NSR top end troubles

[TuonoTom]

A while back I pulled my chambers off to take a look at the pistons, and noticed that the top cylinder had some scratching on the exhaust side. So tonight I decided to pull the cylinders off and take a closer look at both.... This is what I found.

Top piston:
http://farm5.static.flickr.com/4115/...b3285ed4_b.jpg

Bottom piston:
http://farm5.static.flickr.com/4135/...2894af5b_b.jpg

http://farm5.static.flickr.com/4099/...0030300b_b.jpg

Bottom cylinder:

http://farm5.static.flickr.com/4137/...71b7914d_b.jpg

I was hoping I would just have to hone the cylinders and do the usual top end rebuild, but my friend noticed that the top cylinder has a crack in the exhaust port bridge Can this be fixed? Also what's the diagnosis on the bottom cylinder, does it look badly worn? The bike did not seize, I pulled it apart to see what condition the top end was in.

The bike is a 28SE, with TYGA chambers, TYGA reeds, an MC21 flywheel, modded air box, and 152s on the MJ. I have been running BR9ECMs, 95 octance fuel, and castrol TTS oil. I live near sea level, and it's winter here so the bike has been used a lot in temperatures around 0 - 10C.

Check http://www.flickr.com/photos/45725426@N07/ for more pictures.

Thoughts, comments, advice please.

Cheers
Tom

[Crasherfromwayback]

Top piston looks like it's cold siezed. Does the back of the piston have the same marks as the front? A cold siezure will leave marks on all four 'corners' as it were on the piston.

[bogan]

from one of the pics it looks as if the nikasil coating has worn away around the exhuast port? would be lucky to repair the bridge, the casting would have soaked up plenty of oil over the last 20 years and would be a prick to weld. What sorta kms have those cylinders done? and has it been properly jetted?

[Crasherfromwayback]

from one of the pics it looks as if the nikasil coating has worn away around the exhuast port?

I saw that too. But who in their right mind would hone a plated cyl like that?

[TuonoTom]

There is only scuffing on one side of the piston, so I don't think it's a cold seizure. I have always been very careful about warming up the bike.

The bike has done 25,000 km, and had a top/bottom end rebuild about 8,000 km ago. I'm not sure how many ks the cylinders have done but I would assume 25,000 km.

I phoned NZ Cylinders yesterday and they say they can weld up the crack and replate the cylinder, for $650. Ouch.

[eelracing]

I phoned NZ Cylinders yesterday and they say they can weld up the crack and replate the cylinder, for $650. Ouch.

Ouch alright.

You say modded airbox and 152 mains...do you know standard size mj's? I'm not familiar with NSR's but it sounds lean to me.

[TuonoTom]

Standard is 140 and is rich as hell on a warm day.

[SS90]

Top piston looks like it's cold siezed. Does the back of the piston have the same marks as the front? A cold siezure will leave marks on all four 'corners' as it were on the piston.

Not quite.

A "Cold Seizure" is actually when the exhaust port bridge expands faster than the cylinder, due to the engine not being warmed up correctly.

Some performance 2 strokes (I am pretty sure the NSR isn't) have a water jacket running through the bridge, to cool the bridge at high rpm, and the downside is if you don't warm up correctly, you can cold seize, due to the piston heating up faster than the bridge.

A fix (particularly good for uncooled bridges like yours) is to drill 2 1.5mm holes vertically down the front face of the piston in line with the bridge, that allows the fresh air/fuel mixture that is under the piston during the transfer stroke to cool the bridge, and ensure you warm it up before riding. (40deg)

Going by the images you have posted, I would suggest that at some stage, perhaps before you owned it, the bike has been overheated.

Pistons are not round, the are in fact oval, (egg shaped), this term is called "cam grind" (in no way related to 4 strokes)

"cam grind" in terms of a piston means the amount of ovality a piston has, which is required because as a piston slides up and down the bore, it has a thrust face, that is, when a piston reaches TDC (plug fires, fuel/air expands, piston goes down), the pison "rocks" on the small end pin, putting load on the front edge of the piston crown, and the back face of the piston skirt ("thrust face"), this "thrust face" is subjected to more heat than the rest of the piston (uneven load)

This is why a piston is actually oval, it allows for the piston, that has different load (therefore heat) in different areas, so, that when it is hot, it actually becomes round (or close to it) like the bore (which, is actually close to perfectly round)

That is why you see 4 contact points on a piston (or sometimes 2)...... a piston never actually seizes "all the way round" due to heat.... it actually only touches on the 4 points that are the widest of the 2 arcs that form the shape of a piston.

Going by the pictures, I would suggest that it has been pretty hot, (also missing nikasil is a good sign too), and that has, over time, caused your damage.

Oh, the price quoted to fix the cylinder? Right in par with the rest of the world.

Do it, you won't regret it.