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Discussion Starter · #1 ·
Just rebuilt my 250 exc, started it up, let it warm for a few minutes, blipping the throttle every now and then, very low revs though. Turned it off and let cool, thought I'd check under the head to see if everything looked good to go.
Looks to me like I may have been close to a cold seizure.
The cylinder had a fresh replate by apticote, and I sent the piston off with it so the clearance should be bang on.
Checked the ring end gap, x and z clearance, and let it warm very very steadily.
I've pulled the cylinder off, and the streaks look like aluminium deposits from the piston. The streaks stop at where the bottom ring reaches at BDC, which made me think that it may be the rings causing the streaks, but then I saw the aluminum build up.
Any advice appreciated,
Cheers.
 

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Not looking good, obviously.

What is going on in the ring area in the 1st pic? lots of small bits look welded to the piston

The marks on the cylinder don't seem to correspond with anything on the piston.

To me that looks like debris in the rings has caused it

How thoroughly did you clean the cylinder when you got it back, past experience says you need to spend quite some time cleaning them.
 

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Discussion Starter · #4 ·
Cheers Stumo. That's debris looking stuff is tiny beads of coolant from when I took the head off. The rings do have slight aluminium deposits though, although they are very hard to see.
I have bad ocd with this stuff, and so everything was impeccably clean. The cylinder was washed several times in warm soapy water, blown out with compressed air, then cleaned with wd 40 and a cloth. Theres no way anything could have been left in there.

The back of the piston does correspond to the marks on the cylinder, but the front doesnt for some reason. The piston wasnt catching the powervalve either
 

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That looks to me like there isn't enough clearance. It sort of looks like the rings have buckled causing high and low spots, and the Teflon coating on the piston has been worn off.
Has it been re-plated to an A piston size, and the one fitted is a B ?
 

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Discussion Starter · #6 ·
Cheers mate. The piston was sent off with the cylinder, it's an A but it was replated and honed to that size, plus clearance. I agree though, it seems like the clearance was too tight. The piston has worn through the coating and you can see where some of the aluminium has rubbed off on the piston onto the cylinder as well.
 

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And? What did they say?
 

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Discussion Starter · #10 ·
Turned out to be just ring drag. Apparently it's common, spoke to some dealers and they said they've seen brand new KTMs with streaks like mine from when they were dynod at the factory. Dissapears after a short time, and that's why I saw it as I removed the head after 5 mins of running.
 

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What?? Ring drag, never heard of that before! And how do they explain that a load of the Teflon coating has worn away after 5 minutes? That piston looks many hours old.
 

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Discussion Starter · #12 ·
The coating wears away pretty quick but it has worn fast. I think the hone on the coating might be quite rough and the chamfering around the ports isn't amazing so that might be why the ring drag is pretty bad, and the coating has worn pretty quick. Had the clearance checked and it's all good.
 

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What?? Ring drag, never heard of that before! And how do they explain that a load of the Teflon coating has worn away after 5 minutes? That piston looks many hours old.
Seen that once on a mates KTM 300 EXC, but only the marks in the cylinder, nothing with the piston, as we could not look at it while installed and through the exhaust we din't see much.

The bike was running for about 10 minutes after top end rebuild, it hasn't been ridden. The cylinder head seal was leaking so head came off immediately and cylinder looked exactly like the one from Squeakybum. My mate was furious and said he will not go over the top end again and he will ride it until it explodes. Run it in a few hours and all seemed to be normal, so he just rode it. Took the head of after 25 hours just for peace of mind and the marks were gone, cylinder looked like new. I have no clue if that is pure coincidence or not nor do I have a plausible explanation.
 
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I had my head off at 8 hours to fit the TSP head and the bore looked like that, still running fine at 48 hours

2019 250 TPI

Sent from my SM-G960F using Tapatalk
 

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Given the Teflon does not last long and disappears, could the streaks not simply be smeared Teflon pressed on the cylinder wall at points of higher surface loads? When you then take a look later on the piston has very little Teflon on it and the cylinder has been cleaned up by the rings ready for hours of happy ragging?
 

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Discussion Starter · #17 ·
Could be Pindie, but the streaks on the cylinder dont correlate with the streaks on the pistons, plus the streaks the cylinder are only where the rings start and stop.
Runs real nice though, and compression is at 200 psi cold with an sx head so the rings seem like they've seated
 

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If I were you I'd take the head off after about 15 to 20 hours and see if anything has changed. I bet the streaks will be gone. Unfortunately I have no clue why the piston looked kind of used.

In the meantime I talked to a guy who repairs engines for a living and he said it could be that the cylinder is slightly oval, something that is not unusual. and that could cause the marks. But he said it is pure speculation and he would also run it a while then take the head of and measure the bore.
 

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Discussion Starter · #20 ·
Cheers Steve, someone else told me that as well. I did check and put a ring inside the cylinder and shine a light up through to see if I could see light between the ring and cylinder.
 
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