I am rebuilding my 1968 4.2L engine with new piston rings and machine shop restored head. On first extended start/run, it operated (in the shop) smoothly for about 30 minutes and then completely stopped. After checking fuel and ignition, I took off valve covers and found something weird. One of the two bolts that hold the timing chain sprocket to the cam shaft had come out. So the sprocket slipped causing the chain to become too loose and stopping cam rotation (and no doubt causing valves to be bent!).
The strange thing is that the bolt had not unscrewed. It was still connected to the safety wire, which was not twisted. The threads on the loose bolt were somewhat worn not excessively so. I haven’t yet inspected the threaded hole in the cam plate but assume the treads are damaged.
After some thought, my theory is that there was some misalignment between the cam plate and the sprocket plate when I.installed the timing chain. Then when the sprocket rotated, an extra force was exerted on the center line of the bolt on every rotation. Finally the bolt was forced out by what must have been a huge “vibration”
If you have read this sad saga this far, may I ask you:
Have you ever seen anything like this experienced before?
-And do you like my theory or is there a better explanation?
It is not difficult to cross thread these bolts when installing them. The tolerances are extremely fine and you can’t really see the toothed plate well enough to be absolutely sure it’s properly seated.
Coupled with reuse of the original drilled bolts and I can see that you could strip the thread, especially if the toothed plate wasn’t all the way in and you had the bolt only held by a couple of crossed threads.
Once you run the engine, movement of the incorrectly secured toothed plate coupled with significant centripetal force pops the bolt out.
I’ve not personally seen this before but I’ve installed enough heads to be very leery of cross threading the bolts.
Sadly you will have bent valves. You also need to very carefully inspect the threads on the cam. If you have any doubt about them I would replace the cam; used cams are not expensive and are not hard to find.
Finally, where was the retention spring in all of this?
It is not difficult to cross thread these bolts when installing them. The tolerances are extremely fine and you can’t really see the toothed plate well enough to be absolutely sure it’s properly seated.
Coupled with reuse of the original drilled bolts and I can see that you could strip the thread, especially if the toothed plate wasn’t all the way in and you had the bolt only held by a couple of crossed threads.
Once you run the engine, movement of the incorrectly secured toothed plate coupled with significant centripetal force pops the bolt out.
I’ve not personally seen this before but I’ve installed enough heads to be very leery of cross threading the bolts.
Sadly you will have bent valves. You also need to very carefully inspect the threads on the cam. If you have any doubt about them I would replace the cam; used cams are not expensive and are not hard to find.
Finally, where was the retention spring in all of this?
Sorry to hear about your problems with this engine rebuild from hell. I think that if I had the repeated problems you’ve experienced, I’d have given up and taken up gardening . Time to take a step back, a deep breath, and then get back to it…
Do you recall which serrated adjuster you have? The cross shaped ones have far fewer teeth and are (according to legend) less robust than the earlier circular ones with fine teeth all around.
It’s also very possible that the serrated plate was not fully seated when the bolts were torqued. If the teeth aren’t in perfect shape it can get canted and stuck. If it runs like that, the plate can suddenly fall into place, leaving zero torque on the bolts.
When I had my engine rebuilt, the rebuilder strongly recommended replacing the scalloped adjusters with full circle ones. I was told the scalloped ones can and do fail from time to time.
I don’t think we’ve heard from anyone here yet that it’s happened to them, but I can certainly imagine how it could. Didn’t Jag discontinue them?
I’ve no idea if they have or not. Apparently there is a good supply of the solid ones out there at a reasonable price so it seems like a sound upgrade to make when rebuilding an engine.
That said I guess there are thousands of XK engines out there happily humming along with scalloped adjusters. Will they fail? Beats me, but since a stronger part is available and the cost is nominal, I figured why not replace them.
And yet it has come out with the lock wire intact.
“When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth .” ~ Arthur Conan Doyle, The Case-Book of Sherlock
I replaced mine also as per Dick Maury’s recommendation when I adjusted my valves. He was also able to sell me a pair at a reasonable price. I would think that running the engine at high revs for long periods
would eventually ea
t at the teeth of the sprockets which would cause them to slip.
| 
. Time to take a step back, a deep breath, and then get back to it…