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valve hitting piston?

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hi guys, my friends got a yzf250 04, it died suddenly one day and wouldnt start and had low compression, after a bit it would start again but then die again, at first he thought it was the auto de-compressor, so with a bit of a struggle he removed it, we removed the head to have a look at the valves and they looked ok, and piston was like new. after a re-build the bore filled up with water strait away because of a faulty head gasket, but ran for a good 200 yds before dieing full of water, managed to get all the water out, have the valves re shimmed then re-built with genuine gaskets, now it has an awfull noise, we suspect that the exhaust valves are hitting the piston, bacause there is now a slight mark on both exhaust indents on the piston,but the timing is bang on. its just had the valves re-shimed and as there was no valve to cam clearance before re-shimming this should mean that the valves are further away from the piston when open. the cam chain tenisioner is fine and chain tight, the only thing we can think that might be causing this, is the gear on the exhaust cam may have moved due to head gasket failure and filling with water and thus putting the exhaust timing out??

what i want to know is, is it possible to put the exhaust cam out 1 tooth, i mean can i make the exhaust valves open 1 tooth later. will it run?? or do dammage??

regards, stan.

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Off by one tooth won't do damage. The WR cam timing is a tooth different than the YZ and they don't have any problems. Is the head surface flat?

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cheers for the reply swede73, we tried the exhaust cam out by 1 tooth, took ages to start it but the noise was still there, a kind of whirring knock, anyway we stripped the top end down again, and noticed it has had quite a serious boom at one point, there is alloy weld all over the right hand case by the water port, it looks like the con-rod has broke at some point and gone through the caseing, then been repaired, quite a good job tbh, but the mateing surface on the right hand case is slightly lower than the left due to the repair, so it seems the piston has been just hitting the head, you can clearly see the shape of the head on the piston, i have checked the barrel and head and they are fine and not warped, now what can we do to resolve this?? i thought maybe 2 head gaskets to make up the difference but they are £30 each, or 2 or 3 base gaskets??? is it posible to get a steel base gasket, i know i had one on my kxf but that was standard gasket and no faults.

many thanks in advance.

regards, stan..4 stroke mad!!!

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It really depends on the condition of the cases. A good repair? Deck height is wrong...but what else has been bodged up. Not wanting to be the eternal pessimist, but another "boom" may be looming.

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These are not something you just want to be cobbling together. A quick search on fleabay shows at least 3 case sets all under 300 and a few complete bottom ends. Do it right the first time and forget about it. What might cost you say 400 now could end up being a couple grand if you just through it together without actually fixing it.

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hi guys, my friends got a yzf250 04, it died suddenly one day and wouldnt start and had low compression, after a bit it would start again but then die again, at first he thought it was the auto de-compressor, so with a bit of a struggle he removed it, we removed the head to have a look at the valves and they looked ok, and piston was like new. after a re-build the bore filled up with water strait away because of a faulty head gasket, but ran for a good 200 yds before dieing full of water, managed to get all the water out, have the valves re shimmed then re-built with genuine gaskets, now it has an awfull noise, we suspect that the exhaust valves are hitting the piston, bacause there is now a slight mark on both exhaust indents on the piston,but the timing is bang on. its just had the valves re-shimed and as there was no valve to cam clearance before re-shimming this should mean that the valves are further away from the piston when open. the cam chain tenisioner is fine and chain tight, the only thing we can think that might be causing this, is the gear on the exhaust cam may have moved due to head gasket failure and filling with water and thus putting the exhaust timing out??

what i want to know is, is it possible to put the exhaust cam out 1 tooth, i mean can i make the exhaust valves open 1 tooth later. will it run?? or do dammage??

regards, stan.

how full of water was it? If the thing hydro-locked you may have bent the con rod or did something to to bottom end. Water doesn't compress and if it tries to compress it does all kinds of harm below where it's compressing.

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how full of water was it? If the thing hydro-locked you may have bent the con rod or did something to to bottom end. Water doesn't compress and if it tries to compress it does all kinds of harm below where it's compressing.

It was pretty full, pulled the plug and kicked it over and water sprayed everywhere, I've checked over the lot, con rod is fine, as are the big end bearing and piston pin, there's very slight play in the main crank bearings but I've seen worse and are defo not the cause of the knock. We are gonna re build it tomorrow with 3 base gaskets to raise the block a bit, who knows it may have had the block skimmed by the previous owner.

I keep ya updated.

Regards. Stan

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I dont know if its enough to make a difference, but adding extra gaskets will change your cam timing.

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I dont know if its enough to make a difference, but adding extra gaskets will change your cam timing.

TDC doesn't change. Raising the barrel with an extra gasket is not going to alter valve timing.

To the OP....if you are going to go down this route, use an extra head gasket or make a metal base gasket. Multiple paper gaskets in a stack have a tendancy to fail.

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Yeah, I would be weary of a bunch of gaskets stacked together. You could probably get a piece of aluminum sheet laser cut to the right shape if you bring them the gasket. Then you could sandwich it in between two gaskets. There might even be something available online.

Or you could just get new cases. It might be more pricey, but I'd feel safer than with a bunch of spacers slapped together. You would probably want to order a new set of bearings while you're at it though.

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TDC doesn't change. Raising the barrel with an extra gasket is not going to alter valve timing.

To the OP....if you are going to go down this route, use an extra head gasket or make a metal base gasket. Multiple paper gaskets in a stack have a tendancy to fail.

Yes thats true TDC wont change it never will. I guess it would be no more than a tooth difference.

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I would clay the top of the piston to see what the piston to valve clearance is before you go trying to change anything, as that might not even be your problem.

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Hi guys, another update for ya, as we thought the piston was hitting the head we tried 2 base gaskets an hey presto!! It runs fine with no knocks or bangs, not even a grumble from the main bearings, all it needed was that half a mm more clearance to stop the slapping. Fingers crossed she should now be fine again. Gonna take it up the woods on Sunday and see if she lets go or not, but tbh it's a typical Yamaha, takes a hit and keeps on ticking!!! Job done!!

Cheers for all the replys guys,

Regards, stan

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Something is fishy, piston's don't just start hitting the head for no reason. You said the bike was running fine and then one day quit and the piston was hitting the head. It seems like something else is going on here. Did you measure the piston-valve clearance? Deck height?

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Something is fishy, piston's don't just start hitting the head for no reason. You said the bike was running fine and then one day quit and the piston was hitting the head. It seems like something else is going on here. Did you measure the piston-valve clearance? Deck height?

I know it seems strange but the right case sits half a mm lower on the gasket mating face, this is all down to the repair it had before my friend owned owned it, so the block was sitting at a slight angle it looks like the con rod had snapped in the past and gone through the case by the water dowel, even behind the water pump has been alloy welded, so we put 2 and a half base gasket on and bingo! Didnt measure anything or try the clay idea but the block now sits strait and now the piston doesn't stick up above the head surface,

We wondered why there was 2 gaskets when stripping it down, now we know..

I know u guys are thinking omg! What a bodge! But trust me I've seen worse, I once brought a bike that had the flywheel welded onto the crank, along with the sprocket and gear lever. Lol

Regards, 4 stroke mad stan...

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