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Rob Reilly's Moss Gearbox Identification

Moss Gearbox Identification, Prefixes & Suffixes

Car Model Gearbox Type
Pre-war SS, early MK IV double helical gears
Later Mark IV, MK V, early MK VII, early XK120, C-type SH and JH
Later MK VII & XK120, all XK140, XK150, MK VIII and MK IX SL and JL
MK 1, early MK 2 and S-type GB
MK 1, MK 2 overdrive GBN
D-type GBD
Later MK 2 and S-type JC
3.8 MK X and E-type EB
4.2 MK X and E-type EJ
XK140/150 overdrive JLE
late XK120, XK140, MK VII OSL
MK VII overdrive JLN
S-type JBN
XK150S overdrive JLS

Any more?

The serial number is on the top cover, stamped around a large circular ring called a core plug. Also compare with the same serial number which should be stamped on the left hand side of the main cast iron case at the top rear corner. Note the speedometer drive orientation, tailcase mounts (if any), which may indicate whether it is out of a saloon or a sports model.


Letter N, E, or S at end of prefix letters indicate a Laycock-DeNormanville overdrive is fitted.

Suffix letters CR, MS or J after the serial number indicate close ratio gears.

Suffix letters JS after the serial number indicate shaved gears.

Some SL and OSL gearboxes have been reported with suffix A or B, no explanation yet.


Gear Ratios 1st, 2nd, 3rd, top:
Double helical gears - 3.95, 2.43, 1.45, 1
Suffix O or no suffix letters - 3.375, 1.982, 1.367, 1
Suffix CR, MS or J - 2.980, 1.740, 1.210, 1
Suffix JS - 3.378, 1.860, 1.283, 1
First and reverse are always equal.


The gearboxes used in all Jaguars up until 1965 were originally designed and built by Moss Gear of Birmingham for Standard, though beginning in 1948 the actual assembly work was gradually taken on by Jaguar, and the Moss Gear Co. seems to have disappeared by the late 1950’s. Some early double helical boxes are labeled Standard. The prefix S is believed to indicate assembly by Moss, prefix J for assembly by Jaguar. SH may possibly mean Single Helical. All are non-synchromesh on first gear. The fully synchromesh boxes after 1965 made by Jaguar are not part of this discussion.

Early XK120 with SH or JH has a flat mounting shelf on the chassis crossmember, and uses the big rectangular rubber mount and shorter driveshaft.

Later XK120 with SL or JL or OSL uses a longer driveshaft, and has a vee shaped mounting shelf on the chassis cross member, with a sheet metal adapter piece and two round rubber bobbin mounts.

SH vs. JH - The difference is in the internal parts, the SH having a cluster countershaft, while the JH has individual gears splined onto the countershaft. According to the manual the two boxes are interchangeable with each other for early XK120 and MK VII.

The SH and JH for a Mark V have a different input shaft from those of the XK120 and Mark VII, possibly because of the pushrod engine, so may or may not be interchangeable.

The SH or JH box in an early XK120 is not interchangeable with SL or JL boxes, unless you change the mounting pieces and driveshaft as well. However, it IS interchangeable with early MK VII SH and JH boxes. This is up to chassis 660934, 671796, 669002, and 679214.

The SH or JH in a Mark V uses the same large rectangular rubber mount as for early XK120.

The SH or JH in a Mark VII has the flat mounting provision on the gearbox tailcase, but does not use it, instead it has rear mounts on the bell housing.

To prevent a problem of getting stuck in first gear, a stop pin was added to the second synchro sleeve on JL, SL and all subsequent gearboxes. The stop pin was added beginning with serial no. JL13834 and SL6313A, announced in December 1952, Service Bulletin SB 116. This stop pin can be added to earlier boxes.

SL vs. JL - The difference is in the internal parts, the SL having a cluster countershaft, while the JL has individual gears splined onto the countershaft. According to the manual the two boxes are interchangeable with each other for late XK120, however XK140 and XK150 and saloon boxes have different tailcases without a rear mounting, so they will not work for XK120.

OSL is an update of the SL box, with the only difference being different shifter forks.

