XJS V12 camshaft C41249

(foxy99) #1

Hi folks.

My XJ parts lists show 3 different numbers for the RHS (bank A) camshaft: C31835 for series 1; C41682 for series 2 up to 7P7000; C42176 7P7001 onwards.

the number on the cam I have is none of the above.

I’d imagine for a precision part like the cam any changes are important/significant and serious problems could result from fitting wrong one.

Any experts on this subject out there?

NB if you don’t know anything about it please don’t comment. It clogs the thread up with junk.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #2

Roger Bywater is the guy to ask.

(foxy99) #3

Let’s hope he pops in then.

Roger, are you there? :wink:

meantime. here’s a picture of the sprocket end of the shaft, showing date and part no.

(Andrew Waugh) #4

@Roger_Bywater’s last post here on JL was 2004.

(foxy99) #5

wonder if @Kirbert1 is aware of this… :smile:

(Mark Eaton) #6

I do not believe there is a definitive answer. As I understand it, changes have been for refinement and involved changes to opening ramps and possibly a slight increase in acceleration rates where later tappets were lighter. There is also talk of cams with oil holes at the rear of the lobe which were not so quiet. In any event, the following information might provide some clarity …

I think Dick Maury would know and I think he might still be on the list.

Quoted from Roger Bywater … Of course I can’t guarantee any of this information is correct.

"It is never wise to rely too much on figures in manuals.
Circumstances can often cause manufacturers to make running changes that don’t appear in manuals although there may be bulletins to dealers about them. There is nothing unique to Jaguar about that.

The cam timing quoted in the V12 E type manual is slightly different from that quoted later for the HE but there were several detail changes to cam profiles around 1973 into early 1974. From then on the cams remained the same until some further minor changes for the 6 litre. Some early cams were only used for a short period and the part numbers are not found in parts books. The differences were very slight. Significant changes can be made to the acceleration curve of a cam which could be virtually undetectable from basic measurement of the lift curve.

Through the life of the V12, Jaguar used valve clearances in production ranging from 0.010 – 0.012”, 0.012 – 0.014”, 0.014 – 0.016”, but you won’t find anything about the latter in any workshop manual.

There are valid reasons for defining opening periods at cam lift of 0.050”, just as there are for using 0.010” lift , and also for using zero clearance. Using 0.050” gives a reasonable indication of the main lift period but usually misses most of the acceleration/deceleration phases which can make a profound difference. A change in valve clearance of 0.002” can make a measurable difference to performance so none of these is much use really except as a rough guide. "

And also this :

"All changes to V12 cam forms and tappets have been in pursuit of refinement. There were a number of detail changes in the early 1970s ending with C42176/7 which lasted until the 6 litre when further refinements were introduced. The dealer information bulletin about the 6 litre improvements mentions the changes to the cam profile and inlet valve but does not mention any change to the tappets. It may therefore be that the change to a lighter tappet could have been a running change introduced at some other time, possibly as a result of similar changes introduced earlier on the AJ6 engine.

Opening the clearances from 0.012-0.014” range to 0.014-0.016” made measurable improvements to torque, emissions (particulary HC) and specific fuel consumption.
In fact, at one point V12s for the US market were assembled to the wider setting range in order to pass FTP-72 emissions. A higher level of tappet noise had to be accepted for these engines.

The C42176/7 cam form has constant velocity ramps extending to 0.020” lift so any clearance within that range will be collected by the ramp at the same rate of lift.

Because the exhaust clearance closes up with running temperatures it can tolerate wider settings than the inlet without becoming noisy. For most people setting inlets to 0.014” and exhausts at 0.016” would be the best solution."

And also this …

" Walter Hassan’s paper quotes a valve crash speed for the V12 of 7840 r.p.m. and a compressed spring load of about 150 lbs., which suggests that the standard setup was rather over-specified. Early 16CU control units had a fuel cut rev limit just over 7000 which gives some idea of the margin.

In fact the only thing about the V12 valve gear that caused any concern was with regards to noise and refinement and this was largely because of variable side clearance as a consequence of running cast iron tappets in an aluminium carrier. It was a problem the writer spent some time investigating. All changes to tappets and cam profiles were primarily for this reason.

The XK of course had used cast iron tappet guides so did not have the same side clearance expansion issues – but suffered with other problems as we know.

There were a number of changes to the tappets and cams in the first couple of years of production. There were long tappets, some with copper plating, and some with small holes in them, but the standard became the short tappet used from about 1973 onwards. These were sourced from two suppliers during the 1970s at least, these being AE Brico and Clancy.

Brico tappets were plain chill cast whilst the Clancy items were blacked internally. Both were to the same specification, were interchangeable, and were also used in the XK but under a different part number. The next significant change was when the 6 litre was introduced and again the reason was for refinement."

(Mark Eaton) #7

Actual measurements of the CAM were given in 2009 by John Aller. However, I don’t think he quoted the CAM part number that he measured! I guess it was assumed they were all the same.

If you set the lash to zero (or slightly open, insert feeler gage) so you can measure the cam lift with no lash off of the top of the bucket, you probably would set the intake lift at overlap (tdc) to .044''. That is what I did, fast and easy, relatively...... It puts the intake centerline at 111.25 etc., below is seat to seat spec.'s.
            Intake                  Exhaust
         -------------------     -------------------

Centerline 111.25 ATDC 111.25 BTDC
Open 43.27 BTDC 88.33 BBDC
Close 90.05 ABDC 44.42 ATDC
Duration 313.3 Crank Deg. 312.8 Crank De
Area 30.70 Inch Deg. 30.66 Inch Deg
Lash 0.0120’’ 0.0180’’
Peak Cam Lift 0.39217 Inch 0.39159 Inch
Peak Valve Lift 0.38017 Inch 0.37359 Inch
Cam lift @ TDC 0.0444 Inch 0.0430 Inch

The lash settings are what I set mine to as I don’t want to have to adjust them for a while. Not power settings.

Here are the CAM spec.‘s for the stock cams at .050’’:

Checking Height 0.050 Inch
Valve Overlap -3.0 Crank Deg.

     Intake                     Exhaust

Centerline 111.25 ATDC 111.25 BTDC
Open 1.32 ATDC 41.13 BBDC
Close 40.62 ABDC 1.65 BTDC
Duration 219.3 Crank Deg. 219.5 Crank Deg.
Area 29.86 Inch Deg. 29.82 Inch Deg
Lash 0.0120 Inch 0.0180 Inch

(Mark Eaton) #8

Oh well, that formating didn’t work!

(foxy99) #9

Fantastic response @Mark_Eaton

Thank you

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #10

I didn’t know it had been that long, but really I was suggesting contacting him at AJ6 Engineering.

(AndyB) #11

I reckon Roger Bywater has been on JL a lot more recently than 2004 - I hope Roger will chime back in sometime soon - especially now AJ6 engineering’s website has been updated.



(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #12

Y’know, maybe not. The Jaguar V12 can be thought of as two I6 engines, each with its own dedicated camshaft, oxygen sensor, and feedback circuit in the ECU. As long as the camshafts aren’t too different, you’d probably never notice the difference.