Author Topic: valve cap explosion  (Read 1488 times)

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Offline veganxr2x

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valve cap explosion
« on: December 24, 2010, 04:40:18 PM »
Just replaced the cam chain on a 78 550K. When I started it back up the #3 cylinder exhaust valve adjuster cap flew off. The motorcycle still ran/runs fine and the valve is adjusted correctly.

What the hell?

Offline veganxr2x

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Re: valve cap explosion
« Reply #1 on: December 24, 2010, 04:55:14 PM »
I should have said the the cap sheared off. It broke at the threads.

Offline SOHC Digger

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Re: valve cap explosion
« Reply #2 on: December 24, 2010, 05:05:54 PM »
Not good.  Did you look at the cap to see if there are any impact marks where something would have hit it?

Online Alan F.

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Re: valve cap explosion
« Reply #3 on: December 24, 2010, 06:39:53 PM »
Someone more experienced than me will confirm or deny this... but if the old o-ring is crushed down to the max, or if the cap has been overtightened... won't the clearance between the adjuster screw and the cap be reduced, possibly to the point of contact?


Offline 754

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Re: valve cap explosion
« Reply #4 on: December 24, 2010, 10:44:51 PM »
Either something hit it, or it was torqued just under the breaking point at one time.

 Is your adjuster loose?
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Offline Raef

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Re: valve cap explosion
« Reply #5 on: December 24, 2010, 11:03:00 PM »
I am wondering about cam timing and if the piston might have hit the valve knocking the rocker up into the cap??

Mark

Offline veganxr2x

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Re: valve cap explosion
« Reply #6 on: December 24, 2010, 11:08:07 PM »
Timing, valve clearance, etc were all checked and rechecked. I put a cap from a different valve on it and ran it as high as 5.5K Rpms with no problems at all. Maybe there was a hairline crack in the cap and under pressure it failed?

Offline OldSchool_IsCool

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Re: valve cap explosion
« Reply #7 on: December 25, 2010, 12:28:26 AM »
Could there have been a loose object, say a lock nut, that floated to the top of the tappet just as the valve was closing?  If so, it prolly flew out right behind the cap.  coulda been left in there by a PO, but just the same, you may wana check the other tappets to make sure all lock nuts are accounted for.
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Offline faux fiddy

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Re: valve cap explosion
« Reply #8 on: December 25, 2010, 12:47:01 AM »
Timing, valve clearance, etc were all checked and rechecked. I put a cap from a different valve on it and ran it as high as 5.5K Rpms with no problems at all. Maybe there was a hairline crack in the cap and under pressure it failed?

You found the cap to look at? Or is it lost in the flood waters?
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Offline Kevin400F

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Re: valve cap explosion
« Reply #9 on: December 25, 2010, 03:22:50 AM »
The cam cover area is vented to the induction system via a big hose, so that area shouldn't build execessive internal pressure.  Check the venting system, if you conclude there wasn't a "mechanical" impact.

Offline Grabcon

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Re: valve cap explosion
« Reply #10 on: December 25, 2010, 08:19:12 AM »
Over tightening the valve caps is a real and common problem on SOHC motors. People over tighten to stop leaks instead of replacing the o-rings. Take a cap and o-ring to the local NAPA store and find the right size o-ring and replace all. A couple of bucks at best.

Now back to the cap. These things are 20 to 30 years old and have been removed and replaced many times with over torquing everytime and they will break. the big thing is to get the piece that broke off out of the hole without dropping it to the valve train.

Go to the local bike junk yard and by several for spares and get extra o-rings.
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Offline veganxr2x

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Re: valve cap explosion
« Reply #11 on: December 25, 2010, 08:29:54 AM »
Thanks to all for you input. It sounds like it was just a combination of an old cap and maybe a #$%*ty o-ring. Like I said, the bike wasn't and still isn't running funny or making any strange noises. I'm going to get several extras, install one and ride gently for a couple miles before opening it up (I've finally got this thing putting 100% of its available power to the ground--I highly recommend Barnett clutches for anyone that's made their bike faster than the factory intended).

Thanks again.

Offline crazypj

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Re: valve cap explosion
« Reply #12 on: December 25, 2010, 08:47:43 AM »
How 'warm' was it when you fitted cap?
 If you had heater on in garage then it cooled down overnight, the 'shrinkage' could have been enough to cause hairline crack to turn into a fracture.
 Combination of  vibration and internal pressure will be plenty high enough to cause cap to blow off when started
 I've seen it happen a few times after servicing on various Honda's (probably just luck I've never done it to any, although I know the problem area's after 30+yrs)
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Offline Don R

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Re: valve cap explosion
« Reply #13 on: December 25, 2010, 04:09:46 PM »
The caps on my 836 were almost all cracked but none came apart. I posted the part no for parts house o rings in somewhere in faq's.
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