Loose Timing chain

Phillip

New member
Joined
Nov 16, 2020
Messages
2
Reaction score
0
Points
0
Location
La Jolla, California
I bought a 1968 Series 1.5 /4.2 L from a salvage yard with good body and interior but suspect engine. I have no knowledge of history of whether the engine was running before or not. One of the first things I did was test the cylinder compression. Four cylinders had reasonable pressure but two were bad so I removed the head and had it professionally rebuilt.. So I know the valves and valve chains were working before removing the head.

After the head was reinstalled, I set cam shafts correcting and turned engine to tdc on compression stroke. Then I installed one bolt on each cam sprocket and turned the crack to get to the second bolt hole - all carefully following jaguar and haynes maintenance manual procedures.

The problem is that when turning the crank (only about 30 to 40 degrees), the TOP CHAIN DOES NOT ROTATE although the bottom one does and the intermediate gear rotates. I took out the cam sprocket bolts are tried again unsuccessfully about 4 more times. always ensuring that the eccentric sprocket/idler gear was positioned down as far as possible. (I found max up and down of idler gear is about 1 cm, which is about right considering dimensions of the eccentric).

It seems clear it me that the top chain and cam shaft were working ok before I removed the head. Otherwise, for example, the valves would be damaged by pistons during compression tests but they weren't. So the problem must not be due to a worn chain or something like that. The biggest possibility is that I did something wrong in installing the head and chain sprockets but I can not figure what that might have been.

Can anyone suggest why the top chain does not rotate when adjusted as tight as possible and when the crank and lower timing chain are turning ok?
 

Phillip

New member
Joined
Nov 16, 2020
Messages
2
Reaction score
0
Points
0
Location
La Jolla, California
My replies are no I have not had the intermediate sprocket out and, yes, it is clear that the chain is not engaging the intermediate sprocket. That is the basic problem I am posing. The question why not when the chain is around the idler gear and the cam sprockets. Since the valves worked before I removed and replaced the head, it would seem that the answer is not that the chain has stretched too long. Something else is wrong. What?
 

ponsaloti

Member
I Have Donated !
Joined
Jun 6, 2008
Messages
238
Reaction score
0
Points
16
Location
essex riviera, uk
Forget about a stretched chain, it's not that.
A couple of questions.
Is the chain front cover off?
To be clear, are you saying the cam sprockets are bolting onto the cams with the top chain wrapped around them both and you can rotate the crank and see the intermediate sprocket rotating?
If so, what do you estimate the degree of slack on the bottom chain when you rotate the crank clockwise to anti-clockwise?
In other words can you rock the crank backwards and forwards to check any slack in the bottom chain?
 

piman

Active member
Joined
Feb 26, 2016
Messages
1,858
Reaction score
2
Points
33
Location
OSWESTRY, Shropshire
Hello Phillip,

take the camshaft chain wheels off again. I must admit it's a while since I have dismantled an XK engine but I think you should be able to see the intermediate sprocket? You probably will need an assistant to hold a light onto that area of the engine.Take the top chain and manipulate it to pick up correctly on the intermediate gear, you should, I think, be able to feel when the chain actually connects properly on the gear?

Alec
 

ponsaloti

Member
I Have Donated !
Joined
Jun 6, 2008
Messages
238
Reaction score
0
Points
16
Location
essex riviera, uk
Piman, around 25 years since I was rebuilding jag engines but from memory, I'm sure the cam sprockets wont bolt onto the cams if the chain isnt located on the intermediate sprocket.
The front of this sprocket is visible just below the chain tensioner.
 

piman

Active member
Joined
Feb 26, 2016
Messages
1,858
Reaction score
2
Points
33
Location
OSWESTRY, Shropshire
Hello Ponsaloti,

the top chain must be round the intermediate sprocket but misaligned with the teeth, it's the only explanation I can think of except for the almost impossible fracture of the gear such that the inner sprocket is independant to the front.

Alec
 

ponsaloti

Member
I Have Donated !
Joined
Jun 6, 2008
Messages
238
Reaction score
0
Points
16
Location
essex riviera, uk
Agree, it wont be a sheared intermediate sprocket but as said, I'm sure the cam sprockets wont go onto the cams if the chain is not located properly. The chain would be to short.
I'm wondering (I may be over thinking this), if there's loads of slack in the bottom chain and something has dropped down jamming the top chain. Unlikely but you could end up with the scenario as explained by Phillip.
Another thought. Did the cams turn after the re-build and before the head went on?
Also are the cam timing brackets still in situ?
 
Top