Hello folks - ( and apparently I can't type thread titles very well)

In my never ending, and so far completely unsuccessful, attempt to get rid of the chain noise in my '62 MKII 3.8, I need to verify something: Adjusting the tension on the upper chain, does not adjust anything on the lower - correct?

The way I understand it, the two chains are entirely separate affairs, where the only meeting point is a two-piece sprocket in the middle. The lower chain drives one side, which in turn drives the upper chain from the backside. This intermediate sprocket is in a fixed position, which is why you take slack out of the upper chain using the idler sprocket, which is mounted on an eccentric shaft. This should also mean that if you make the upper chain tight, or loose, it has no impact on the lower chain at all, because lower chain tension is a factor of the fixed intermediate sprocket, crank sprocket and the hydraulic tensioner taking up slack between them - the eccentric has no impact.

I need to confirm that thinking is correct because I can change the chain noise by making adjustments to the eccentric. I can make the sound change pitch, occur at different RPMs, go away at idle or be heard at idle, but cannot make it go away entirely. I am trying to make sure its an upper chain issue so that I know whether I should bother trying to change it, since if its lower chain, it will be going to the shop. Currently, the sound can be heard at idle intermittently, then constantly if RPM is raised to say 1200 - 1500, but seems to go away above that. I can also hear it on overrun, and it sounds exactly like my 61 MKII used to, the difference there is that the chain was quite loose on the 1961, and tightening it up resolved it entirely - but no so such luck with the '62. Sprockets look to be in good shape, at least compared to pictures of "bad" sprockets. Might it be the chain is just worn and needs replaced?

I am almost prepared to take on that job, replacing the upper chain and perhaps sprockets, but only if there's a high likelihood doing so will resolve the problem. Anything more and I'll need to take it to the shop anyway, which means towing the car about an hour away, setting up an appointment for drop off, getting a lift back, and doing that all over again for the return trip.

Thanks,
Allan.

 

Frome what you describe, I think you really do need a new lower chain, tensioner, and chain dampers as well as replacing the top chain. You'll probably find the hydraulic tensioner is at the limit of its travel. Unfortunately nothing lasts forever. When were the timing chains last replaced ?

 

Consider the chains as separate entities.

I'm inclined to agree with Fraser. What is the difference in noise between hot and after cold start? Oil pressure after cold start will provide maximum pressure to the lower chain hydraulic tensioner until you reach full operating temp & a while after. Bulk oil temperature rise lags coolant temperature quite considerably.

At some point in the lower chains life you will go through a transition period of brief rattle on start up that shuts up the minute the oil pressure comes up.

 

In a static sense, the tension in one chain doesn't affect the other. In spite of that, if one is not under sufficient tension, (over-)tightening the other might well influence the noise from both through a dynamic coupling between the two. However, it's not a good way to make the whole system quiet and may well result in more problems. The camshaft drive chain system in XK engines has a very good reputation for function and longevity. As Fraser wrote, when the chains, tensioners etc need to be changed, it is much better to bite the bullet and change them. And then forget them for at least 100,000 miles.

 

No idea, unfortunately. The tensioner slipper pad etc looks good on the scope, but hard to tell how far its traveled and or whether its doing its job.

@Glyn M Ruck : There is no change in hot vs cold. The only way the noise changes is if I actually tighten or loosen the upper chain eccentric. Otherwise the sound will stay the same, hot or cold, only varying with RPM - which itself changes depending on how tight or loose I make it. If I tighten the chain all the way as much as I can without getting a lever, the noise will go away, but is replaced with a sort of low whine / whirring sound. Perhaps I'll take a sound byte of both an post them so you can all hear it.

@Peter3442 : Yes, if indeed the entire assembly needs done, or just the bottom chain, it will be off to the shop. Its just not at all convenient and sort of annoying given that the car was reportedly free of such issues - perhaps I am resistant out of spite or stubbornness - maybe both.

 

I'm surprised temperature makes no difference (cold thick oil vs hot thin oil). Either the bottom chain is fine or everything is so worn that the tensioner can't do it's job at all.

Can you get an engine stethoscope onto the lower chain case carefully? Without getting it into rotating parts. Safety is all important. It would be great if you could listen to the lower chain.

Your other option is to pull through a new top chain. Observe influence properly tensioned (not over tensioned). If noise remains take it to the shop & let them do the lower chain & it's paraphernalia. (I have only ever done a complete XK rebuild. I am assuming you can pull through a new top chain with the old one a la Alfa etc.)

BTW ~ when you get the top chain to that low whine it would suggest to me over tensioning.

 

If you have a suspect bottom chain don't ignore it. I did that once on a 3.8 engine and it spat the hydraulic tensioner and jumped one tooth on the timing system, with subsequent poor running. However I was extremely lucky as it could have jumped a number of teeth with catastrophic and expensive results for the inlet and exhaust valves. Spending some dollars now will save you heaps of dollars you would have to spend if the bottom chain does "jump".
Bill Mac
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