I have a pretty new rebuilt 68 400, maybe 400 miles on clock. The other day as I pulled in the drive I noticed a tapping noise like in the valvetrain. Took off the valve cover on passenger side and the very first push rod is mushroomed on the tip. Curiously, the rocker stud has a small hole in it about a 1/4 inch from the casting at the bottom of it. There is no corresponding hole in the casting though. I sort of assumed that the stud may have pulled out and that possibly it was pinned in a prior life. Than I took off the next push rod and rocker assembly and measured both of the stud lengths and found them to be identical or very close to, at around 1.72 inch exposed. Now I'm not sure if the stud has been pushed out or and wanted some advice on what to do next. I have no indication why the pushrod mushroomed, but would you get a new single rocker set and pushrod and replace that to see if it torques down correctly and runs, or is there another process that I could use to help identify the root cause? Any helpful idea will be greatly appreciated.
First, what lift cam? Stock?
Second, what is that mark I am seeing on the rocker arm near where the rocker arm strikes the valve stem?
Third, not sure what the hole in the rocker stud is. Doubt it is from pinning it as pinning it would go through the rocker arm stud pedestal lower down.
Fourth, is that pushrod shown in the photo still in the engine the one you are using now? Or is a damaged one? The end does not look correct at all. Should look like a ball on top.
Fifth, what is the mark that I am seeing in the top of the valve stem in the pic?
Sixth, I measured a press-in stud on one of my 1972 455 heads. From the pushrod plate that goes over the stud (and is seen in your photo as well) to the very top of the stud, it measures 1.67". Breaking this down further, I measure .894" from the plate to the edge of the 7/16" stud right where the machining is done where the taper starts. Then I measured .642" off that same edge up to where the 3/8" threads end, not including the non threaded shank at the end.
I would check all rocker arm geometry. If heads have been milled or even deck surfaced, it could change the length of the pushrod and a stock length may be too long.
You may have the incorrect ends on the pushrods.
If a high lift cam, you need to check for coil bind and clearance between the spring retainer and the rocker arm at full lift.
Check to see if the pushrod is binding/hitting on the head where it comes though below the pushrod plate. This could deflect the pushrod and not allow it to seat correctly into the rocker arm pushrod cup.
No or little oil going through the pushrod. Wrong lifters, or too long of a pushrod bottoming out the valve lifter, or even a collapsed lifter.
Piston-to-valve issues I would think would have bent the pushrod as well. If it is straight, then this is probably not an issue.
It looks like you have the Competition Cams 1.5 long slot rockers? If you have a high lift cam, you need long slot rockers so the rocker arm does not bind on the rocker arm stud at high lift.
Just a few things to check, but I would not be driving it until I found the cause of the destruction. I would pull every pushrod and look at them all. Then check rocker arm geometry.
If you had a local machine shop rebuild your engine, bring him on it as he may have some things to check or may be tearing it down to find the problem.