Ring clearances

[(F5)]

pretty mint, no scraping at all really.

PS found gasket stripper works better than oven cleaner on PVs

[Jakobi]

Gasket stripper works a ripper! Costs more than oven cleaner but gets the job done quick and easy!

[(F5)]

just did my mates 300 & set them at .5. Safest & don't want to take chances with his bike.

Man I need to get my head mod sorted out to reduce the silly 2mm squish. but it can wait

[Wheelieman69]

I am working on a 1999 xc250 (possibly a 2000)
I believe this is a wosner piston
Top ring end gap is .015 mm
Bottom ring end gap .010 mm

Cylinder wall is nice n smooth no gouges marked more on the exhaust side

Piston has marks on the side but pretty good looking still

After reading up these end gaps seem super tight and this is after an entire season of riding the bike and pretty hard too ! Maybe 80-110 hrs

Should I just put everything back together and ride on or open the end gaps before putting it back together ???

Bike ran really well all summer then the flywheel nut backed off pinning the fw and breaking the key. I decided to see what the top end looked like while I was wrenching. No damage to anything thankfully just replaced $10 key and adding a 8 oz fww as well see how it changes her. First gasser and I love it !
Pictures to come shortly..

[pscook]

Originally Posted by Wheelieman69

I am working on a 1999 xc250 (possibly a 2000)
I believe this is a wosner piston
Top ring end gap is .015 mm
Bottom ring end gap .010 mm

I question these numbers. Why? 0.015mm = 0.0006". I am pretty confident that you won't get a feeler gauge thinner than 0.002" into any gap, let alone one that is that small. I believe that you are measuring in inches, so you have a metric gap of 0.381mm on the top ring, and 0.254mm on the bottom ring.

If you look at post #10, Jake gives a good representation of a good gap. I think you are fine with your numbers, assuming that you switched measurement modes.

[Jakobi]

I'm guessing he meant .015" and .010" (0.38mm and 0.25mm)

That would make more sense. Still on the lower end of spec. Maybe last rebuild someone forgot to spec the end gaps. In saying that if it was built with tight tolerences and never seen many revs, kept a clean air filter, a quality oil, and never been drowned, it wouldn't be unheard of. The first set of rings I pulled out of my 300 at 75hrs came out with 0.4mm gap. The next one saw 85hrs and came out also only at 0.5mm and had seen much more throttle application. I'll be pulling the current 250 piston down soon and be interested to see how much wear they've had. Its at 100hrs now and has been ridden harder than I ever had any of the 300 top ends.

[Wheelieman69]

yes jakobi that must be. but my feeler gauges say mm on them so thats why i put that???dgcues@hotmail.com
hers some pics
how do you know when the barrel needs to be redone as well ?

[Jakobi]

The real way is to have someone with the right tools mic the barrel and make sure its still round etc. The backyard mechanic usually just gives it a good cleaning with an abrasive pad to remove the glaze. If you've worn entirely through the cross hatching then its seen some time. If you can still see them its usually good enough to go another round. Thats assuming no obvious damage. Cracks, flaking, scoring etc.

[(F5)]

I've just lost tips of rings on an aircooled 52mm bore that had the gap in the boost-port. Evidence was it was bulging when hot (airs get a lot hotter it has to be said). but that was set at 0.3mm. I'd always gap the rings now to be sure. its a big bore & 0.5mm isn't going to cause any performance drop off.

Light rub of scotchbrite & if not grey ally showing through you should be right for this age bike where you're not chasing 0.1hp.

[Wheelieman69]

Thanks f5
I used the scotch brite and all seems good. I'll send out for nickasil and rb mod next time apart.