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Discussion Starter #1
Hi everyone, I'm new here and I have a few questions regarding my 400ex. I bought it and it was already bored to a 440, I prefer the 416, but it's fine. Bought it anyways, ran good, and then I blew it up. Something made my intake valves go through the piston. No big deal right? Wrong. I got new valves, piston, valve seals, rings, head gasket, wrist pin, everything. Put it all in to spec and she run great. Started right up and went like a dream. I had about 4 hours on the rebuild, still breaking it in, going light, and I took a rest by a brook. Go to start her again about 20 minutes later, starts up, but the ticking somehow went from like a 3 to an 8 really fast. Shut her off, and she wouldn't start again. Took her apart, and my intake valves are bent and stuck open. Why? I think it's my rockers, because they seem a bit worn, and I just picked up a parts machine for the plastics, so I'm putting the rockers from that in the good one. No hole in the piston, but the cam is about 1/4 of the way off of timing as well!! The chain is tight, and everything seems fine with that, I just don't know why it would skip timing and bend the valves again. Hoping that when I put the rockers in it might help. Aren't the chain tensioners on these faulty? I think I heard that somewhere. I have one off of the parts machine I could try. Any help would be greatly appreciated. Thanks!
 

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There are many reasons to bend valves and none are related to rocker arms unless the rockers are not moving freely on their shafts.

I just had a guy bring me a head cover off his Chinese 440 kitted race quad. He got a Niche 440 cylinder kit and Niche 'Race Extreme Ported Head' as he called it - the sub rockers were so tight that they would hold the valves slightly open. After a thorough inspection, I determined there was a .020 discrepancy in the thickness of the casting where the sub rocker shaft screwed into the cover. I first tried just a new copper sealing washer as old washers tend to compress and reduce the clearance. I fixed the problem with 2 .010 stainless steel machine tool shaft shims.

If the problem is not in the head cover, it may be the valve guides have insufficient stem to guide clearance. When the engine was shut off the valves may have been open - when the engine is not running, the heat increases before it decreases and the valve may have just stuck in the guide allowing the piston to hit the valves when the engine was cranked to start.

Now, the cam may have jumped time for an unrelated reason when the engine was shut off and cranking the engine to start resulted in the valves opening at the same time the piston was traveling up.

The cam chain tensioner is not prone to failure, but can be damaged when the cam is prevented from moving as is the case when the valves are pushing on the piston as the piston is rising in it's stroke. Whenever there is a catastrophic failure as you described, the cam chain and tensioner should be replaced as a preventative measure. Also, be sure to set the tensioner properly during installation.

The mechanic working on the engine will inspect all the parts including the cam chain sprockets and guides to assure functionality. Also, assure the cam chain is the correct chain for the 400EX and not the so called 'CRF450 upgrade' - while the CRF450 chain is cheaper, it is also wider than the 400 chain, does not fit the sprockets or guides properly, was designed for a race engine and made from thinner and lighter materials as regular replacement of the chain in the CRF is mandated as routine maintenance due to the cost of racing. Just having the CRF chain in the EX may be a cause of failure.
 

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Discussion Starter #3
Thanks for the reply! I thought about valve guides after I posted. I'll have to look at them when I get home. The cam and chain are pretty new, and have no real wear. Top sprocket just replaced, but I haven''t looked at the bottom. I think I'll replace the valve guides, valves, and just to be safe throw the rockers and arms on. They seem to be less tight than the other ones. The ones on there weren't too tight, but it didn't feel right. I'll let you know what happens!
 

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If you replace the valve guides, the new guides have to be reamed and honed and then the valve seats recut - take it to an automotive machine shop to get it done after you get all the parts.
 
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