Topic: valve cap explosion

[veganxr2x]

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?

[veganxr2x]

I should have said the the cap sheared off. It broke at the threads.

[SOHC Digger]

Not good.  Did you look at the cap to see if there are any impact marks where something would have hit it?

[Alan F.]

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?

[754]

Either something hit it, or it was torqued just under the breaking point at one time.

 Is your adjuster loose?

[Raef]

I am wondering about cam timing and if the piston might have hit the valve knocking the rocker up into the cap??

Mark

[veganxr2x]

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?

[OldSchool_IsCool]

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.

[faux fiddy]

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?

[Kevin400F]

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.

[Grabcon]

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.

[veganxr2x]

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.

[crazypj]

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)

[Don R]

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.