Hi all My mate has bought an ns250r of a 1984 vintage, its been sat in a barn for 12 years since being imported..

Anyway, finally got it registered yesterday, and we went out for a spin.

Unfortunately, we got a bit carried away and ended up going a little bit too quick for the bike seeing as it had dont 60 miles in 12 years well I was flat out on my tdr250 and the ns was still gaining distance on me! I came through a cloud of blue smoke, which smelt rubbery, and when we got back it turned out the bike had locked up

Quick check, still running, no unusual knocks or noises but not very happy on the rear cylinder...

Barrel off today, seems to have gotten a bit hot in there, the spark plug electrode had melted a little on the tip sure signs of overheating inside, a little 'dish' in the centre of the piston and a 5mm piece of piston vanished off the crown on the exhaust port side. Plus the usual seizure marks, luckily bore seems good!

Anyway... any ideas why this happened? The bike is bog stock, runs fine, 8000kms on the clock.

Also, where can we get pistons/gaskets from? Dave silver didnt seem to want to help much!

Thanks!
Paul

Well i would think that you are looking at a weak mixture.
How well did you clean the carbs? did you give the power jets a good clean through?
As for pistons, Honda or Tyga. Use 28 pistons, search the forum for posts on sizes.
Oh! and fill out your location. you never know, help could be just around the corner

Cheers fella, dont think there are powerjets on the carbs! I'll put pictures of the damage up later, seems very rich in there!

Am I right in thinking the mc 28 is crankcase induction? The '11 is manifold to barrel jobbie so I doubt the '28 pistons will work without holes being put in them?

Thanks!
Paul

That will be me not putting on my glasses thought it said 1994.
Pistons will be a big problem for the MC11. don't know of anyone stocking them

Hmm and it seems honda have them as NLA Unless I can find some old stock it seems I have 2 options;

Match the sizes up with another bike, TS125R seems to be similar in dimensions..

Have a piston made from a block of alloy. Expensive but a consideration..

Unless anyone else can come up with any suggestions?

Paul

rgv, kr1s are 56mm bore, i havent got any knackered rgv pistons lying around but im sure if you asked on the rgv forum someone would kindly measure gudgeon pin size and post a pic for you,think the old rg125 was 56mm as well

Try a look on the Wiseco site:

http://www.wiseco.com/Catalogs/DirtBike.aspx

While they may not have something for the NS250R, the PDF catalogues are good for cross-referencing piston and gudgeon pin sizes. That could narrow down the selection to a few pistons. You may find you need to modify the barrel if you use a different manufacturers piston, so it would make sense (if that is the case) to do both barrels so matching pistons are used.

I would hate to say all is lost though, when you see people with all kinds of old vintage 2-strokes restored and running. The NS isn't that old just yet!
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Andy.
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jezzbennettsm wrote:rgv, kr1s are 56mm bore, i havent got any knackered rgv pistons lying around but im sure if you asked on the rgv forum someone would kindly measure gudgeon pin size and post a pic for you,think the old rg125 was 56mm as well

I have a few pistons kicking around, the rgv one is close but is flat topped, also being crankcase induction there are no port windows in the skirt.

The closest I found was out of my old tz250, but again no rear port windows although I could possibly machine some in if I got desperate.

Gutted a piston sold on ebay last night, doubt one will come up again in the near future!

Thanks Andy, I'm going to take a tdr250 for a mot this morning, my friend who does the mot's recons he can match the piston up for us to something similar. Fingers crossed!

Paul

If you can not get an MC11 piston , fine one that is neares, note positions of ring pegs. ( they need to be the same or very similar. Do not worry too much about there being no piston skirt window this can be cut very easily. Make a template and reference it from the old piston crown. Then carfully cut new window in piston with die drinder ( dremell) make sure the window is very smooth and radiused in the corners and polish off any "machining marks".
I have done this many times when racing a 500cc maco in MX side car racing, I NEVER had a skirt failure as a result and my 500 reved to 16,000 rpm.
Try looking in Mahle piston catalouge or Karle Smitt pistons.
Good luck,
Dave.

Thanks dave, I think I have found a good match

NS125 pistons are more or less exactly the same dimensions, pin dia etc just the crown is about .5mm too high. Have ordered one to try, hopefully find a good solution to this problem!


PAul

Paul,

With regards to the piston, .5mm is a fair bit, I would advide that you try and calculate the amount of additional squish you have with this piston and have somone carfully remove the corrisponding amount from the inner surface of the head OR try and find head gaskets that are .25mm thicker than you currently have. I say .25mm as the actual area of the gasket is much more than the area of the piston and .25 should accomadate the additional squish volume.
IF you run the pistons as is with the .5 mm hight increase and nothing else then i would make sure you run super unleaded in it at all times this should prevent detonation.

Good Luck.

BR
Dave.

You may also consider a slightly thicker base gasket.
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Andy.
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Andy wrote:You may also consider a slightly thicker base gasket.

I was thinking that, mind you would be worth measuring the squish as all too often I have seen 2mm+ on engines as standard! Could work out ok with the higher pot (although probably not with my luck! )

If that (0.5mm) is the difference in overall height from the pin to the outer edge of the crown it's huge and needs to be compensated for by an equally large increase in gasket thickness to retain stock compression.
A 0.5mm taller piston will sit 0.5mm further up the barrel and reduce squish by 0.5mm.

If the piston dome is of a different radius but the pin to crown edge measurment is the same you need to work out the new compression ratio and make sure you don't change it too much.
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Ah well got the piston for the ns125, quite different really, think old grampian need to update their book!