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.
You are of course quite right, Frank. What I was trying to get at was the amount of wear on the cam lobes - I may have misunderstood what Rob Reilly was looking for ICW his database.
The only explanation I have for the greater wear at the front is that the oil supply comes in at the back and on start up the front cams will get less lubrication for the first couple of revolutions of the engine. I don’t know how oil in the “system” drains back into the sump following switch off - all I know is the oil light stays on and the gauge rests on its stop for a couple of seconds when starting from cold, so oil from somewhere has drained back. Certainly puddles of oil sit in the head next to the tappets and I assume these are supplied indirectly with oil escaping from the cam bearings - which are pressure fed. But just my guess really.
Yes, I am quite happy with the cams and won’t be replacing them.
I would plasti-gauge the bearing clearances once you receive the new ones since when I received my new shells, the assembled clearance was WAY too great…had to surface-plate sand the bearing ends down to attain the correct clearances. Of course, I did this work before valve installation so it was relatively simple to find and fix the problem during “mock-up”.
Thanks for this interesting contribution. What we were looking for on the XK forum is the difference between heel-to-lobe and side-to-side. We found it was slightly more than the published lift, because you have to allow for the tappet clearance.
I found one early XK120 cam with very severe wear.
Here is the thread.
hi…just saw this thread…interesting…so jumpin in…maybe with info you don’t need…but here oes,. as an XK person…I don’t know the cam-valve timing specs of the car in discussion here…but curious about it,. The XK is 15 57 57 15, while Mk 7 was 10 50 57 15, and.,.,In 1969, Jaguar changed the cam lobe
profile to quiet down the tappet noise and the clearances went to
0.012’’ Intake and 0.014’’ Exhaust. The change was made at engine
number 7R8687/7R8854 (E Type) and 7L8343 (XJ-6) around the time
they switched from two to four bolt flanges. The lift spec is the
same but the duration changed to 17/ 59, 59 / 17. The cam timing tool fits when #6 piston (front) is at TDC compression,. The #6 cam lobes will point outward, so that valves are closed. The exact position of all the cam lobes depends on those valve timing figures above, so 15 57 57 15, will have slight difference in lobe position from other specs. By the way,you do know that all cam bearing caps are numbered and they must be installed EXACTLY in same place…and same orientation as original and marked on the head lip, I replaced cam bearings and found the press in fit varied slightly by manufacturer. Dick Maury recommends slightly less than the spec 175 inch lb. In any case, do not exceed,. Use an inch lb quality torque wrench.,.when it clicks STOP., (tighten down slowly across all caps a little at a time each as there will be pressure on the lobes that are in contact) Timing chain adjustment if substantial will change the cam alignment-recheck with the tool,. Measure with micrometer…all parts…write down old, and new, the thickness of old tappets, and new, check shims, old and new even tho marked for size., yes the shims must rock on the top of the valve stem. Block the abyss with towell and saran wrap…: use new cam lube on lobes. In Saloons Forum.,.see P,. Saltwick’s Valve Adjustment article.
Nick
Setting the later cams to tighter clearances will cause so much
duration at idle that the car will probably not be able to idle
much below 1000 PRM’s and roughly at that. The later Series 3 XJ6’s
had a tendency to wear the front cam bearings and when the chain
was tensioned, the clearance on the front of the cams was changed.
If it was as low as .009, the front cylinder would be dead at idle.
They are set .012 - .014 for a reason. I have found that setting
them at the wide side of the clearance range gives the best idle
quality. --"
Thanks Nick - much helpful information there. It is a 1971 engine with 12-14 thou clearances and 17 59 59 17 timing.
Before disassembly some clearances were as little as 9 thou which together with the out of spec valve timing may be a significant cause of the lumpy idle I was experiencing - I hope for improvement after re-assembly. The bearing cap nuts were very tight - needed an extension bar to break loose - I suspect well over 175 inch lbs. I will set clearances at 14 thou on reassembly. All this together with the copper lock wire suggests to me that the last intervention was less than professional.
Will put it all back together in correct location/orientation/numbering and will be interested to see how the replacement (Mahle) bearing shells match in (collecting tomorrow).
take care pressing in the bearing shells…sometimes at first it seems they just won’t fit…,but they will…too much pressure or using a tool will likely damage the bearing, the sets do not come with a spare…I advise to have a second set on hand…(and you do know how I know this,and I now do have a set that is one short ) this a quote from Dick Maury, Coventry West the Master.,., ,"11 lb/ft or 132 in/lb. 175 is a bit tight. "
Nick
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The clearance is spec’ed to ensure that the it does not close up as engine heats up, Frankie - which may cause burnt valves.
Excessive clearance may cause abnormal valve noise, but it is better than too tight. And a periodic ‘ticking’ may come from a wandering shim which is not perfectly flat…
The new bearing shells and tappets are in, I have corrected the valve timing to specification and I have turned the engine over a couple of times by hand. I checked the camshaft bearing clearances with plastigauge and got .002 - the old clearances were similar at .0025 and I was a bit disappointed not to have reduced the clearances more (old shells worn) (specification is .0005 - .002). I laid the plastigauge under the bearing caps and my checking was done with the valves fitted so I suppose the pressure of the springs tending to lift the camshaft may have caused unreliable readings? I torqued the bearing caps to 9 ft lbs (jaguar specification) but I may increase that to 11 ft lbs and see if it makes any difference to the clearances. Not keen to go higher than that. Using the old shims for the time being the valve clearances have barely changed (+/- .001 or .002).
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As the clearances are to specs, Frankie - there is no point in doing more. That the clearance changed minimally just indicates that there was little wear - as also indicated by little change in valve clearances…
And increasing torque beyond specs should not change clearances…
I had assumed (possibly incorrectly) that where a range of bearing clearance values is given (in this case quite a wide range) there is built in allowance for wear in service. Otherwise, if bearings with the maximum allowable clearance are fitted from the outset they will be out of spec relatively quickly. AFAIK the shells should press into final position under load from the cap - it was that effect I was thinking of maximising by applying a slightly higher torque. However, if the resistance is between the cap and head nothing will happen and no harm be done.
I’ve followed this with academic interest. No “cammers” in the house at this time.
I’m thinking that the process of seating the cams in their bearings is being over thought. The wide range of acceptable clearances support this. Weak pun, huh? The load is relatively light. Unlike a crankshaft bearing.
If the clearances are close and they should be with the caps torqued to spec. Note the relatively low value and the cams turn smoothly, it is time to button up…
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The wide tolerance reflects that clearances are not critical, Frankie - except for too tight…
If the clearances is near the outer limit it’s the owners choice to reinstall - or replace. Bearing in mind that reused parts must(!) be returned to their original positions - as stressed in the workshop manuals.
Bear also in mind that all bearing shells are made to be flush with the caps and base - otherwise shells will deform (or ‘yawn’) as bearings are tightened. Increasing torque will be taken by the shell and base metal - not affecting the shells…
Thanks Frank - it’s always helpful to receive good practical engineering advice. I’m a relative novice compared with most contributors to this site, and am learning a lot.
Bearing shells are NOT quite flush with the head surface or cap…in fact,the modern way to measure this resulting “bearing crush” is to torque both cap bolts to spec, then loosen one nut and then measure the resulting gap between that side of the bearing cap and the head’s surface…should see about 2 or 3 thou of clearance there. That much cap “bend” is perfectly fine and desired. Same is true with rod bearings, though this bearing crush is much more important. The same principle is applied to the cam sprocket retainer plates…they bend slightly at the bolt holes in order to exert pressure on the inner radius of the sprocket, holding it tight to the cam’s snout.
