Tappet cover exploded, advice?

DA_2067

DA_2067

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Hey all! , to begin this is my first post and being somewhat of a newbie I was hoping someone could help me gain some insight into fixing this problem.

I have a 1979 XS400 2F that was running very well as of recently but as I went to go for a ride yesterday I simply kickstarted the bike 3-4 times. While getting it to start I saw a plume of smoke followed by a large pop coming from the head of the engine. Upon further inspection I found one of the exhaust tappet covers had literally exploded off the head and shot 100 ft down the road :doh: while damage is minimal (no cracks in the head just the remainder of the bolt threaded in and cracked) I was wondering if there was a preferred method for extracting the remainder of the cover that's still threaded in place.
Also, if anyone has a suggestion for a website where I could get a new set of tappet covers, they seem to be scarce on eBay.
Any help is appreciated, thanks!

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Sorry I don't.Best idea I have is to check bone yards near you or in other states call them for what your need.You can try emc on ebay but their prices are stupidly expensive.Also put a wanted in the classifieds.
 
I would guess a bike wrecker which might have engines that are blown but were broken elsewhere.

Is it possible this part might be common to more than one Yamaha engine? If so, it would make the search easier.
 
The only way I how it could have happened is both valves were closed on the cylinder and when kicked back it created the pressure to blow the cap off.
 
16VGTIDave said:
... The crankcase shouldn't have any pressure in it, but obviously did...
Click to expand...
I have a theory.
If the cap is from the access portal for adjusting the valves, then isn't that chamber above (and not part of) the combustion chamber? So, when there is a normal ignition of the air/fuel mix, both valves are closed. And then, after the piston is on its way down, the exhaust valve opens.

Prior to normal ignition, the intake valve is open.
What if there was a premature ignition of the air/fuel while the intake valve was still open?
Then the expanding gases from the explosion could blow past the intake valve into the chamber above and pressurize it. Since normally this isn't under any pressure, it would blow the weakest link (presumably the access cap).
 
There is very little space (mm's) between the valve and the valve guide. So I highly doubt it was combustion gasses unless the head gasket is blown. My guess is the the cover was already cracked and ready to fall off and with the normal oil/air pressure in the motor it blew it off. The only other thing I could think off is the rocker arm is out of adjustment and it came up just enough to pop it off.
 
Sorry guys, but I don't think so. The valve stem is a fairly tight fit in it's guide. I can't see how enough gasses could make their way up to pressurize the cam cover.

It would be more plausible that there were gasoline fumes in the crankcase that somehow were ignited. If the cover was already damaged, possibly from being over tightened, it could have been blown off from the resulting pressure. But that is just a wild guess...
 
16VGTIDave said:
Sorry guys, but I don't think so. The valve stem is a fairly tight fit in it's guide. I can't see how enough gasses could make their way up to pressurize the cam cover....
Click to expand...

Agreed. There should be atmospheric pressure...in other words, NO pressure other than what's in the world around us, where the valves are. The valve guides are designed to seal that passage. Also, if your exhaust and intake valves have that much overlap....you have trouble, my friend.

What caused it? I have no idea....but if it were me, I pull the valve cover and look to see if there anything loose that could have gotten between the rocker and valve cover...though that is probably down the road as well. I suspect a mechanical impact rather than an explosion.
 
I'd expect that if the crankcase vent tube were blocked, a gasket would be more likely to blow or even a seal rather than shatter a cap and eject it with that much force.
I'm with Hough on a mechanical impact over a buildup of pressure. That being said, if gas and oil were to mix, and a spark were to ignite said mixture in the upper cylinder head, a very rapid expansion of hot gases could, in theory, shatter the cap and fling it.
 
Those caps are pretty fragile (cast aluminum) so it's definitely not out of the question that it was already cracked. In that case, it could go before a gasket. I broke a cap trying to take it off once without much force at all. Thankfully, XSChris has enough spares to go around.:)
 
Thanks for the advice! While I have the tank off for replacing the carb holders I will definitely remove the valve cover and check the status of it. It seems like mechanical impact is pretty sound logic at this point. I'm also assuming that bolt had taken too much abuse seeing how badly it sheared and cracked. I ran into this website http://www.goodguysmotorsports.com and they appear to have what I am looking for. We'll see how that goes. as for what else it could be I haven't run into any problems yet that would indicate anything is horribly wrong. Hopefully a basic tune up, etc will get me to a better place :bike:
 

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