Causes of apparent piston seizure?

[Tumbles]

Ok, won't be able to pull my engine apart and troubleshoot it until next weekend, but wondered in the meantime if anyone knows of any other potential causes of my problem.

http://www.kiwibiker.co.nz/forums/showthread.php?t=74908

All I can think of is piston seizure. No fuzes broken, still has full electrics, bike rolls with clutch in or in neutral.:argh:

[Ixion]

Catastrophic piston seizure is uncommon on a four stroke outside racing circles.

Could be big end, or cam chain jumpe doff and jammed, or valve hit a piston. Any noises when it happened?

Piston seizure on a four stroke is seldom instantaneous, more a very rapid slowing down like putting the brake son hard. But , when an engine blows they tend to disregard rules

You won't really know till you pull the heads.

Have you tried to see if you can drag the rear wheel backwards in 5th gear. Sometimes doing that (trying to turn the engine in reverse) will unlock a nip up.

[mowgli]

Ok, won't be able to pull my engine apart and troubleshoot it until next weekend, but wondered in the meantime if anyone knows of any other potential causes of my problem.

http://www.kiwibiker.co.nz/forums/showthread.php?t=74908

All I can think of is piston seizure. No fuzes broken, still has full electrics, bike rolls with clutch in or in neutral.:argh:
Sounds to me like a broken conrod but then I'm no mechanic.

[Tumbles]

Good info cheers.

It was an instant thing so that gives me hope that it is a snagged cam chain like you say or perhaps a camchain guide going tits up rather than a full blown welding of piston to cylinder :sweatdrop:

I'll definitely be pulling off the cam chain covers before the whole engine comes out. Will keep you posted on results.

[speedpro]

They have a roller bearing crank I think. Could be a bit of "stuff" from rebuild has got somewhere it shouldn't be. As has been said, it's pretty unlikely to be the piston/s if it was sudden, more likely crank or cam bearing. Even there the cam bearing is more likely to flog out than jam the whole engine. Be intersting to find out.

[Tumbles]

They have a roller bearing crank I think. Could be a bit of "stuff" from rebuild has got somewhere it shouldn't be.

Yeah it is a 2 x roller bearing support crankshaft. They were both new and I am 99% positive that no crap got in on the rebuild. Is it bad that there were a few o-rings spare afterwards though? (jokes)

Still thinking cam chain/guide at this stage.

[Squiggles]

Oh noes, looks like you'll be joining Drider (he's having his SRV worked on at the moment)

[Katman]

Piston seizure, particularly after having major work has been done can be caused by something as seemingly unrelated as an incorrectly positioned clutch cover gasket. I recently had a DR650 in with piston seizure that I finally tracked down to that very problem. Sometimes within the structure of the clutch cover is a gallery that forces oil through a jet that squirts oil onto the underneath of the piston. If the gasket is not sealing that gallery properly not enough oil pressure will be built up to provide sufficient 'squirt'. Overheated and seized piston results.

[Tumbles]

Piston seizure, particularly after having major work has been done can be caused by something as seemingly unrelated as an incorrectly positioned clutch cover gasket

Well, she's a o-ring around the clutch cover, no gasket. But I hear what you're saying about gaskets, o-ring etc and how they act to direct oil into specific places. I'll be having a quick look at anything that I can get to without pulling the engine out f the frame. Will let you know how I go.:niceone:

[Tumbles]

Ok, I shot up to Palmy ast night, grabbed my tools, came back down to Welly this morning, and got stuck into it.

The cam chain and guides look fine.
The engine can be turned over using the flywheel nut in reverse only. When you got to turn it forwards, it locks INSTANTLY It doesn't lock when you turn it back to where you started from, it locks up straight away.
Exactly the same results are gained when turning the her over with both cylinders removed (the whole things down to the trans).
Valves, cylinders, pistons all look fine.

So: Its definitely not anything to do with cams or pistons. This leaves!: The bottom! Yay! I'm so happy right now!:no:

So. Anyone got any more ideas on where I should be looking first? I'll be attacking this tomorrow night.

[98tls]

Have you drained the oil on this thing yet?

[Tumbles]

Yes.

blah blah blah to make up enough characters for the forum to accept the message :buggerd:

[98tls]

Yes.

blah blah blah to make up enough characters for the forum to accept the message :buggerd: So if anything what was in it?

[Tumbles]

Oh I see where you are going now haha. Nothing was in it. Not a thing. No rats, rags, small babies or engine debris.

