Can anyone tell me what piston this is?i was told it would be printed with the letter a,b,or,c on it.Really struggling to get the piston kit I need.And she’s smoking badly also spitting oil out of exhaust.....Help...

Should be stamped on the crown of the piston,but can't see 1 on your pics,But what ever size you fit ,the barrel will need to be re nicosealed to suit.https://www.splatshop.co.uk/montesa-cota-315r-piston.html

Might be stamped on the barrel - above the gearbox sprocket  - seem to ring a bell.

Does the bore look worn  - damaged  - might just need rings .

If its just smoking and blowing out oil are you just putting too much into the fuel - is it sounding rough or down on power.

If oil mix is ok check crank seals to make sure its not drawing oil in from there

 

 

 

Oil coming out of the exhaust has nothing to do with rings or the piston on a two stroke. Jimmy is correct its either too much oil in the petrol or the crank seals are gone.

If its constantly running at very low rpm it may just need to go for a blast down the road.

its hard to tell from that picture but the piston & bore look ok?

take a picture with the piston all the way down 

 

Edited May 24, 20206 yr by suzuki250

All of the above, check if your gearbox oil is down pointing to crank seal failure.

Looks like a single score mark on zooming in

2 minutes ago, Bilko said:

Looks like a single score mark on zooming in

Think its dirt as its hiding the ring.

The piston was full of black sludge when I took the barrel off.I was hoping for a quick fix?

4 hours ago, NickMacro said:

The piston was full of black sludge when I took the barrel off.I was hoping for a quick fix?

You may well get one not involving a new piston. Apart from smoking badly and oil coming out of the exhaust are there other symptoms ? When a new piston is required the engine will rattle at tick over. If it's not doing that then it doesn't need a piston. 

Things to check 

Is the float set correctly in the carb

Has it got the right jets in the carb

Is it using gear box oil

How much 2t oil are you mixing with the petrol ?

Edited May 24, 20206 yr by baldilocks

Out of interest, did you remove the piston to clean the black carbon.off the top of the piston or clean it while in the barrel?

I just gave it a wipe inside the barrel.i was just cheaking the piston size and top see if that was the problem.i cleaned it.Started it and still smoking and spitting black oil out of exhaust.Ive was trying to see what piston I needed?Still don’t know ?

Have you read all the suggestions and ruled them out, one by one ? 

Not yet I’m going to start tomorrow ruling them out one by one.Ive only just got the Bike a couple of weeks ago.Trying to make her run perfect 

Was it running like that when you bought it? Or has the problem developed since, if since have you mixed too much 2/ oil? Maybe if you ride it for a bit it'll clear if the exhaust is soaked in oil? If the bike sounds ok I'd run it around a bit, maybe build up throttle to see if it clears.