourts so it was guaranteed to match.

Works fine, no issues.

https://www.sadboyracing.com/online-shop/dirt-bike-parts/#cc-m-product-6770788011 

The racing one i got is out of stock but they still have the three ring piston kit which I can't vouch for, but there are few options.

 

Edited by dwayne
Info needed

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On 20/02/2018 at 11:26 AM, splatshop said:

The Honda CR250 piston is 66.4mm so way too small for a 315R, the Montesa has a 72.2mm piston.

Can the cylinder be bored to take a different piston like a beta for example

 

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19 hours ago, Andyb1987 said:

Can the cylinder be bored to take a different piston like a beta for example

 

Some one mentioned something related to a Beta piston fitting a 315 but no evidence appeared with the exact piston so you are on your own with that experiment. As far as I know the pistons are available but rings are difficult to obtain although dykes rings that will fit if they are modified are or were available at splatshop in Derbyshire..

If you are a polisher/ prettifier then it will mean either spending half of what the bikes worth to fix it in time and money  or if you want to actually ride then the answer is to just buy a bike that you can get spares for in your price range.

Edited by oni nou
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The original piston has a unusually long distance from the centre of the gudgeon pin to the top edge of the piston, on the original piston it is roughly 32mm.

Most other 250cc trials bikes are around 28-29mm so to keep the squish clearance correct and the port timing you'd need to measure both pistons very accurately and then get the bottom of your cylinder machined down to suit.

I've never done this so there may be other reasons why this not work, for example there might not be enough room for the front pipe to fit, there might not be enough material left to keep the coolant water ways the correct shape and the cylinder to frame mounting bracket would need altering to suit.

You'd also need to make sure the piston ring stop pins are at the correct angles, the piston skirt doesn't hit the crank when it's at BDC and it doesn't open the exhaust port at TDC. The radius on the cylinder head would need checking and you'd need to re-machine the cylinder head and gasket to suit the larger diameter piston.

So yes it may be possible but check everything first before making the plunge as it's not so easy to put material back on after.

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4 hours ago, splatshop said:

The original piston has a unusually long distance from the centre of the gudgeon pin to the top edge of the piston, on the original piston it is roughly 32mm.

Most other 250cc trials bikes are around 28-29mm so to keep the squish clearance correct and the port timing you'd need to measure both pistons very accurately and then get the bottom of your cylinder machined down to suit.

I've never done this so there may be other reasons why this not work, for example there might not be enough room for the front pipe to fit, there might not be enough material left to keep the coolant water ways the correct shape and the cylinder to frame mounting bracket would need altering to suit.

You'd also need to make sure the piston ring stop pins are at the correct angles, the piston skirt doesn't hit the crank when it's at BDC and it doesn't open the exhaust port at TDC. The radius on the cylinder head would need checking and you'd need to re-machine the cylinder head and gasket to suit the larger diameter piston.

So yes it may be possible but check everything first before making the plunge as it's not so easy to put material back on after.

Thanks for the reply I think thats to complicated for me it will be easier to buy the dykes ring etc and have them machined to suit.

 

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Bought a 315  kit off Albert at Sadboy. Not inexpensive, but not as dear as other providers. Its a  3 ring kit, Size "B" . Came with all gaskets, etc. Circlips, pin, rings, piston all individually packaged with Honda part numbers. Piston itself has "Gilardoni" cast on the inside, plus a part number. (Gilardoni make all of the Beta barrels / pistons & probably other makes too. Have never had any bother with Beta.). Quality looks fine.  Barrel off to Langcourts for Nikasil, & will no doubt have to take a great deal of care to ensure rings are in correct location, gapped, fitted right way up, etc, etc. Quite fortunate to have access to a Honda trained master Technician, so he will be doing the final fettling & build.  

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make sure its not one of their own, they have been know to give up after a couple of hours. If it is a gilardoni and a part number can you share these please?

Can you compare to the original and either post a photo side by side or let us know if its the same?

Does it look like the piston has been machined?

 

Sorry for all the questions but if it fits and works = major breakthrough in the 315 world!!!

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Will do. Glad to help. You'll have to be patient though, as I am unavailable until later on this month. Will provide all part number & hopefully some pics.      

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