[Dak]

Starter motor? One way clutch or sprag clutch, don't know what they use, might of destroyed itself.

[Sully60]

Starter motor? One way clutch or sprag clutch, don't know what they use, might of destroyed itself.

Yeah, it sounds more like a transmission, drive, starter type thing than the engine. Two gears meshing that shouldn't can give the one way only impression.

[Tumbles]

Starter motor? One way clutch or sprag clutch, don't know what they use, might of destroyed itself.

I see where you're going here, I'll check it out.

[Tumbles]

Yeah, it sounds more like a transmission, drive, starter type thing than the engine. Two gears meshing that shouldn't can give the one way only impression.

Yeah agreed. BTW another signal was she rolls forward in neutral sweet as. Its just when its geared it doesn't. Also just selecting gears feels completely normal. There is none you can't select and no unusual clunkiness. Therefore I think I might check the flywheel/starter side of things first. Besides, getting to the gears on these means splitting the crankcase so I'm keen to make that the LAST option. :msn-wink:

[Ixion]

Ok, I shot up to Palmy ast night, grabbed my tools, came back down to Welly this morning, and got stuck into it.

The cam chain and guides look fine.
The engine can be turned over using the flywheel nut in reverse only. When you got to turn it forwards, it locks INSTANTLY It doesn't lock when you turn it back to where you started from, it locks up straight away.
Exactly the same results are gained when turning the her over with both cylinders removed (the whole things down to the trans).
Valves, cylinders, pistons all look fine.

So: Its definitely not anything to do with cams or pistons. This leaves!: The bottom! Yay! I'm so happy right now!:no:

So. Anyone got any more ideas on where I should be looking first? I'll be attacking this tomorrow night.

Can you turn it back indefanately (ie more than one revolution)?

The only thing that locks in one direction only by design is the starter drive. Though sometimes a loose wotnot can jam in such a way that it forms a sprag . I doubt it's transmission.

[Tumbles]

And that's exactly what it turned out to be.

The centrifugal clutch designed to let the starter motor drive the crank shaft but not let the crank shaft drive the starter motor has failed, meaning that the engine was constantly spinning the starter motor for a period of time. This fragged the starter motor to the point where the starter motor seized, siezing the whole engine.

Once I realised the starter motor had siezed, I removed it and took it apart to check the damage. I had to bang apart the start motor with a hammer and I estimate 10-15% of its total mass came out as dust. Results below.

So now its off to TSS for a new (second hand if avail) starter motor and to borrow their flywheel remover. I'll keep ya all posted if anything obvious comes out of the centrifugal clutch inspection.

:rockon:

[Ixion]

Check trademe . There's one of the wreckers puts up an XV250 starter (should be identical) every so often. (Never an XV750, alas)

EDIT. Found (http://www.trademe.co.nz/Trade-Me-Motors/Motorbikes/Parts-accessories/Engine-drive-train/auction-157579214.htm)it. Bucket-n-bits, down your way.

[Tumbles]

Check trademe . There's one of the wreckers puts up an XV250 starter (should be identical) every so often. (Never an XV750, alas)

EDIT. Found (http://www.trademe.co.nz/Trade-Me-Motors/Motorbikes/Parts-accessories/Engine-drive-train/auction-157579214.htm)it. Bucket-n-bits, down your way.

Cheers mate, I have asked them previously and didn't get a reply. I'll try and make contact again.

[FJRider]

Cheers mate, I have asked them previously and didn't get a reply. I'll try and make contact again.

Check starter button switch for continued contact, dirty or wet sometimes does it. OR broken spring in button

[Tumbles]

Update:

Well, I've taken the flywheel of and the one way starter clutch at the back of it looks fine. Turns out its the gear behind this, turns the starter clutch during start, has locked itself onto the driveshaft effectively bypassing the clutch. Despite my best efforts with a bearing puller this afternoon, I have been unable to remove this gear from the driveshaft. Therefore its time for TSS Red Baron to do what I cannot and fix my bike. :argh:

Also managed to pick up a starter motor from Buckets and Bits. Cost me $200 though which is gutting.

Thanks to TSS again for letting me borrow their flywheel puller and bearing puller no questions asked. You guys rock.

[Tumbles]

Well, turns out the gear can't be saved, the copper(?) bush had welded itself to the crank. A new gear costs $360 so seeing if Buckets and Bits can do me a second hand one for a bit less than that. Anyone got any advice on how that could happen? I'm worried that it means the oil isn't tracking through the engine properly. Any ideas on how to check that?