I have taken the head off my 68 E type engine to have it rebuilt With advice from this forum, I have reinstalled the head and installed one bolt on each of the camshaft sprocket pla tes and the cams. However when I.turn the crankshaft to get to the other cam bolt hole, the lower chain turns but the top chain does not rotate. Then when I turn the camshaft, the sprocket and top chain rotates but the bottom chain does not move.
The top eccentric adjuster is in the tightest position but changing the eccentric does not help. The head is bolted down but not torqued tight yet.
This same problem was posted two years ago but responding suggestions do not work for me so I am looking for fresh ideas.
As far is i know, there should be a “key” - so to say a pin between the top and bottom chain sprockets that keep them connected. Maybe it got missing after the rebuild?
SNG offers a fixed set of sprockets, without the key to avoid this.
First, I hope you stopped immediately when you realized this, otherwise you could have had a collision between a piston and an open valve. Did you stop quickly? 10 degrees maybe 20 is safe but beyond that could be contact.
I’ve never looked at the inside of a 4.2 engine, but on a 3.8 that isn’t possible so long as the upper chain is engaged on the intermediary sprocket. Both upper and lower chains engage there on two sets of teeth.
Can you confirm with a light or maybe a camera that the upper chain is engaged there? I’m not speaking of part #9 in that diagram. They only provides tension. The intermediary gear is below that one and has two tooth rows. It’s part #15 there.
edit: I guess on the 4.2 it’s parts 15, and 16. On the 3.8 it’s one part. It looks like they’re connected by a woodruff key. I wonder if yours sheared off?
………“It sounds like the upper chain is simply hanging slightly below the teeth at 6 o’clock on part 15”…….
That is exactly what can happen on the 3.4 ltr and 3.8 ltr fitted to the S type and Mk2 engines. I presume that can also happen on the 4.2ltr.
Most certainly Jaguar modified the XK engines in the models mentioned above with the addition of a “retainer for top timing chain” part number C25818, about August 1964. Might it be possible they modified the 4.2 ltrs?
On the earlier unmodified 3.4 ltr it is possible to have the top chain drop off the sprockets if no upward tension is kept and applied during fitting.
Doubtful a sheared woodruff key is the issue. It’s not the idler gear that’s not engaged but the intermediate gear below. Try disconnecting the cam sprockets, allow the upper chain to fall below the intermediate gear then pull it back upward. This is done by feel. You will know when the chain is correctly engaged when you are unable to move it when pulling up on either side. I expect you will need to back off the idler eccentric completely in order to have enough slack in the upper chain to get the cam sprockets back on (which is to say, one will go on easily, the other won’t).
The engine schematic posted above is not for the 4.2 litre engine. The intermediate sprocket in the 4.2 is one piece - #14 below. It is therefore impossible for the top chain not to move at the same time the bottom chain moves unless one or the other isn’t engaged in the intermediate sprocket. And since it’s only been the head that’s been removed, it must be that the upper chain is not engaged.
Ah, okay then, it is exactly like the 3.8 in that regard. It seems odd that there would be room for the chain to not end up on the teeth. Mine has never once ended up in such an orientation. Do you figure it’s skipping over them because it couldn’t get sufficiently tight? Maybe some gunk blocked the adjuster gear so it wouldn’t tighten?
I suspect the chain is tight up top but slack below because the eccentric is set at it’s lowest point on the idler. So, you will get a tight fit between the cam sprockets but lots of slack across the lower section of chain. One link out, especially in an old upper chain, will be enough for it to hang loose below the intermediate sprocket
The five or six posts above seem to accept my contention that the intermediate gear is not engaging with the upper chain.
However, only Nick proposed a solution but he acknowledge s that I may be able to connect a bolt on one sproket/cam but not the other. What then, is the next step?
That would be easy to feel on the outsides of the cam sprockets, and yes, that does happen.
I doubt it has much to do with chain wear, a chain that far gone can’t look useable anymore?
Have a look with your fingers. If that is it: Loosen the eccentric and align both cams. Tighten the eccentric and see if you have engagement. Hope for the best. Would that work?
If the chain doesn’t engage with that lower gear then bolting the gears to the cams only permits you to tighten the chain to the max, either to the max tightness of the chain, or to the max throw of the gear. From there you’ll know if it engaged or not. But it seems to me that you’ve already tried this right?
@Nickolas If the eccentric tensioner gear was at the end of its range of motion and the chain still had slack, would the adjuster stop or just flip around in a 360 degree circle? It rides on that simple oblong shaft so without the chain to restrict it I’d think it would simply return to its original position?
Only if there’s a master link in the chain. All of those I’m familiar with don’t allow separation. Not necessary anyway.
There’s also wear in the three sprockets that comes into play. It doesn’t mean that they’re unusable, so long as there’s sufficient adjustment left in the eccentric to adjust the chain tension. When I rebuilt my 4.2 I installed all new sprockets and chains and even with the eccentric in it’s slackest position I was just able to get the cam sprockets mated to the cams. As time goes on there will be wear in the sprockets and chain wear/stretch, but plenty of adjustment in the idler eccentric to accommodate it.
The adjuster would stop and you would be unable to turn it 360 degrees. With the cam sprockets bolted in place the range of the eccentric is confined to the length of chain between the two sprockets. The chain would be tight between the sprockets but slack below. The only way you’d be able to take up the lower slack would be to rotate one or both of the camshafts out of phase such that valve timing was compromised.
Disconnect the cam sprockets, ensure that you are at #6 TDC, set the valve timing and go from there. Make sure the upper chain is engaged in the intermediate sprocket per above then play with the idler eccentric till you can get both cam sprockets mated to their cams, checking and double checking that you have not moved one of the cams out of phase while you do. After a few tries it will come as second nature and you will wonder why it wasn’t so simple before.
So, as you’re mounting the gears on the cams, keep tension on the outer chain segment by rotating each gear towards the center? I think I’ve always done that.
Yup. The objective is to get the chain tension close with both cam sprockets in place but before the retainers are aligned and bolted to the cams. If there’s too much slack in the long section of chain below the cams there will be no way to take it up with the eccentric without putting the valve timing out of phase.
