LBZ: Headgasket Curiousity

[twister773]

So i should be finally getting my truck back thursday :happy2:

It is in the shop for headgaskets, the shop had the heads tested and they tested fine. When the tech was putting them back together he noticed a cracked valve guide and a bent valve. Is it common to bend only one valve? Think the valve bent then cracked the guide or visa versa? They already got the new valve and guide in and are gonna start reassembling tomorrow, i was just curious.

 

[twister773]

well truck wasn't done today but I am still curious about the one bent valve...ideas?

 

[RKTMech]

Is the valve really bent or is it getting replaced because the guide is damaged? Just had a set of LLYs done and they found the bottom of 2 guides broken off but no bent valves on a first time apart engine!?
A little weird......

 

[twister773]

Yep for sure bent, only one valve, first time engine has been apart.

 

[68skylark455]

The only way to bend one is contact with the piston. Did they look to see if it damaged the piston? Strange it wasn't found with checking the head. I have seen some shops not back off on the rockers and when reinstalling them it bends a valve. Also, if the valve is bent hows the pushrod?

 

[twister773]

Didnt touch the piston, and I also thought it was weird it tested fine. Also it didnt bend while putting the heads on because they didnt even tighten them up they went to put the heads on and noticed one of the valve guides being cracked. Not sure.

 

[stacks04]

The only way to bend one is contact with the piston. Did they look to see if it damaged the piston? Strange it wasn't found with checking the head. I have seen some shops not back off on the rockers and when reinstalling them it bends a valve. Also, if the valve is bent hows the pushrod?

Just out of curiosity how would that happen? We do a ton of them here and never had this happen. We generally dont touch the lash adjustment. The valves should not come near the pistons because the cam/crank timing.

 

[68skylark455]

Just out of curiosity how would that happen? We do a ton of them here and never had this happen. We generally dont touch the lash adjustment. The valves should not come near the pistons because the cam/crank timing.

I agree with you but in some instances ie resurfaceing, valve job, guide or seat work will change the original settings before removal, any binding, or miss alignment could bend one, GM is very specific on thickness of headgaskets also, Guy Trip could answer theses questions alot better then I can i was just offering some suggestions, also, just out of curiosity, when bolting on a cylinder head, complete with valves and springs installed to where you just install the rockers and push rods, how do you see a cracked valve guide? I may be wrong but on most engines you have to be looking very hard to be able to see the top of the guide thru the spring right? Just asking some questions my customers would ask me in this situation. I have only had one LB7 apart a couple of times so I may be way off base and if so please tell me. But as a tech/mechanic for the past 20+ years I would never reinstall adjustable rockers back onto a head I removed without backing all of them off and then resetting them after they were reinstalled especially ones mounted onto a shaft. To much variances in surfaces, adjustments plus some lifters bleed down and some stick up and don't bleed at all. Stuff bends pretty easy in a bind under torque. If you read the GM service manuel about removing the cylinder head it does tell you to loosen all rockers first before removing. http://www.duramaxdiesels.com/forum/showthread.php?t=6155&highlight=rocker+arm+shaft .
I am still trying to learn these things too I just think it sounds fishy to say a crack in a valve guide was seen while it was being installed.

 

[JoshH]

I agree with you but in some instances ie resurfaceing, valve job, guide or seat work will change the original settings before removal, any binding, or miss alignment could bend one, GM is very specific on thickness of headgaskets also, Guy Trip could answer theses questions alot better then I can i was just offering some suggestions, also, just out of curiosity, when bolting on a cylinder head, complete with valves and springs installed to where you just install the rockers and push rods, how do you see a cracked valve guide? I may be wrong but on most engines you have to be looking very hard to be able to see the top of the guide thru the spring right? Just asking some questions my customers would ask me in this situation. I have only had one LB7 apart a couple of times so I may be way off base and if so please tell me. But as a tech/mechanic for the past 20+ years I would never reinstall adjustable rockers back onto a head I removed without backing all of them off and then resetting them after they were reinstalled especially ones mounted onto a shaft. To much variances in surfaces, adjustments plus some lifters bleed down and some stick up and don't bleed at all. Stuff bends pretty easy in a bind under torque. If you read the GM service manuel about removing the cylinder head it does tell you to loosen all rockers first before removing. http://www.duramaxdiesels.com/forum/showthread.php?t=6155&highlight=rocker+arm+shaft .
I am still trying to learn these things too I just think it sounds fishy to say a crack in a valve guide was seen while it was being installed.

Even if the rockers were backed all the way off, that wouldn't be enough to keep valves from being opened when the rocker shafts are bolted down.