What causes this?

[evo550]

Only over exhaust port, 2 brand new cylinders and pistons,both lasted about 1 hr riding.
Standard jetting.
Holes drilled in piston. Getting hot there but why ?

[bazza]

do ring pins line up with centre of port bridge?

[TM BILL]

Havre you relieved the bridge a bit  drilling holes in the piston is only half the job.

[evo550]

do ring pins line up with centre of port bridge?

Pins are at the back of piston.

Havre you relieved the bridge a bit  drilling holes in the piston is only half the job.

Not sure what you mean by " relieving the bridge" Bill, both rebores and pistons where done by Serco and I would assume what ever needed doing was done by them.

[Simo63]

do ring pins line up with centre of port bridge?

Pins are at the back of piston.

Havre you relieved the bridge a bit  drilling holes in the piston is only half the job.

Not sure what you mean by " relieving the bridge" Bill, both rebores and pistons where done by Serco and I would assume what ever needed doing was done by them.

I wouldn't assume that at all.  Infact my last rebore from Serco I recall bevelling the port edges myself.  Not hard to do but it must be done.

[Lozza]

Looks like shoddy  chamfering you can also V the ring groove in front of the ex port and the main culprit will be ring end gap

[Nathan S]

Not sure what you mean by " relieving the bridge" Bill, both rebores and pistons where done by Serco and I would assume what ever needed doing was done by them.

Sand down the bridge a bit - give the piston more clearance over the bridge when its cold. It's the hottest part of the barrel and will expand when hot.

Google "TM300 ring life" or similar - they (ad the 250s) have got a too thin bridge that expands back into the bore. Then the ring wears out over the bridge (ie: The bridge wears a notch into the ring) - if you ignore the loss of compression, eventually the ring snags on the exhaust port, wrecking the barrel as well as the piston and rings.

So... have a close look at the ring wear when you get it apart.

[evo550]

OK, all the ports have been bevelled at Serco.
As for ring gap....well there isn't any to speak of, both times it was a new rebore with max of about an hour on it.
First time I sent it back they said looks like overheating, which I thought was a possibility as my raditors where clogged with magnesium rot powder.
I have since replaced the rads with nos ones (thanks Hoony), but to no avail.
Is it possible that a non working powervalve (fully closed) could cause this area to overheat at High rpm?

[Hardex]

What Bike is this? We had problems back in the day with RMX250's doing the same thing after resleeving them.I see you have a wiseco piston in there .The problem was that the sleeve thickness was thicker than it should have been and the sleeve wouldn't get hot enough being water cooled ,and so we had to hone it out more than usual to allow for this problem.

[TM BILL]

http://www.thumpertalk.com/topic/713203-cr500-exhaust-port-bridge-relief/

I would be cleaning it up an.d checking that exhaust bridge , or going back to your engineer and asking what they recomend

[Curly3]

I reckon it's a case of all of the above, all could be contributing factors but I'm surprised the re-borer didn't advise you.
Or, ride a 4 stroke.

[OverTheHill]

Is that a 'wiseco' old style piston number [just visible] wiseco's [i was told] require more clearance than the original piston it would have been running due to there different expansion rate. To shoot that theory down though--if it had been running fairly hard for an hour or so then--don't know. Did totally remove a bridged exhaust port [LA or Wiseco sleeve in my GYT DT1 Kit cylinder] about 40 years ago, bit of a brave [stupid] move but still going. Have a fairly thick ring compared to modern thin ones though & never got snagged. [lucky], Made a big exhaust port though. Anyway-back to the problem, mix it 'oily' & richen up the main & needle a bit then put run it on av gas for a try. One other thought, did you say power valves are not working?, wonder if you tie the linkage in the fully open position--put up with a rush of power & see how she goes. [i missed what make the bike is but] Goodluck. ps, hasn't popped a crank seal out from the magneto side?, be running bad & gaspy lean if it had, couldn't see just a simply worn seal leaning things out 'that' much as would always have an oily mix keeping seal trying to do it's job. Sorry, one more thought [you might have said] have the power valves been relieved [if necessary], probably not a lot of clearance between them & piston when shut & think you're on second oversize which [not sure] could be 20 thou or 40 thou depending on how the oversizes go for your model. Goodluck twice. Damn it, more thoughts before i go, had a KX80 when the kids were young [40ish now jeez] & the water pump impeller had become detached from the shaft so just basically therm-siphoned around, did fix it but must've been like it for ages and never gave up.

[OverTheHill]

Hardex is on to it i think---ignore most of my irrelevant comments. Cheers.

[Davey Crocket]

Thats seems to be a fairly common fault with the guys who did your work, a couple of mates had the same problem recently and the main culprit was not enough bore/piston clearance....a wieco piston is the last brand I would use....go Wossner or ProX first and seizeco last.....also with the cold weather we have been having lately make sure you warm the engine up properly.

[evo550]

What Bike is this? We had problems back in the day with RMX250's doing the same thing after resleeving them.I see you have a wiseco piston in there .The problem was that the sleeve thickness was thicker than it should have been and the sleeve wouldn't get hot enough being water cooled ,and so we had to hone it out more than usual to allow for this problem.

1987 cr 250, still on original sleeve although out to max oversize.

Thats seems to be a fairly common fault with the guys who did your work, a couple of mates had the same problem recently and the main culprit was not enough bore/piston clearance....a wieco piston is the last brand I would use....go Wossner or ProX first and seizeco last.....also with the cold weather we have been having lately make sure you warm the engine up properly.

Wiseco was the only manufacturer that went that big O/S, to save on resleeve....which now is the only option.
Ant idea on what the clearance should be..and ring end gap?
Cold weather not an option here 35deg in the shade