Bent Valve Phenomenon

[propeine]

So my car decided that it wanted to sound like a subaru after my timing belt replacement/BC 272 install. DSMLink was showing wacky numbers and the car sounded more than just "cammed". Decided I'd run a compression test. 180, 178, 178, 28. Well what the hell is up with that? So I do a leakdown test on the suspect cylinder and everything is coming out the exhaust. Must be valves right?

Pulled the head today and sure enough both exhaust valves are bent but...the kicker is all the timing marks lined up before and trust me I checked 100 times and rotated the motor over by hand probably 30 revolutions total. The other thing I don't understand is there isnt a single mark on the piston or valves that indicate they came in contact with one another. Finally its only that one cylinder. Anyone else ever have something like that happen?

 

[VETTE_50_TH]

Hmmmm. But how many degrees is it until the 1st cylinder valves start to open? Cant be too much after. I would say that if he bent those, he would have bent the 1st cylinder ones too.

James

 

[propeine]

Hmmmm. But how many degrees is it until the 1st cylinder valves start to open? Cant be too much after. I would say that if he bent those, he would have bent the 1st cylinder ones too.

James

The lobes are 180 degrees out of course. While one is on the compression stroke the matching cylinder ie 1 and 4 is on the exhaust stroke. So its quite a while til the cyl 1 valves open. Only takes maybe 20 degrees of counterclockwise rotation to have the cyl #4 exhaust valves at full open. I'd have to go look again but I'm pretty sure the intake valves would be most likely to bend on cyl #1 with the motor at TDC

well at TDC, if you crank counter-clockwise on the cams, the camshaft will open the #4 cylinder exhaust valves (could be #1, can't remember firing order).. If the belt is off they'll hit the piston... I'm pretty sure it's #4 cylinder

#4 is correct, exactly where my two bent valves are. cyl 1 is nearest the timing belt.


I'll take pictures tomorrow

 

[VETTE_50_TH]

Right. So your intake should be bent as well. Thats why it is so confusing?

James

 

[propeine]

Right. So your intake should be bent as well. Thats why it is so confusing?

James

Probably had a better grip on the wrench I used to counteract the torque when I did the intake cam. I get what you're saying though. Who knows maybe that wasnt the cause of all this. Just seems logical.

 

[VETTE_50_TH]

Probably had a better grip on the wrench I used to counteract the torque when I did the intake cam. I get what you're saying though. Who knows maybe that wasnt the cause of all this. Just seems logical.

I get what your saying now. If you didnt have the belt on, i bet that caused it. Yea, there is no way for 1-4 and 2-3 to have the exhaust ports open at the same time. So that makes sense now.

James

 

[Freerevving]

yeah with the belt off and the pistons at TDC, rotating counter-clockwise on the exhaust cam, the #4 exhaust valves would open

 

[DSM1G90]

you didn't mistake valve for compression and bend them lightly?

(a little late here on this topic) For when I ever do a belt, I take the plugs out to get rid of compression along with making the crank rotation easier.

This way, if I feel any tension, I immediately quit in fear of a valve to piston contact.

Thus, when doing the cams like that, get the crank 90* either way from TDC so if valves do come down, the pistons are out of the way since the tops of the pistons are all in a straight line halfway down the bore.

-DSM

 

[propeine]

I was following the RRE method of cam removal so put everything at top dead center. The VFAQ for changing cams is no longer around as far as I can find. Trust me from now on any time I'm doing head work I will have the crank 90 degrees ATDC.

If I had grabbed a buddy to lend me a hand this probably wouldn't have happened. Holding the cams still and trying to get off that stupid cam gear bolt is quite the workout. With a friend to hold the cam still while I cranked on the bolt I'm pretty sure I would have been ok. Live and learn right? Just a good opportunity to clean out all the carbon buildup, do a new headgasket and ARPs.

 

[Freerevving]

Yeah live and learn

If you are applying torque counter clockwise, then advance the camshaft clockwise before placing a wooden block under the wrench on the camshaft. Removing the spark plugs might have prevented this problem altogether, as the valves would have pushed the piston down