The ratios were changed simply by increasing the number of teeth on the primary (input) pinion gear from 25 to 28 teeth, and correspondingly reducing the number of teeth on the mating layshaft gear from 39 to 37 teeth. The effect is to make the indirect gears higher and the gears close. In Jaguar terms, the high ratio gearbox is termed the "close ratio" gearbox. So, in interchanging parts, it is only necessary to ensure that the two parts match.

2. Provision of a stop peg on 2nd gear synchrohub, and corresponding relieved tooth on 1st gear/synchro sleeve. This positively prevents the sleeve moving too far and going over centre of the synchro balls. If the peg type synchro hub is used, then the corresponding sleeve/1st gear must also be used. (The later sleeve can, of course, be used with the earlier hub.) The peg can easily be seen on one of the outside  flats of the hub.

3. Provision of 3rd/top synchro hub interlock plungers (recognizable by having two holes through the synchro hub) with corresponding recesses on the mainshaft and sleeve. This positively prevents the hub moving too far in either direction. The parts sre interchangeable but the plungers can only be used if the later mainshaft sleeve and hub are all fitted.

4. Improved oil seal speedo "driven" gear and bush. The earlier gear and bush car only be used together as can only the later driven gear and bush be used together. (Both types require different speedo drive cables too.)

5.  Synchro hubs - further modifications. Ear1y hubs had 36 splines and later ones have six sets of four splines with "flats" between them. Parts are interchangeable.

6. Morgan gearbox rear plates later had a reinforcing web added to the left mount lug.

7. Later gearboxes had an oil shield fitted immediately behind the rear mainshaft bearing to prevent oil flooding the speedo drive and oil seals.

8. Later gearboxes incorporate a reversing light switch in the top cover.

Now, on to the Jaguar gearboxes. There are basica11y three types of Moss-type gearboxes and these can be identified by the prefix and suffix to the gearbox number. This number is to be found on the top cover, the casing side, and also on the engine compartment identity plate.

"N" added to the prefix indicates overdrive, and therefore a long mainshaft. (Any mainshaft longer than Morgan length is very apparent. No suffix, or "O" indicates the gearbox is a low ratio type. A "CR", "KS", or "J" prefix indicates a high ratio type. Other letters identify the gearbox as one of three types listed below (see also table).

1. MOSS BUILT (both low and high ratio)

Other than the obvious differences already mentioned, these gearboxes are identical with the Morgan versions. They mainly have number prefixes: "SL) (the mainshaft is identical with the Morgan one) or "SH", which has a short rear extension, and therefore a longer mainshatt. A "SLN" prefix also indicates a longer rnainsbaft, or, of course, overdrive. You may come across some similar versions with an "OSL" or "M" prefix. These are also the same as the Morgan gearbox and have the same mainshaft and are all high ratio.

The "SH" type gearbox is to be found on early XK120, Mark 5, 6 and early Mark 7 Jaguars.

The "SL" type gearbox is to be found on later XK120, 140 and later Mark 7 Jaguars.

The "OSL" type gearbox is to be found on Mark 8 and 9 Jaguars and is a high ratio gearbox. The
"M" type gearbox is to be found on XK150 Jaguars and is also a high ratio gearbox. (With XK150s, the letter "S"  added to the prefix is often used to denote overdrive.)

2. JAGUAR BUILT (Both low and high ratio)

These gearboxes are also found on Mark 5-9 and XK120-150 Jaguars (suffix "JL"or "JH") and early
Mark 1,2, or 3.4 Jaguars, suffix "GB" (but not all "GB" gearboxes - see later).

In addition to the previous1y mentioned differences, there are seven parts that are Jaguar-made, and consequently have different part numbers. The difference in some of these parts is merely one of wider tolerances and selective grading (e.g. 3rd top sleeve has part number C4049 or C4049/1, according to grading) but this should not be a significant difference when rebuilding a used gearbox. The real  differences are:

a) One piece (i.e. combined) primary shaft and gear. Hence these gearboxes will not provide a primary gear for Morgan use.

b) Built up (from 4 separate parts) layshaft. Only as a unit can this be used to replace the one-piece Moss Morgan cluster gear.

c) The two synchro hubs and corresponding mainshaft splines are different from the Morgan parts. These three parts can be used together to replace the Moss parts. A mixture of Moss and Jaguar made synchro hubs and mainshaft must not be used. The only mainshart from this type of gearbox that will fit into a Morgan is the JL non-overdrive version, the others being too long.

The "JH" type gearbox is to be found on early XK120, Mark 5, 6 and early Mark 7 Jaguars.

The "JL" type gearbox is to be found on later XK120, XK140 and XK150, later Mark 7, 8 and 9
Jaguars.

3. JAGUAR JS TYPE
This type of gearbox is to be ftound on later Mark 1, Jaguar 2.4 and 3.4 all Mark II and.sorne XK150 Jaguars.

Do not be deceived by this gearbox. Although both inside and outside it looks like a Moss gearbox, it is completely different from the other Moss/Jaguar gearboxes,. The teeth profile is different and the ratios are different. This type of gearbox is easily identified by the  suffix "JS" after the gearbox number. So avoid all gearboxes with this "JS" suffix after a GB prefix (on Mark I and II) or a "JL" prefix (on XK150).

The Jaguar prefix really denotes the gearbox "application" (i.e. the position of tbe mounts,  lever, rear extension, etc.) Avoid also all gearboxes with "JC", "ES", "EJ" prefixes as these are all-synchro gearboxes and even the casings are different fromthe Moss types.

These tables ahould assist with your hunting for gearboxes. I made my spare gearbox from a mixture of JL and SLN gearboxes for a cost of $15 plus new bearings, seals, primary shaft and end plates. So hunting can be worthwhile!

JAGUAR GEARBOXES
 

Recommended Fluid for Moss Boxes

Moss boxes will never be the smoothest of transmission. The latest recommendation for improvement is the use of Redline or Amsoil MTF. The newer fluids are excellent for manual transmissions while being less temperature sensitive and balky than the thicker, vintage fluids. That being said, the original recommendation was for a 30 weight fluid (wt) , essentially motor oil (any oil designation ending with a "30" is a 30 weight oil. 

MOSS BOX NOISES

Whining and grinding noises point to different problems.

If the box seems to winge while stationary in neutral with your foot off the clutch pedal, a good guess would be that the large bearing on the input shaft is worn.

If the box grinds going into first and reverse, the guess would be the clutch is not releasing as much as it should. Check that first.You could also check the posiblity of a finished throwout bearing (clutch).

If you decide the clutch is not the problem, then it sounds like your box may need its synchros lapped and get new bearings. You could alsol suspect the thrust washers and shims on the main and constant pinion shafts as the synchro sleeves do push these fore and aft when the shift lever is being moved. You could diagnose this by removing the interior to get at the gearbox cover and checking the endplay of the shafts while the box is still in the car. If things are worn enough to produce that kind of noise, you would probably even be able to see some damage. You could also try fishing around in the bottom of the box with a small magnet to pick up pieces of demolished parts to aid in the diagnosis. 

Different noises mean different things. Always try to identifying under what conditions it makes certain noises as that  will help to pinpoint the problem. 

Morgan Moss Gearbox Capacity