Lower Timing Chain Tensioner Spring

As I continue my hunt for the source of my strange engine noise, I have dropped the sump and pulled the timing chain cover with hopes that I might find a potential cause of the noise… having pulled the head last winter and not finding anything significant.

Nothing really obviously amiss in the lower timing chain either, but since I’m here, I’m reassembling with new tensioner and guide etc…

what is quite disconcerting, and where I am looking for advice, is the new spring is way longer than the old one, and does not put the blade under enough tension as can be seen in these photos…

  1. I refitted the original assembly to remind myself of how much tension it was under… a lot!

  2. these are the two assemblies on the bench, the left new one with the much longer spring…

  3. the old / original spring looks like it has been shortened at some time in it’s life…

  4. new assembly installed, obviously lots of slack…

  5. even the top guide does not stay in place with the new assembly!

so lots of questions!

  • am I supposed to have to adjust the length of the spring when new?
  • if so, how short / tight should it be? can it be too tight?
  • maybe my old one was too tight due to its apparent custom length and that might be leading to my noise?

any experience or suggestions (other than going hydraulic, I’ve got these parts, I’d like to use them!) ?

I’m thinking of using the old spring with the new blade…


Many years ago my spring broke, and I recall it was worn thin at the loop, which may be why the one end is gone.
I found one of the loops on the tensioner blade was also half broke off. The wear patterns in the blade are maybe .005" deep.

Here are some notes I made at the time.
wire dia. 0.049
coil OD 0.310
coils 2-1/4", 46 active coils
loop-to-loop 6-5/8" (I must have measured the new spring from Welsh Ent.)
spring rate 10.2 lb/ft
Associated Spring equivalent E0300-049-3000 (I looked that up in a spring catalogue as a backup plan in case Welsh was out of stock and I had to make one)

I’m going to offer the opinion that your old spring was shortened too much.
Also that your lower chain may be longer than it was originally, due to a tiny amount wear in each link, multiplied by the number of links.

Your new spring may indeed be too long. The way these things are made is they make the ends of those two straight wires wider inside the beehive ends, so they don’t pull out. How I don’t know, possibly a little blob of weld. The blob is still inside the broken end of mine.

So then the question is, how much tension.
Enough to keep the chain from bouncing.
It looks to me like either your new spring is too long, or your lower chain is too worn. Is it new?
I don’t think the C4820 block has any provision for installing the hydraulic tensioner.

Likely unrelated, but are you certain the noise is from the lower and not the upper chain? The SOP is to not get it dead tight, but it still should be tight with essentially no slack between the camshaft sprockets and just a little slack on the outside runs. I generally turn the concentric counterclockwise firmly then check the slack of the outside runs on both sides levering with a long screwdriver and if they flex just a cm or so declare it good. You should also be able to shift the chain a little from side to side on the camshaft sprockets. Tight, just not dead tight.

I do the same as Nick, pretty much.
I spent a long time pondering my 140’s lower tensioner. The original parts were completely shot, the spring broken and the slipper worn through in several places. I bought new parts for this, but from memory wasn’t happy with the new spring and general fit of the parts. After a lot of deliberation I went with a hydraulic conversion, buying the best quality parts I could find which came from Rob Beere. It’s a big(ish) decision, because you do have to do a little work to the front of the block, but I think in the long term it’s worth doing. As ever, it depends on what you plan to do with the car.

I agree with Roger, however it does make sense to change the chain at this point. you can use a split link chain (discussed many times on this forum) to save work.
when I rebuilt my 120 I wanted to keep everything standard so I bought replacement chain and spring tensioner, spring etc from coventry auto components and when installed everything was/appeared to be in the correct contact and tension. its certainly runs well.

Hi Phil and Dave,
Yes I certainly agree on replacing the chain. In fact I would take this opportunity to replace both chains and inspect all the sprockets and chain guides for wear, and make the judgement whether to keep or replace. Unbolt the entire upper assembly on the front of the block and disassemble it and clean everything and assess, The teeth on the center double sprocket appear to be very pointed from what I can see in the pictures. And now is the time to upgrade to a lip seal on the timing cover, discussed recently on the forum.

Regarding changing the blade type spring tensioner to hydraulic, I feel it is a tossup. I have both styles on my cars and have had a failure on the hydraulic type versus none on the blade type. As the lower chain wears the hydraulic tensioner piston moves further out until it simply falls into the sump and you have no tensioner at all. The blade style will fail in a manner as Rob experienced, the spring breaks and you have little to no tension. So it’s a matter of miles til failure, most likely not an issue with most XK’s driven a thousand miles a year. So a personal choice.

Best regards,
Tom Brady

Hi Dave,
I had similar problems with my lower chain tensioner. I purchased a new blade and spring from CAC but like yours my spring was far too long and exerted no useful tension on the blade / chain.
Fortunately I don’t live too far from Coventry so I returned the items in person and spoke to Stewart about the issue.
Sadly I can’t remember the detail of what he said, but I can tell you that I was persuaded to come away with one of their hydraulic conversions!
I installed the said item - it did require some fettling (mentioned in their guidance leaflet) but my noisy chain is now quiet and has been for a couple of hundred miles so far.
The tensioner picks up its oil feed from the pipe (now replaced) that fed the lower sprocket.
Hopefully the pictures that follow are self explanatory.

thanks for all the input!

