50% down on power

[john 4d]

Now I know why.

Eight miles from home I felt a sudden loss of power, not total, leaving enough to know that something was up and enough to get home. Eight miles!

I posted the other day that on reassembly of my engine, only a few weeks ago, that it ran better than it had ever done before in my ownership. The only issue being that I had a tapping noise. Well that noise has gone now and I've narrowed the search down to where it came from, the LH head.

This morning, on removing the LH cam cover, I discovered that the Cam sprocket had detached itself from the cam shaft. Therefore the cam wasn't turning, hence no tappet noise.

This is the second time in my ownership that this has occurred on the same cam. Since learning the hard way the first time I made doubly sure that the sprocket was reassembled correctly on the rebuild. Which leads me to believe there is something else amiss that causes this failure, other than me and other than not tightening the two screws, holding the sprocket, enough and locking them in place.

This time I guess I was lucky, although the end result and upcoming amount of work required will be the same, in so far as the sprocket and chain didn't de-rail this time. It just spun, floating and rotating on the tenuous shaft engagement with the cam shaft end and the support plate. Allowing the engine to continue running albeit on four cylinders, who knew it could do that? Getting me home on 50% power.

Bent valves? Pierced pistons? Sprung chain tensioner? Oh Joy! Still, at least I made drive it day this year.
John

[tjk]

HI, Could the bolt threads in cam be damaged or badly worn alowing bolts to pull out under load???

[garyf]

If it is worn out threads in cam could it be helicoiled or best to fit new cam?

[flying farmer]

I can guess now what the tapping noise was. Your bolts that lock the sprocket to the cam had come loose.

This happened to me a few years ago on my P reg Stag.
I had taken it for its MOT about 4 miles from home after a 6 month Winter lay up, and it was running quietly on the way there, but while idling at the testing station it started developing quite a loud tapping noise from one of the cams.

I drove it home slowly and investigated and found the bolts were loose even though the locking plates were still in place and all the threads were undamaged.

This can only mean that the bolts had stretched. I have noticed that some cam to sprocket fixing bolts I ordered from a major supplier (can't remember which one) only went half way through the threaded section of the cam and had the strength of cheese.

I did wonder whether the 6 month lay up had allowed some of the cam buckets or valves to stick and overload the fixing bolts, but now all my cars are tax exempt they don't end up not being run for that long.

I am now careful that any bolts I use go all the way through the end of the cam and pull up tight rather than feel like they are stretching.

I would imagine your loose bolts have either pulled out of the threads, or just fatigue failed in the middle due to being hammered back and forth several times each rotation.

If you are lucky you will have got away with just bent valves, I had 5 bent ones when my timing chain snapped, but sometimes the valve guides get cracked as well if the valve stem bends. I didn't suffer any significant piston damage, just an assortment of semi circular dents in the piston crowns, but it will depend a lot on how much piston to valve clearance you had before things went wrong.

I am just sorry I forgot about this problem when you first mentioned the tapping noise you were experiencing.

Neil

One final thought, did you check the cam rotated freely in the head while it was all in bits and no valves were fitted? Heads warped from overheating will have the mating face skimmed flat but the alignment of the cam bearings can be compromised leading to a tight cam and heavy loads on the timing chains, I have seen this loads of times.

[Mark Serj]

I always use a longer bolt with lock nuts on the rear side of the cam, dispensing with the bent locking tabamobob