I am about to adjust the valve clearances and before I started I checked the alignment marks and was surprised to find a discrepancy from “spec” as follows.
In order to fit the camshaft position template snugly with the shoulders on the shafts and flat of the head, and with the tongue in the cut outs of the camshafts the crankshaft must be at 10 degrees ATDC for the inlet cam and at 6 degrees BTDC for the exhaust cam. That seemed to me to be quite a bit “off” but I would appreciate feedback from some of you more experienced guys.
The engine has done 61,000 miles and has been running fine.
My experience is with older XK engines but I thought it was always Top Dead Center.
You have to take off the sprockets and reposition them.
Do not turn the crankshaft when either of the cams is disconnected.
Timing chain tension also has to be considered; have you checked that?
While you are in there, if you have a 1"-2" micrometer, you might want to measure each cam lobe peak-to-heel and side-to-side, just to see how they are wearing.
I would be interested to see the results.
We have a project going on the XK forum examining wear on cams.
I would definitely be doing this much. Our XJ6 has original camshafts and head has recently been off for skim etc. Data point: no measurable cam wear - they have done 140,000 miles plus. Paul
Frankie, no the grooves in the cam are there for TDC on both of them. Either they were timed and then the chain was retensioned or somebody was doing a bit of experimentation.
Another possibility is timing chain wear.
I don’t remember the number of links, but for example suppose there were 100.
If each link and pin were to wear say .0005" and there were 100 links, the chain would be .050" longer than when it was new.
This could change the timing of the cams by a few degrees from their setting when new.
I think someone has been in there before - the lock wire is soft copper rather than proper steel wire. It looks possible to adjust the serrated plates with the head in place - I’ll just be careful not to drop anything into the sump.
I checked tension before releasing the tensioner pin and it seemed too tight to me - needed a screwdriver to lever the chain to show movement. The manual says slight flexibility but not taut - sadly I don’t have enough experience with these engines to gauge the right “feel” for the required tension. Any tips?
I assume it makes no difference whether I set the valve clearances before or after adjusting the valve timing.
I’ll be back in there on Tuesday to have the camshafts out and will take the lobe measurements and report back.
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Experimentations on valve timing has been known as Robin implies, Frankie…
However, 10 deg off is nearing a point where valves may hit pistons. Besides, there is a 16 deg difference in sum between inlet/outlet - which doesn’t seem to serve performance enhancement. Inlet valve closing 10 deg after TDC makes little sense…
I think resetting cam timing is called for, including verifying chain tension as Rob mentions. As an aside; you should also verify that the timing pointer is correct - by checking actual piston (#1 or #6) TDC versus pointer…
You are correct that it should be steel wire, not copper. I like to stuff a rag down in there to catch any stray dropped bolts.
The factory workers had a tool like this to set timing chain tension.
Other versions are on the market with Jag parts vendors.
Or you can make one by grinding away on a cheap socket.
Condition is not as bad as I thought it would be. The cam lobes and journals are near perfect. Several tappets show ghosting of lobe contact and a couple fail the fingernail test on the tops and skirts. All rock slightly in their guides but I guess that clearance is necessary for lubrication - the diameters measure to within spec - only a couple are out to a max 0.01 inch. The bearing shells show wear and a couple fail the fingernail test (worst at chain end where pressure is greatest). I can’t measure the tappet guides but they are all in their correct positions.
I am inclined to replace all bearing shells and tappets now I have got this far - any contrary opinions based on the photos below?
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Pictures doesn’t tell much, Frankie, but shells are not a great expense - ‘for want of a nail’ springs to mind…
However, a bearing alignment check and ditto for camshaft straightness might be added. After all - the old stuff was imbedded, and worked in their proper places.
But then we move into ‘shipwrights disease’ territory…
However, a bearing alignment check and ditto for camshaft straightness might be added. After all - the old stuff was imbedded, and worked in their proper places.
But then we move into ‘shipwrights disease’ territory
I know - that’s the trouble - I’m doing only half a job. It started with only adjusting valve clearances - but then it expanded into assessing the tappets - then the bearings…………. My dilemma is how far to go (I’m not into taking the head off at the moment). On the one hand I could leave the shells and tappets as they are and let wear progress until head overhaul time, or I could replace the shells and tappets now. But why? Will further significant damage will result if left for another 10,000 miles? If the wear in the cams is typical of general wear in this engine it is certain that the bottom end will be in a similar state.
While you are in there, if you have a 1"-2" micrometer, you might want to measure each cam lobe peak-to-heel and side-to-side, just to see how they are wearing.
_ I would be interested to see the results._
Results attached.
Interesting pattern emerges - progressively greater wear towards front of engine - or coincidence?
Ah - the site won’t accept a word document from my PC - I’ll have to find a work around
…but my initial reaction is there is nothing wrong with the cams they are too even. However, the measurements, heel to toe, is not given in specs - someone with a new cam may have something…?
The spec give is valve lift; 0,375" or 9,5 mm - which is the relevant figure; lift is the only thing the cams do.
In theory; the difference between lift and heel to toe you measured is the latter, minus the cross diameter of the cam - which you can also measure. Also possibly deducting the valve clearance of 0,300 mm…
I suspect it will be close to your heel to toe measurement - indicating the amount of cam wear…?