Rob… thanks for your spring measurements, my new one is 2.5" for the coils, 5.75" end to end so likely NOT too long…hmmm…

I can envision some previous mechanic dealing with a slack chain by shortening the spring / increasing the blade tension rather than replacing the chain… probably would have been done in the 70s , these cars weren’t worth anything back then!

And so maybe that short sighted solution got the car through another 40 years of driving!

But maybe that now leads to my noise issues… if the lower chain is stretched now beyond the ability of the customized tensioner to accommodate for the wear that would presumably give me my rattle, which only happens when the engine is not under load, down shift and or put your foot on the gas and the noise goes any.

A couple of further questions…

this chain has a joining link in it, I’m guess therefore it is not a factory original chain? Someone previously pointed out that is is on the wrong way around!

How can I asses the stretch / wear in the chain to decide if it needs replacing? Is there a specification for how much defection or??

Also, here is a photo of the wear on the tensioner and the guide for what its worth…

So I’m thinking that its time to replace this chain. The current one has a link which implies it can be changed without any further dismantling of the engine (head is still on) but a quick look at Moss and Barretts show continuous loop ones which would require pulling the head. I’m also thinking that the wise move at this point is to pull the head and replace the upper chain and guides as well as the sprockets etc, basically everything, and I doubt anyone here is going to tell me that’s “over servicing” but before I start ordering up all the parts, any other wisdom?

Also interesting to note that basic lower chain is (only!) about US$15ish, way less expensive than I would have thought, but the “upgrade” is closer to US$100… big upgrade! I’d hate to cheap out on parts at this point, don’t want to be opening this part of the engine up again too soon! And I do spend @ $200 every couple of years on a motorcycle chain so from that perspective this is inexpensive, but boy, what a price range!

Nick, re the upper chain: I pulled the head last winter and serviced it. I may not have got the upper chain perfectly tensioned ( I think I did but…) but even if I didn’t, presumably I would have changed things enough to change the noise I’m chasing, but the noise did not change…

There is another issue that cropped up during the rebuild of my XK140 engine, concerning the upper timing chain. On dismantling mine, I noticed that the shaft the centre idler sprocket runs on had been spinning in the alloy casting support it runs in, causing wear in the casing which allowed vertical play in the shaft itself at one end. It seemed odd to me that this shaft was essentially cantilevered, and had no anti-rotation device to stop the shaft spinning with the sprocket. The wear caused by the shaft on my engine allowed the shaft to tilt upwards, meaning I had to locate another casting. At some point later than my car, this problem must have become apparent to Jaguar - I bought a used, later (I think it was an E-type) idler sprocket bracket, which had a roll pin added to the flange of the shaft which engaged in a slot in the casing. Unfortunately this was a different shape where the upper chain guides bolted in, so I decided not to use it.
Then I discovered that Crosthwaite and Gardner (well-known C- and D-Type preparers) made and sold these components for the earlier XK engines. They too had addressed the problem, and their new aluminium idler shaft casting was the correct XK (C-Type) shape but also included an anti-rotation device. This fixed the problem, so hopefully my idler shaft will not get loose in its mountings any time soon. I was surprised how much wear was evident in that casting on a car that seemed to have covered only around 70k miles, having been in storage since 1983. It would have been impossible to tension the upper chain accurately with the shaft as loose as it was, and things would have been a lot noisier with the sprocket able to move up and down.


This subject has been dealt with before, but there are many aspects of it.
My timing chain was rather “loose” and I ordered a new timing chain. It was made by IRIS, a Spanish manufacturer, and was slightly shorter than the old chain (only 49 links iso 50).

We discussed this in May 2021 on the Forum. The supplier stated that this particular chain was used on all XK engines without problems. There were a number of “warnings” from Forum members that this might create problems.

I did a lot of measurements and the chain fitted fine, probably because of some wear of the sprockets (no sharp or damaged teeth, so still reliable). I fitted that somewhat shorter chain and replaced the “old blade” type of tensioner and guide. The tensioner works well and I think it has the correct level of force to keep the chain in position. See picture.

The engine runs fine with very little noise from the front of the engine. I understand that “time will tell” whether this was a good decision but so far it runs beautiful.

Bob K.

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The original manufacturer of our chains was Renold. They are still in business in Manchester, and they have a subsidiary Renold Jeffrey in Morristown Tennessee.

The standard Roller Chain Trade Size is 06B-2 and can be bought in 1 ft increments up to 20 ft, then 50 ft and 100 ft.

Some years ago when I needed a timing chain for my Mark V and my favorite Jag parts vendors had none, I measured the link size and bought a length of stock chain with master link from McMaster-Carr and it turned out to be made by Renold.