Bent valves from lean mixture or back fire?
 

bumble bees do hit sound barriers.

Bent mechanic, coming up with a story like that ?

Cannot think of any possible way to bend a valve other than contact .

Makes no sense to me. I can’t see a bent valve coming from anything but contact.

Burnt > lean
Bent > valve float/contact

Well, if it. did hold it open from a backfire, it would still be contact

Over-rev due to a downshift?

How are they sure it didn't contact?

Only from mechanical contact. But I have seen broken stem / failed keepers where then the valve drops and in that short frame of time, backfires through the carb as the carnage took place.

In my experience there had to have been a close encounter of some kind.

In a back fire the flame front starts at the valve and pushes out the intake, so the pressure wave is going the wrong direction to load the valve. (Referring back to some research in the engine lab when I was in Engineering school decades ago....)

Certainly a sticky valve guide or a foreign object holding the valve open would cause the bent valve but valve to piston contact is pretty much the only cause..

I would have to agree. I have yet to go into a M8, but I highly doubt in stock form HD would have valve to piston clearance that close. Has to pass Fed smog. With that said, if it already has higher compression pistons in it, and of course a larger cam, it is possible with severe valve float, or a cam installed out of time.


Backfires don't cause bent valves. Contact does. If they were trying to tune it for more HP, and it went very lean, it will easily stick a guide or tulip a valve. Ask if the pushrod was damaged. ;)

Doesn’t a bent valve cause a backfire, not the other way around??

Nut falling into the intake?