My view on these is that while they may well be more likely to fail than the round ones they are still quite unlikely to fail in the great scheme of things. Personally I would leave them alone and replace them at the next engine rebuild.
It would be reasonable and easy to source a good used pair. The teeth don’t see any wear and we routinely reuse them; we have dozens of them. We routinely bead blast both them and the sprocket (after checking the sprocket chain teeth for wear). It is important to ensure that they mesh together easily as you don’t want to find out they won’t fit when you are setting the cam timing.
I know of one occasion where the toothed plate did slip. It was a ‘63 3.8. The sprocket itself cracked allowing the toothed plate to slip a couple of teeth.
If the data was available I suspect that Jaguar had half a dozen engines wrecked in the 50,000 or so they fitted these to. Good reason for them to change but not a big reason for Bob to lose sleep.
You would suspect that it had been incorrectly fitted. They can be tricky to align and you can see someone inexperienced or just not paying attention tightening up the bolts with the sprocket not flush, and it cracking under strain as a result.
Thanks Andrew, I confess that’s my inclination. After 50 years in service it doesn’t sound like it’s going to fail any time soon. Also I’ve not read of people actually having it occur. So at some point, if I pull the head or something, or perhaps just get bored, I’ll deal with it. I tend to drive my cars fast, but not harshly. So it shouldn’t be unduly stressed.
Have you verified your cam timing? If it’s fine, then yeah, I’d leave them alone for now until the head comes off.
If it’s not fine though, then you’ll be removing those round clips and disengaging the adjuster plate anyway. It wouldn’t hurt to try nudging them out at that point. I’ve never touched one like that but others have said they can slip out.
Hi Erica
Yes, sort of. I’ve used the standard Jaguar valve gauge with the notch to make sure inlet and exhaust are properly synced. I’m not able to tell if that is exactly at tdc since my car doesn’t have the timing pointer on the bottom, instead it’s on the side and I use #3 cylinder to set ignition timing. But I’m assuming things are ok…
I agree though that if I were to have to loosen them I’d be inclined to replace them now.
Some of the earlier ones, the one in the top photo with the removable nut, had oil tubes drilled out to the teeth. It could have sacrificed strength for lubrication.
Are not the sprockets actually held in place by the clamping force of the bolts clamping the toothed disc to the camshaft, rather than by the teeth on the toothed disc? If so, then the strength of the toothed disc becomes an issue ONLY if the bolts are not properly tightened.
I think the toothed drive sprockets are secured to the cam shaft by bolts (2 or 4) but are secured to the chain sprockets only by the little teeth… thus the teeth are a driven component and a tooth failure (however unlikely) would cause slippage.
Yes, however the sprocket fits snugly on the cam flange, so if the adjuster is pinching it there tightly enough it should in theory not slip. But ultimately it depends on how snug that fit is, and there is plenty of oil lubricating it.
I’m betting that is not true, for several reasons:
Look at the ID of the sprockets - there is an internal flange, the back face of which rests against the front of the flange on the cam itself. The toothed disc also has a flange, which rests against the front face of the flange on the sprocket. I’d bet a dimensional anaysis would show that the flange on the sprocket ID is wider than the toothed portion of the toothed disc, so that when the bolts are tightened, the toothed disc is drawn towards the front of the cam and clamps the sprocket to the cam securely.
If what you’re saying is correct, then there is only a loose connection between the chain and the cam under the best of conditions. Not good. And the entire torsional running load of the cam sprockets would be carried by a) the little teeth on the sprocket and the toothed wheel, and the small contact area (and imprecise fit) between the bolts and the bolt holes in the toothed wheel. This would pretty much guarantee torsional slop in the connection between the cam and chain, and would lead to wear of both the teeth, and the bolts and/or bolt holes, as well as noise.
I believe the spring clip is there purely to keep the toothed wheel in position when the bolts are removed. Once bolted up, it could be safely removed and tossed.
Does anyone have a cam, sprocket and toothed disc to check this out? All you have to do is bolt them up (in ANY position), snug up the bolts, and see if the sprocket is locked in place. I bet it is. If so, then all this fretting about the fit of the toothed is much ado about nothing.
I believe there would be easily detectable slop both between the teeth, and the bolts/holes. Put in the bolts finger-tight, wiggle the sprocket and you should feel some slop. Snug the bolts with a wrench, and there should be NO slop at all. MY clear impression is the teeth are anything but a precision fit to each other. The teeth in the sprocket are broached, but the ones on the toothed wheel appear simply stamped.
I could be wrong, but depending on those teeth to carry the cam load would seem to me an incredibly poor design.
No slop with mine, maybe with the scallop one. There is in/out slop but not rotational, and yes that definitely gets snug when tightened. I’m just not sure that it’s so snug that it couldn’t slip over time under constant stress.
Funny thing - look at the photo in post #5 of this thread, showing a manual page with a drawing of the sprocket, disc, and snap ring - the sprocket is drawn backwards! The toothed side is to the left!
Now look at the photo in post 6, showing two toothed discs. Notice the one of the left, the contact area where the disc contacts the cam is clean as a whistle, where it was obviously making solid contact with the sprocket. That would seem to me to support what I’m suggesting.
The diagram looks right to me. The tooth side inserts into the sprocket first. Once installed the teeth are no longer visible from the front because they’re covered by the outer lip.
The odd thing is that the scallop one seems to lack this outer lip as shown in Andrew’s post below it. Or at least it lacks the lip where the teeth are. As shown in the photo in the first post, it only has a lip on the center reinforcement. Seems like a hinky design to me.
This excerpt from S2 Service Bulletin B.102 may help, or confuse the issue more…
*Camshaft Sprocket Adjuster Plate*
Some time after the introduction of the fabricated adjuster plate which has lobes in place of the original type vernier teeth, cases have occurred of damage to the lobes and loss of camshaft drive. This condition arose as a result of lack of clamping of the sprocket to the camshaft flange.
It is evident that this possibility exists and therefore the following action should be taken on all cars coming in for service which have not been fitted with the modified adjuster plate assembly, C.26688/2… blah blah blah
Subsequent Service Bulletins for the 2.8 and 4.2 engine in the XJ6 (B.102) includes the following fascinating sentence “This condition arose as a result of lack of clamping of the sprocket to the camshaft flange but investigation has failed to positively establish the exact point at which the clamping effect could vary.”…
So, my impression is that, as Ray says, the bolts that secure the adjust plate (the one with the teeth) to the camshaft should clamp the sprocket to the camshaft, without the need for the teeth to take the load, but Jaguar found that for one reason or another that they could not determine (or weren’t prepared to publish) this sometimes didn’t work, and they therefore changed things.
