Matt @ Tyga (or was it Paul?) told me he used to run D pistons in larger cylinders for extra clearance. Since I had a piston nip up on me last year I decided to do that even though it was more my fault than the bike's. However, I forgot to ask what rings to use - do I use the ones for the D piston or the appropriate cylinder, or doesn't it matter? Thinking about which would work better makes my head hurt. I'm putting the engine together now and have the D rings on the D piston so please help!

Thanks for any advice.
Craig

The right ring is the one which when fitted gives the right ring-end clearance...
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In other words, you will want to size different rings, and make sure your ring end clearance is proper. You can use a small file to take some off to make it just right.

That and the NSR250 only has once set of rings (rings are not matched to pistons/cylinders).

Of course I typed this out as fast as I could so it was posted while I checked the online engine rebuild section. And naturally it had my answer - all the rings have the same part number So now all I have to do is fit them up and I should be good to go! Yay, don't have to wait for another order to finish the engine.

You got to watch our for those tyga boys, they tried to sell me a set of power bands and some muffler bearings...

At least I think that's what they said, I couldn't really understand their typing, it was all in that british language or something.

Tyga has bower bands??!! I'll put 2 on each cylinder!!

What ring end gap are you working to ?

The online rebuild section says .30-.45mm so as long as I'm somewhere in there I won't touch them. (I'd rather have a factory end than one I've carved myself.) Any suggestions for a "race" setting? The tracks I'll be at are for the most part short (1/4 mile straights) but I'm hoping to visit some longer ones this summer, namely Road Atlanta. Whether or not Charles will admit to knowing me when I get there is another question...

I work towards 0.24mm and change rings when i get to 0.35mm . I notice a difference of 6bhp on the dyno with fresh rings only .

Wow....ok thanks. Maybe someone will chime in with power vs reliability (while flogging like I plan to).

The engine is in and I'm connecting everything up now.

You don't want the piston too loose as it will rattle in the bore and break the piston skirts . Make sure the bike is at running temp before you speed off . I have never measured the piston to bore clearance of a worn piston to compare results .

My method was:

Short twisty circuit with no long straights = tight piston/bore, advanced timing, high comp.

Long fast circuit, flat out forever = loose piston/bore, retarded timing, low comp.

Reasoning:

On a short circuit with only short bursts of acceleration there's little chance of excessive heat build up/piston growth, so you can run a tighter clearance. The high comp and advanced timing is relative of course, but this gives good punch out of corners, but little over rev.

On a fast flat out forever track the opposite of above is true.


As for ring end gap, affecting the power. I'd say not a lot, if you're talking the difference between 0.24mm and 0.35mm on a new ring. The power loss comes from the loss of pressure of the ring against the bore when the ring is worn. A keystone ring does help here of course, as it is mechanically forced against the bore wall by design.

We use the same advice here in darkest africa !

Matt@TYGA wrote:
Long fast circuit, flat out forever = loose piston/bore, retarded timing, low comp.

I never knew you could fit a Seizuki ignition to an NSR!
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Andy.
NSR-WORLD.COM

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