JL, SL and OSL boxes with a tailcase mounting are for XK120. JL, SL and OSL boxes without a tailcase mounting are for Mark VII and XK140/150.

JLN is for MK VII with overdrive. The tailcase has no mountings at all. The speedometer drive is on top, angling off to the upper left, with a centrifugal governor attached. There is a spacing part between the top cover and the maincase, which raises the top cover up about an inch. The top cover extends rearwards over the Laycock unit. The operating solenoid is vertical.

JLE is for XK140 with overdrive. The tailcase has a horizontal mounting surface on the underside with no bolt holes. The triangular rubber buffer under this is shimmed up to the required height. The speedometer drive is on top angling off to the upper left and has a small right angle drive attached. It has the same top cover and spacer between as the JLN, and the same vertical operating solenoid. XK140 FHC has a different shift lever than 140 OTS and DHC, and both levers are different from MK VII.

GBN is for 2.4 litre saloon with overdrive (and possibly 3.4 and MK 2 as well, but the reference doesn't say). The tailcase has a tab mounting sticking down with two bolt holes. The speedometer drive is on the bottom, angling down and to the right. The top cover has no spacer and does not extend back, the shifter is directly above the maincase.

GB is for 2.4, 3.4 Mk1 and Mk2 without overdrive. The tailcase is incredibly long, equal in length to the Laycock unit, with the same tab mounting as GBN. Same top cover as GBN, but I can't see the speedo drive in my picture. The driveshaft connection is a splined tailshaft rather than a 4-bolt flange.

Note to the XK list on shaved gears from Klaus Nielsen 8-97:

Gear finishing is carried out by a fine machining process called "Shaving" if the teeth are in a soft condition i.e. before hardening by
heat treatment, and by "Grinding" if the gears are finished after hardening.
There are several approaches to this process, aptly named parallel, transverse, diagonal or plunge shaving. All employ an axial feed motion
relative to the gear blank cylinder. Their common characteristic is that
the shaving tool cuts extremely small chips away from the work surface
in a sliding motion, thus generating the desired smooth surface along
the correct geometric path.
In either case, whether shaving or hardening, the purpose of the fine
machining step is to eliminate the unavoidable surface irregularities
left on the gear blanks by the coarser shaping methods, be they milling,
shaping or hobbing.
Removal by shaving of these small geometric deviations from the ideal tooth profile also reduces distortion due to hardening by initial correction of the mating gear surfaces.
The sum effect of shaving improves the running, the noise generation
characteristics and the stress behaviour of the gears,eg. the quality of
the end product.
Why the "Shaving " process is occasionally associated with gear box
ratios escapes me. Gear ratios depend, not on the degree of finish, but
solely upon the number of teeth in each gear in the pair in question.
Baggage from the Lyons era....perchance?
Klaus Nielsen


The MK VII/XK120 manual and most of the parts catalogues have good pictures of some Moss gearboxes.

Use SAE 30 or 10W30 motor oil, not SAE 90 gear oil.
The 30 viscosity is important to get the synchros to work properly.

A comment on putting XJ6 standard gearboxes in XK140/150. Some UK based modifiers do this, as stick shift XJ6’s are common there.
For XK120 the cross member has to be cut away and re-configured, so being one for originality I cast a vote against that.

"Jaguar Cars" by C.L Vandiest c1961
"Jaguar Saloon Cars" by Paul Skilleter c1980
“Original Jaguar XK” by Philip Porter c1990 & 1999
Service Bulletin SB 28, Jun 1948
Service Bulletin SB104, Feb 1952
Service Bulletin SB112, Oct 1952
Service Bulletin SB116, Dec 1952
Service Bulletin SB133, Sep 1953
Service Bulletin SB140, Jan 1954
Service Bulletin SB145, Mar 1954
Service Bulletin SB95B, Apr 1953
Service Bulletin SB163, Feb 1955
Service Bulletin SB179, Jan 1956
Service Bulletin SB196, Sep 1956
Service Bulletin SB214, May 1957
unnumbered Service Bulletin from JCNA Sep 1957

Send additions and corrections of the text to Rob Reilly

We need photos of example gearboxes; right rear and left rear angle shots are best.
Send photos to Ted Uiterwyk


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