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The Experimental Jaguar Mk10s

(The following findings are very much a “work in progress” and work is continuing in trying to fill the gaps – all contributions gratefully received!)

Mk10, Jaguar, Mk10 Jaguar, Building The Legend, tera, quad-cam, V12

Mk10, Jaguar, Mk10 Jaguar, Building The Legend, tera, quad-cam, V12

The Mk10 Jaguar was introduced in 1961 as a replacement for the MkIX – itself following on from the MkVII and MkVIII. The Mk10’s design was light-years ahead of the “stately” MkIX and made the most of Jaguar’s 6-cylinder 3.8 XK engine with its body construction and independent rear suspension. However, despite its relatively low £2,400 cost at launch in 1961 compared to its limited competition, it was destined to never become the success that Jaguar anticipated. It had all the trademark “accroutements” such as plenty of chrome, svelte styling (for its size), the famous leaper mascot, excellent performance and “space, grace and comfort” by the bucket-load. However, it never really succeeded in its target US market because the Americans were looking for something typically “British” in the mould of the preceding cars and the Mk10, perhaps, didn’t deliver that.

It did, however, offer Jaguar a tremendously accommodating vehicle for testing of all sorts. During the late 1950s/early 1960s Lyons was looking for a new powerplant to power his new range of performance saloons. The Mk10 fitted the bill as a testing mule for his new designs perfectly.

Mk10, Jaguar, Mk10 Jaguar, Building The Legend, tera, quad-cam, V12

The cachet, association of “luxury” and promised effortless performance of a V12 was felt to meet the requirements of Lyons’ target US market. The fundamental configuration of a V12 provides perfect primary and secondary balance no matter which V angle is used and therefore needs no balance shafts endowing the V12 with great refinement – just what is needed in a luxury car. Also, because Lyons had a return to racing in mind, the fact that a V12’s rotating parts can be made much lighter and therefore more responsive, made it an ideal base for a racing engine. A large-capacity and torquey V12 also has another advantage in that it can run slower than smaller engines, providing greater engine longevity. The fact that the engine can achieve the same level of performance as its lower-capacity cousins at less frenetic engine revs contributes greatly to its suitability as a “luxury” powerplant.

After its launch in 1961, a small number of Mk10s were earmarked by the factory as “testing mules” for a range of duties. These duties included the road testing of Jaguar’s new powerplants – including their legendary V12. Some of these cars were given the designation “XJ5” (to eventually become the Mk10 replacement – the 420G) and, as far as I am able to piece together so far, consisted of the following:

XJ5/1    Green

XJ5/2    Grey - Reg 6100 RW – Mr Knight - Engine ZA2305-9

XJ5/3    Maroon – LHD – Reg GDU 333D - Body 4D50154

XJ5/4    White - Automatic

XJ5/5    Golden Sand – Reg AWK 134B - Manual

XJ5/6    “LUCAS PI”

The records also contain references to other test Mk10s (which may also refer to the above cars at different times in their development):

Body 4D50067 – Production Exp.

Body 4D50251 – Production Exp.

Reg AHP 304B – Sir W Lyons

Reg 7910 RW - BLUE

Reg 1196 KV – MAROON

Reg 5437 RW – “Test”

Reg QH 3241 – Mr Luttens

Engine ZA5601-8 – Lightweight Body Panels

Reg HWR 760C – Mr Butler

Reg GYM 345C Shell Car

The man largely responsible for the design and development of Jaguar’s engine range was the late Claude Baily - the main architect of Jaguar's quad-cam V12, their legendary XK engine and quad-cam 90° V8 military engine. The Mk10, of course, provided a wonderfully capacious test-bed for his DOHC designs from 1961 onwards until after Walter Hassan joined the company in 1965 and used the car to test his SOHC developments of the V12 engine.

