Another Timing Chain Question

Hello everyone, so I have been doing research about this lovely v12 and I had a question. I am about to acquire an H.E. out of an 87 XJS, it is supposedly in working order. My question is as the title hints, should I tackle replacing the timing chain and tensioner while the engine is out and on a stand, as sort of preventative maintenance? or should I stick to the old saying of “don’t fix it if it ain’t broke”? I would hate to get it into the car and find out 500 miles later that I should have done it. Are these chains and tensioners known to break? or is it a set and forget kinda deal?


I would check by removing tensioner access rubber plug. This should be renewed anyway since it will be rock hard and likely leaking.
If tensioner is OK l would leave well enough alone. Most tensioner problems occure when they are disturbed and the chain itself will likely outlive most of us forum members.

Good to know, thanks.

The chain itself is fairly bulletproof. The issue with the tensioner depends on whether the engine has been overheated. If it hasn’t been overheated, the tensioner is probably fine. If it has been overheated, it’s likely brittle and will snap at the earliest provocation.

If it’s not too difficult, you might remove the A bank cam cover and try retracting the tensioner. If you can retract it a ways and then gradually return it to engaged position with no problem, it’s probably OK for the foreseeable future. If it snaps when you try to retract it, though, well, you know it needed replacement!

Will do, I was planning on replacing it for good measure until i saw the cost of the tensioner. So just to be clear I should try to “cycle” the tensioner to see if it still has some flex to it? And if it doesn’t snap then leave it, and obviously if it does snap then replace it? One more question, I have heard that you should not rotate the engine backwards at any time, what is the reason for this? what will it damage and how?

I think Kirbert means for you to partially retract it. Full retraction involves totally straightening the tensioner and is a step too far if not required for further work.
Tensioner is positioned on slack side of drive chain not drive side, by turning the crank backwards will apply real tension to the tensioner with the risk of breakage but because of inbuilt chain slack will also result in a change in valve to piston timing with risk of damage there also.

Good to know! so even if i turn the engine back just a little, say 10-20 degrees it can screw stuff up? if so good that I found that out now and not when it was too late.

The above might help In visualizing how the tensioner works. This was one of my earlier links and in re-reading I realized that I could have been more descriptive.
Hope this helps.

got this old pic, may help!


Ok so If I am seeing this right, then turning the engine counterclockwise (facing the front) would permanently change the position of the tensioner? thus changing cam to crank timing? Or do I have this wrong?

The first thing that would happen is the bow in the tensioner would be flattened by the tension now being applied to chain. Until that tension is sufficient to remove chain slack and turn the cams the crank will be turning but cams stationery resulting in a timing change of a few degrees but only while the rotation is counterclockwise. TimING will still be correct on clockwise rotation but the tensioner will have been subjected to a straightening that is not good.

Ok, that makes more sense. good to know, I suppose that straightening effect would probably break an old original tensioner but not a new one right?

Right. They are pretty flexible but over time become less so.
I prefer not to release the tensioner even for head removal and have found with use of the cam sprocket hanger the sprockets can be removed and replaced on the cams without any problem.