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."