QuestionResubmit - forgot car model: 1976 TR6 - dual carb.
The engine had been missing and spitting when accelerating in
1st or 2nd to restart from slowing down. Carbon buid-up and
some oil was noted on spark plug from # 6 cylinder. New hotter
plugs were installed and a Crane electronic ignition. Engine ran
great for a day or 2, then got very rough and spitting. After
being driven again - inspection revealed a hole in valve cover,
and removal of the valve cover revealed the rocker assemblies
for #6 loose and shifted. The pushrod had caused the valve
cover hole and the stud threads into the cylinder head looked
fused or stripped. And the rocker shaft between 5 & 6 was
sheared (broken). Obviously all this has to be repaired.
The car has about 46K original miles - and has been driven and
had regular maintenance over the last couple years (when I
purchased it). My question is what could have caused this to
happen? And if I replace the rocker shaft, retap the stud threads,
replace the stud, replace the valve cover will this fix the
problem, without immediate worry of it recurring? All the other
parts appear to be intact - as well as the rocker assemblies on
the other 5 cylinders. My concern is some underlying cause that
will cause repeated issues with #6.
AnswerHi Raeann,
Rocker arm shafts are hardened to resist ware but thus they are brittle and the constant pulsing strains put on to it by each push rod trying to overcome the force of each valve spring is a lot of strain. Then add to this a stud pulls out and then the strain is too much and it breaks. My guess is that the nut came loose first or was left loose if it was removed at any time in the last few years, then the up and down hammering striped the threads of the stud and then started flexing the rocker arm shaft and the brittle shaft then broke. Keep in mind rocker arm shafts are not routinely replaced and yours could be 30 years old. It is no use worrying if it may happen again. Just carefully examine the rest of the studs to see if any are loose in the head and look at their threads on the head end. I also would pull a compression test when you replace the rocker shaft and correct the threads in the head. Also keep in mind that a valve adjuster left out of adjustment for a length of time could also have started the sequence of events. If the car does not have a CAT, I would consider decarbonizing the combustion chambers. The factory usually recommends removal of the head to do this but it can be done with water if you don't have a CAT in the exhaust system. Don't attempt it yourself unless you have done it before as it can be dangerous to the engine if done wrong.
Let me know what the results of the compression test is.
Howard
