Hi,
Im new to this forum. My name is Micke and I living in Sweden. I just bought a Nsr 250 se mc21. Havent owned a 2-stroke for about 20 years
It dont started so good, after 10min in my first ride the bike died the bike was a bit weak from the beginning, high revs to take off. So i think it was damaged when i bought it.
My problem is that i have H2 barrel rear and L1 in front, but they have compitable top. R top for R barrel and SE/SP top for SE/SP barrel. The L1 is the damaged barrel. I dont understand how I know which piston size I need for that barrel? Some people says i should have a, b or c stamped on the barrel, but i dont. Just L1 and a long number "1ds1L15".

Hope you understand my bad English

Kind regards
Micke

The original marking for the barrel size is on the rear of the barrel at its base. L and H determines the barrels original hight not its bore size.
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If I have to take the carbs off once more...

The best way to know your cylinder bore (diameter) is to measure it. There were markings that did apply when the cylinders were new, but it is over 20 years and you do not know the history of the cylinder, like if it has been replated or not.

If the cylinder is damaged and the lining is not repairable, then it needs to be replated. When you get it replated, you will usually send it away with the new piston and they will get the plating/piston within tolerance.

Thank you very much! Found it! It was "b"

I cant measure it my self right? I need special tools for this?

There are 3 pistons available, B, No mark, and C,
A barrel =B piston
B barrel= no mark
C barrel =D piston.
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Proud Father of , 05 ktm 400exc supermoto 2018 honda crf rx supermoto

You need a bore gauge and a micrometer. Once you know your bore size you can calculate what size piston you require depending on the clearance you would like to run.
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If I have to take the carbs off once more...

Sorry to bring up an old thread , I'm looking at doing the top end on my mc18 Stumbled across this thread trying to decide what size pistons to order. Once I got the barrels off I noticed a very vague c marking on one and none on the other. Both Pistons have b markings on top and are only single ring pistons. From what I have researched on here so far (new to nsr's, excellent site but) This isn't Quite correct? I am guessing this means I will need to measure the barrels? If I was to get the barrels re-platted can I buy any A, B or C piston and Barrels will be matched or do I need the smallest piston of the lot?

Do any of the aussie bloke on here know of workshops that could replate the barrels for me?

Thanks guys

If you go down the replating path, buy yourself a set of pistons and you send them off to the replater and they will match the cylinder bore to your pistons.

There are a couple of places local that can replate, but I have been told that the best are overseas.

If you think that the cylinder is suitable to reuse, measure your cylinder and then find the piston to match. Off the top of my head I don't know if the piston types have been measured.

You want to double-check what you have there. Single-ring pistons aren't just a simple case of "pop them in", so you may find that stock pistons don't fit!
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Andy.
NSR-WORLD.COM

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