Overdrives

Does anyone have the Laycock de Normanville Overdrive Service Manual? I
haven't been able to find a copy and am in desperate need of it. Thanks -
Brian Pel

Brian, I can photocopy one from a Jaguar Mk I shop manual for you, if you
like. As long as you're in no immediate rush for it, in any case. Mike
Plechaty

Some models of Volvo and Triumph also used the Laycock de Normanville
overdrive, and the repair of these is covered pretty well in their Haynes
manuals, which might be found in your local library. The Volvo dealer can
order parts, but you have to guess which model Volvo would use the
equivalent part to the 140. Be careful taking it apart, there are a lot of
strong springs inside all trying to push the sections of the aluminum case
apart. I put one together once and I stood it on end and used the weight of
a car to compress the case sections together so I could get the bolts in. I
would think the reverse of this process would be the best way to take one
apart. Publication J.18 that Roger mentioned has pictures to identify which
tailcase and thus which model the gearbox is intended to fit. Brian, since
yours is an early car and I believe you said the number matched the ID
plate, is there any evidence to support a theory that JLE boxes were not
available until after your car was made, thus explaining why you have a Mk7
JLN box? - Rob Reilly - 120 FHC 679187

George: my car is a very early 140 DHC and has a gearbox with a JLN suffix
(which leads me to believe it is a Mark 7 gearbox) The overdrive unit looks
to be identical to the Laycock unit in the Mark 2. The 140 supplement to my
Mark VII manual references a separate service manual for the overdrive unit
which I am hoping will give the information I need. Thanks.  - Brian Pel

There are a number of relevant publications that cover Overdrive unit.  I
do not have a specific Jaguar Workshop Manual covering just Overdrive, as
referred to in XK140 Supplement, although it most likely was issued prior
to this detail being included in overall Workshop Manual issued for (mark
1), Mark 2, S-Type, 3.8 Mark 10. Similarly there was a specific seperate
Manual for Automatic Transmission issued in 1950's.  As above, the easiest
source of Workshop Manual detail for virtually identical OVERDRIVE unit as
fitted to XK140 is Section F of 2.4 and 3.4 SERVICE MANUAL (publication no.
E/120), or in Mark 2 SERVICE MANUAL E/125, or 3.8 Mark 10 SERVICE MANUAL
E/125, or S-Type SERVICE MANUAL E/133. Jaguar SPARE PARTS MANUAL #J.18,
covers all aspects of OVERDRIVE fitment, including Unit itself, but also
linkages, relays etc, for Mark 7, XK140 and 2.4. (I have a spare copy of
this if anyone wants it - mint original pages, but no card covers - $20 +
post)  I also have an "Operating and Maintenance Handbook for the Laycock
de Normanville Overdrive. As fitted to the JAGUAR. X.K.140" which is an 8
page booklet covering basic 'how it operates' and 'how to use it'  And then
also very accurate and useful are the LUCAS publications:  * ELECTRICAL
EQUIPMENT FOR LAYCOCK de NORMANVILLE OVERDRIVE p.1 - 12 Technical Service:
Electrical Equipment (Sept 1954) p.13 - 18 Electrical Equipment and Spare
Parts and       * SECTION S, out of the LUCAS Master Catalogue No.400E (and
other editions), covers very detailed Spare Parts/ photos of Overdrive for
XK140/150. - Roger Payne

Some models of Volvo and Triumph also used the Laycock de Normanville
overdrive, and the repair of these is covered pretty well in their Haynes
manuals - Rob Reilly

Old Ferrari's (250 GTE & 330 America) also used this unit, but info is
sketchy at best from these models. I doubt the modern Ferrari dealers would
even have a clue about overdrives.... - Mike Plechaty

Believe it or not, the more humble marques of Sunbeam Alpine and Rapier
also used the Laycock overdrive unit. As I recall, this unit is described
in detail in the Haynes, Clymer and Rootes repair manuals for these
vehicles. - William C. Burke

I am moving my XK120 to Florida this summer and also would like specific
info as to what kind of electric fan to install?? Is it worthwhile to
completely remove the mechanical fan and just use an electric one?? Second
question: Is there an overdrive unit that can be retrofitted?? Sure would
be nice to cruise at a lower RPM. - Steve McDonald

To fit the Laycock overdrive unit on a standard Mossbox you would need to
change the mainshaft (that's the output shaft of the main gearbox case)
because the OD one is longer. The OD unit won't fit in the 120 chassis as
is, you would have to chop out a notch in the crossbrace and finagle some
kind of rear rubber mounting that holds it firm, or use bell housing mounts
like a 140 or Mk7. Being one for originality I vote against it. - Rob Reilly

Hi, Steve - Depending on the year, a Laycock de Normanville overdrive unit
may/might be retrofitted to your transmission, but a complete overdrive
transmission would be a 'better' approach. You'd also need a different
(shorter) drive shaft. However, the XK-120 has a frame cross-brace behind
the trans for the rear trans mount, and this brace would interfere with the
overdrive transmission. If the car's anywhere near 'nice', this
modification, while satisfying to the current owner, could materially
affect its market value downstream. A better solution might be the fittment
of a different rear axle with a lower numerical ratio. I believe as low as
2.93:1 were available for the XK-120 (lower yet for the XK-140, bt that may
be a different rear, depending on the year). A lot depends on what you have
on the car now; if it's the later type, a small embossed matal tag on the
differential housing should give you the number of teeth on the ring and
pinion gears; divide them to determine the ratio. It's a good goal. Good
luck! Where in Florida are you moving to? I spent seven happy years in
Satellite Beach (Melbourne), just south of Kennedy Space Center (of "I
Dream of Jeannie" fame!).  - Larry Schear, Twin Cam, Inc.

If you have any questions or comments send e-mail to: ted@jag-lovers.org
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