In 1960, Baily was proposing the following Jaguar engine range and the experimental Mk10s played a significant role in the testing and development of his ideas:

Proposed 6-cylinder engines:

  • 83.0 x 76.5 mmstroke                 2.4 litre
  • 87.0 x 76.5                                    2.7
  • 87.0 x 87.0                                    3.1
  • 87.0 x 106.0                                  3.8

Proposed 12-cylinder engines:

  • 83.0 x 76.5 mmstroke                 4.9 litre
  • 87.0 x 76.5                                    5.4
  • 87.0 x 87.0                                    6.2
  • 87.0 x 106.0                                  7.6

Mk10, Jaguar, Mk10 Jaguar, Building The Legend, tera, quad-cam, V12

Of these cars, I shall concentrate on two of them for now – XJ5/4 (an automatic white car) and XJ5/5 (a manual golden-sand car). These two cars featured prominently in the testing of Claud Baily’s quad-cam V12 engine which, as well as hopefully bringing Jaguar success in return to Le Mans, would also form the basis of the luxury V12 Lyons was hoping for.

The quad-cam V12 engine project was given the code "XJ6" - not to be confused with the saloon of the same name. Two Mk.10 cars (XJ5/4 and XJ5/5) were to become mules for the production variant of the "XJ6" racing engine. These two cars had already led chequered lives testing out various powerplants including enlarged versions of the 3.8 XK engine. The following excerpts give an indication of the “day-to-day” life of these cars:

The engine as installed in XJ5/4 and XJ5/5 development Mk10s – a big engine for a big car!

XJ5/4 (White)

As well as being a “testing mule” for the quad-cam V12, this car saw duty as a mule for development engines E5054-8, 7D50010-8, 7D50003-8 and E5043-8.5. Close to the start of its life as a test car, the following entries were recorded:

  • 9-4-64     Speedo 1,730. Engine number 7D50003-8 production is same number as experimental built engine in XJ5 No.2. Valve timing altered at Mr Knight’s request to inlet opening 12 degrees BTDC exhaust showing 16 degrees ATDC”
  • 10-4-64  E5043-8.5 4.2 litre engine from 7910 RW fitted”
  • 24-4-64  Speedo 2,353 … engine E5043-8.5 removed, damaged beyond repair … 7D50010-8 production built 4.2 engine fitted
  • 5-8-64     7D50010-8 engine removed … E5054-8 4.2 test engine with Mahle pistons fitted “
  • 30-8-65  engine removed and stored in Experimental fitting shop … car to be prepared for V12 engine installation”
  • 25-3-66  No.2 V12 engine handed to Mr Weaver for installation in car”

By all accounts, this resulted in a nose-heavy, badly-handling car. To make matters worse, the width of the quad-cam racing engine meant that full steering lock couldn’t be attained. However, in a straight line on the M1 close to Newport Pagnell, development Aston Martins were given quite a fright when this exceedingly rapid imposing car wafted past!

  • 4-12-67  engine removed from car”
  • 29-4-68  No2/10 V12 Heron (SOHC) engine fitted in car”
  • And the final entry in the log book for this hard-worked car:
  • 18-6-68  Speedo 13,435 No2/10 engine removed from car”

What happened to this car afterwards? Has it survived? Was it scrapped (most likely)? ….

XJ5/5 (Golden Sand)

  • 15.4.65XJ6 No.2 12 cylinder engine installed in car”
  • 6.10.65   No1 V8 engine handed to Mr Weaver for installation in place of No.2 V12”
  • (The V8 engine referred to above is actually one of the quad-cam V12 engines internally modified to run as a V8)
  • “10.9.66   No9 V12 HERON (SOHC) engine fitted”

The above is only a small selection of entries made in the car logs which have, thankfully, survived. Logs for the other cars have also survived and form a valuable source of information for Jaguar historians/researchers. 

As previously mentioned, this study into the history of the development Mk10s is very much in its infancy and research is continuing. If any readers have any recollections or information to share it will be gratefully received and included in this story of part of the history of these wonderful, imposing and stately cars.

The author can be contacted by email at neville@buildingthelegend.co.uk

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