Merc 5.7L V8 bents/breaks push rod

[dmitrev]

I saw interesting thing - due to worn stem, where the rocker arm pushes the valve, in some position of the valve, rocker arm starts to go outside of the stem, and on rpms more than 2500 push rod is bent or broken. This is because push rod contacts the head, and angle is too big. Fixed with the cap on the stem, and due to my err in measuring, cap contacts on the locks, not on the stem, but Merc now goes to 5000 without push rod problems.

 

[76SeaRay]

Sounds like you need guide plates on the push rods....

 

[Bondo]

Ayuh,..... Have to know exactly what head ya got,....
'n how ya got to where you are now,..??

 

[flashback]

My 79 sbc pushed the rocker stud out of the head. In those days they were pressed into the head.

 

[dmitrev]

1978 Merc 260 V8 5.7l. Changed cracked head with the second hand, "rebuilt" with the old valves, and, one valve stem - missed to sander. Guides probably fix the pb too. Design seems to be some self-centering rocker arm, and it works on the other valves, with planar stem surface. Old man on quay said - Chevy broke push rods often. I was shoked, because as former "socialist" country we see Chevy on films only, but if he recognize engine - he know it And if it is common pb - there should be explanation. And - voila - worn(not planar) stem, possibly worn rocker arm too. After change of the places of the rockers, seems, problem is bigger

 

[Bondo]

Ayuh,...... Intake, or exhaust valves,..??

 

[Silvertip]

Lots of possibilities here: 1- worn rocker arms, 2 - worn rocker pivots, 3 - Improper adjustment, 4 - partially pulled stud, 5 - weak valve springs causing float, 6 - worn or sticky lifter and 7 - mismatched parts

 

[Scott06]

Wonder if the proper retainer to valve seal clearance is there? If when reconditioned they didnt cut the tops of valve guides down and fitted modern seals the retainer can kiss the seal and break $hit… had this exact issue on my 65 GTO, valve job done years ago cut guides for teflon seals… running stock cam… find out retainers were kissing the seals… more lucky than right was just barely kissing the seals be ding them so they didnt seal…

 

[dmitrev]

Exhaust valve. About seals - must inspect them too. About the lots of possibilities - worn stem surface - solved with the stem-hat made on lathe