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Old 10-02-2011, 01:43 PM   #1
wmax351 OP
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Pissed EEEEEKKK! Airhead Valve/Piston Interference

Just took the top end off of my rebuilt '71 R75 after 100 miles due to a ridiculously loud banging sound from the right side. Seemed almost like rod knock, but it wasn't. Turned out to be this...

Right Side:






Left Side:





Any ideas what can cause this? I used the original, .5 mm base gaskets/shims rather than the low test, 1.2mm shims. Could that have done it?

It seems as if the valves only hit when they were seated, as it would have likely grenaded due to a bent valve otherwise.

What should I do here? Any chance of the heads or pistons being salvageable? They are a fresh rebuild with under 100 miles on them.

What should I do to make sure it doesn't happen again?
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Old 10-02-2011, 01:54 PM   #2
Renner
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who did your heads?
have you asked them about this?

your good luck served you well regarding this bit of bad luck
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Old 10-02-2011, 01:54 PM   #3
Beater
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Quote:
Any ideas what can cause this? I used the original, .5 mm base gaskets/shims rather than the low test, 1.2mm shims. Could that have done it?
(snip)
What should I do to make sure it doesn't happen again?
Absolutely that could have done it. Any narrowing of that gap can do it. But holy crap, I am shocked you didn't snap a valve. I would put in the thicker gasket and go from there. It's a good thing you pulled that apart when you did ...

On a side note, I thought the space between the piston at top dead center and the valve was like 3 to 4mm .. How long have you had the bike? Do you know any history on it? Did anyone shave the cylinder head?
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Old 10-02-2011, 01:57 PM   #4
Jon-Lars
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What cam is in there? and is there an offset cam key?
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Old 10-02-2011, 02:07 PM   #5
wmax351 OP
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Quote:
Originally Posted by Renner View Post
who did your heads?
have you asked them about this?

your good luck served you well regarding this bit of bad luck
Heads were done for the PO, by an unknown source. They were professionally done though, and I have all the original parts. I assembled the rest of the bike myself.




Quote:
Originally Posted by Beater View Post
Absolutely that could have done it. Any narrowing of that gap can do it. But holy crap, I am shocked you didn't snap a valve. I would put in the thicker gasket and go from there. It's a good thing you pulled that apart when you did ...

On a side note, I thought the space between the piston at top dead center and the valve was like 3 to 4mm .. How long have you had the bike? Do you know any history on it? Did anyone shave the cylinder head?
I have had the bike since a year and a half ago, when it was in 3 Rubbermaid boxes. Didn't appear to have a shaved cylinder head.

Think it is fine to run these parts without replacing them?

Quote:
Originally Posted by Jon-Lars View Post
What cam is in there? and is there an offset cam key?
AFAIK, it is a stock cam, with no offset at all. Set up by Dave at Recommended Service.
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Old 10-02-2011, 02:14 PM   #6
Renner
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I'd run them down to Scotts Valley to talk about them with Ted Porter at the Beemershop.
You could call and offer to send pics first.

My money is on the piston being ok and a valve job required.
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Old 10-02-2011, 02:31 PM   #7
AntonLargiader
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Looks like an oversized exhaust valve. Can you measure its diameter?
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Old 10-02-2011, 02:57 PM   #8
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Quote:
Originally Posted by AntonLargiader View Post
Looks like an oversized exhaust valve. Can you measure its diameter?
I measured them. ~38 mm (1.490") exhaust,~42 mm (1.650") intake. They appear to be standard. Could the seats be protruding too far?
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Old 10-02-2011, 03:23 PM   #9
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Your intake valve is hitting too! You are running short reach plugs in long reach holes!

I am thinking it has to be a too thin base gasket.

I am not so sure your other cylinders valves aren't touch too?
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Old 10-02-2011, 03:31 PM   #10
caponerd
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Quote:
Originally Posted by Jon-Lars View Post
What cam is in there? and is there an offset cam key?
Looks like the exhaust valve is the one that's hitting. I'd be looking at anything that can cause a delay in valve timing. Would a loose cam chain cause that?
Maybe just a couple of degrees retard in the valve timing would allow the valve to just touch the piston without bending the valve?

Come to think of it, with those valve pockets in the piston top, can the piston even hit a closed valve without first hitting the head itself?
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Old 10-02-2011, 03:40 PM   #11
wmax351 OP
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Quote:
Originally Posted by supershaft View Post
Your intake valve is hitting too! You are running short reach plugs in long reach holes!

I am thinking it has to be a too thin base gasket.

I am not so sure your other cylinders valves aren't touch too?
I'm pretty sure the other side is touching, just not as much.

Quote:
Originally Posted by caponerd View Post
Looks like the exhaust valve is the one that's hitting. I'd be looking at anything that can cause a delay in valve timing. Would a loose cam chain cause that?
Maybe just a couple of degrees retard in the valve timing would allow the valve to just touch the piston without bending the valve?

Come to think of it, with those valve pockets in the piston top, can the piston even hit a closed valve without first hitting the head itself?
That's what it seems like. It doesn't add up.

Its a fresh cam chain, sprockets, and tensioner, installed by Dave Gardner of Recommended Service. I'd say its highly unlikely that that is the cause.
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Old 10-02-2011, 03:40 PM   #12
supershaft
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Who says the piston is hitting closed valves? It (they?) isn't (aren't?). Pistons do not hit closed valves.

Look closer. The intake is hitting too!
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Old 10-02-2011, 03:53 PM   #13
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Quote:
Originally Posted by supershaft View Post
Who says the piston is hitting closed valves?
It was mentioned by the OP earlier but that's possible w/out crushing the exhaust valve on the exhaust stroke.
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Old 10-02-2011, 03:55 PM   #14
wmax351 OP
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Quote:
Originally Posted by supershaft View Post
Who says the piston is hitting closed valves? It (they?) isn't (aren't?). Pistons do not hit closed valves.

Look closer. The intake is hitting too!
That's a good point. It does seem as if the marks are exactly where the valve would be if it were at least mostly closed. Otherwise, the marks would be closer to the center of the piston.
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Old 10-02-2011, 04:01 PM   #15
Airhead Wrangler
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Any chance you're a couple of teeth off on your timing chain??
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