The bike ran good before I took it apart. It did have trouble near the redline. It looks rich to me & not sure if the oil below the rings is due to this.

http://im1.shutterfly.com/procserv/47b6df28b3127cce96d035f4c86300000016108CbsmTVqzbw
http://im1.shutterfly.com/procserv/47b6df28b3127cce96d035f0c86700000016108CbsmTVqzbw
http://im1.shutterfly.com/procserv/47b6df28b3127cce96d04648094e00000016108CbsmTVqzbw

OK i'll start! Looks like a bit of ring gumming and a fair bit of blow past. how worn are the rings, and what type of oil are you running.

Is it my imagination, or is there only one piston ring?
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Andy.
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I think Andy that there are 2 grooves and only one ring there.....

Am I right in thinking you ucan get some race pistons with only 1 groove?
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j911brick wrote:I got second dibs on those books you already sold.

Has it got the right size pistons?there are 3 different size barrels identifiable by the letter A,B,or C stamped somewere around the bottom of the barrel ,there should be a letter printed on the top of piston but its usually burns off but ive washed off some old pistons and its still been there.Andy should be along soon to tell you how to measure the pistons the were the tec info is for pistons sizes( because hes good like that)

I'd say the pistons are shagged!

One ring doesn't help either, and personally, if I was going to consider running one ring on a two ring piston, it'd be the top ring I'd use. As Wayne says, there is so much evidence of blow-by, that a set of pistons and rings look like a must.

Piston to bore clearance should be 0.039mm ~ 0.045mm. Service limit is 0.09mm. Anything greater than this and the piston(s) must be replaced.
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Last edited by Andy on Tue Jan 10, 2006 12:54 am; edited 1 time in total

Yes, there is only one ring. I'm using the other to measure ring gap. It looks like I'm due for new rings. The feeler guage .508mm fits tightly between the ring. Shouldn't rings last for quite awhile? I'm not exactly sure how many miles this set has on them since my speedo hasn't worked for a while. If I were to guess I would say less than 4k. I will look at the part #'s used on the last rebuild. I know the cylinder is the A type. I'm running BelRay synthetic.

0.5mm is the service limit for the rings. The gap should be 0.3mm ~ 0.45mm.
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Andy.
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hey spengilly, which nsr do you have?

sorry to invade in your thread but does anyone know if the top and bottom rings are the same? I'm about to order a topend kit (from tyga) and there is only one kind of ring available.
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1994 NSR250SE

big pete wrote:
sorry to invade in your thread but does anyone know if the top and bottom rings are the same? I'm about to order a topend kit (from tyga) and there is only one kind of ring available.

The ring set is the same for the top and bottom cylinders, the top ring and bottom ring per piston are different. The top ring is a "keystone" shape, the bottom ring is plain.

Only one part number is now available, and is suitable for all NSR250 models.
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so when you order the rings, they come in a set of two?
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1994 NSR250SE

A set of 2 rings for one piston.

You need to order the part number x 2 for two cylinders.
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Andy.
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Thanks Andy. I just thought it was a bit weird when I looked at Tygas site and only saw piston rings X2 on a topend rebuild for two cylinders
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1994 NSR250SE

I took the other cylinder off & the piston looked the same as the first. I used the B type piston with the A cylinder the last time it was rebuilt which was approx 2yrs ago.

How do you measure piston to bore clearence?

Could the excessive blow by be due to rich jetting? Would the excess oil in the mixture find it's way past the rings through time? I know the rings need to be replaced but they are not too far past the limit. I also noticed the limit is .65mm in the parts manual.

Thanks for the help.

You need a bore guage. First measure the piston and lock your mic' to its size. Then using the mic', set your bore guage to read zero when inside the mic'. Put the bore guage down the bore and it will read the clearance. Take note in 4 corners top and bottom and work out if the clearance is too big due to bore wear, piston wear, or alittle of both.

Dont hold the bore guage too long as the heat from your hand will distort the reading and let it stand alittle while before rechecking the next piston and testing the bore.

Also check that the cylinder are still round (using the bore guage), if they arnt this will lead to fast ring wear and early blow by.
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If I have to take the carbs off once more...