Note that my repaired gearbox has not been road-tested at the time of writing (August 2019), but I wanted to make my findings so far, available to you.
The main repair covered here is fixing the synchromesh. The Moss box has a bad reputation for this, and many think you have to live with the issue. My grateful thanks to SS Jaguar owner Peter Scott for tipping me off that it can be repaired quite easily. Easily, except for the small matter of dismantling and reassembling the entire box!
From the serial number, my gearbox is a type JB, which differs from those I've seen listed for various Jaguar models that also used Moss boxes. I noticed that there's an AC part number on the top cover.
As always, I read the AC handbook, had a good laugh, and then worked out for myself how to do this!
Firstly, dismantle the clutch release mechanism, by releasing the 2 sections of shaft. Then you have access to the bolts holding on the bell-housing. You have to apply an open-ended spanner at an angle to some of those bolts, as a socket can't get in there.
Unbolt the top cover and the complete selector mechanism lifts out and lets you gasp at what a wonderful work of sculpture those gears are. At the rear of the box, remove the speedometer drive shaft/gear by removing the locking bolt. Remove the speedo gear casing, gear and washer.
Unbolt the rear extension casing and remove the locking plate for the layshaft and reverse shaft. The casing will require a bit of prising, because it brings the rear bearing off the main shaft. Slide the distance tubing off the main shaft.
The top row of gears can't be released until the layshaft is dropped down out of the way. So, remove the layshaft spindle. Next, the primary shaft (front shaft) has to come out. This is where it gets interesting! Remove the seal holder, which covers the bearing. The handbook recommends a special extractor tool for the bearing or else hammering from the inside, destroying the oil-thrower in the process. The "Ian" method is to carefully remove the external circlip from the bearing. This clip is similar to a piston-ring, but normal circlip pliers are effective for its removal. Then use the circlip groove to lever out the bearing, bringing the primary shaft, gear and synchro cone. There is a needle-roller bearing (in a cage) between this shaft and the main shaft.
To get the main shaft out, use a soft mallet to knock the shaft rearwards until the main bearing comes out of the case. Be careful not to use too much force, as a ball-race bearing can be damaged easily. Once free from the casing, persuade the bearing to come off the main shaft. It can be prised away from the shoulder on the shaft. Once the bearing is off, the shaft - complete with gears and synchros - can be lifted out through the top of the gearbox. Be careful when removing the 1st gear/2nd synchro unit, because there is a peg and ball that will drop out from the inside.
The layshaft can then be lifted out. Be careful not to drop any of the needle-rollers from inside the ends of the shaft. Also, note the order of assembly for the thrust washers at each end. The reverse gear can remain unless it needs any attention. If you need to renew the needle rollers, the size is 23.7mm long and 3.0mm diameter, and there are 58 of them.
The synchromesh units have inner and outer components that slide against each other. 6 spring-loaded ball bearings inside, hold them in place until you apply sufficient force to make them move. The intensity of this force, helps to set how effective the synchronising of the gears is. It also sets how much effort you have to apply to the gear-lever, so you don't want an excessive load. Data from the Jaguar Mark V gearbox, suggests a load of 42-45 pounds for 3rd and 4th gears, and 62-65 pounds for 2nd gear.
I tested mine by resting them upon bathroom scales, and placing a small piece of wood packing under the synchro unit. The thickness of the packing needs to ensure that you don't slide the unit too far and have all the ball-bearings and springs fly out! The measurements of mine were: 4th gear 45 pounds, 3rd gear, 40 pounds, and 2nd gear 47 pounds. My 3rd and 4th were consistently different despite being on the same set of springs. The dimensions of the springs are: Mean diameter=6.2mm, outside diameter 7.45mm, wire diameter=1.25mm, number of free turns=5.5. The height of my worn springs was 16.6mm, and I calculate the correct height to be 17.0mm. The ball-bearings are 7.9mm diameter.
Classic Jaguar parts suppliers might have some parts, including the springs, but check that they are of the correct size, since there are several versions of this gearbox.
Alternatively, you can shim the springs by inserting a 4BA washer into some/all of the spring holes. I found that I only needed to shim 3 out of the 6 springs (on both synchros) - shimming alternate springs, to maintain balance. This increased the measured loadings to: 4th gear 48 pounds, 3rd gear 44 pounds, and 2nd gear 62 pounds.
Dismantling the synchros needs some care. Wrap them in a strong, clear bag, to catch the springs and balls. I read a few techniques for reassembly that either did not work, or were too complex. My method for the 3rd/4th synchro was to lie the parts on a bench and insert the springs and balls. Then I used the large ring washer from the primary shaft bearing, which was just the right size to help retain the balls while you wrestle with them. The inner part on my synchro tilted just enough to be able to push each ball in, in turn, while pressing it together. This was done with it all wrapped in the plastic bag. This technique did not work on the 2nd gear synchro. So, I placed it in a soft bench vice (wrapped in the bag), and pushed each ball in while gradually tightening the vice. Oh yes... one important thing: The 2nd gear synchro inner and outer parts need to have the inter-lock hole aligned with the shorter internal tooth on the outer section. That ball and peg mentioned earlier, is the inter-lock, and it has to locate against that shortened tooth.
Above is the 1st gear with the 2nd gear synchro inside. Also, the peg and ball for the inter-lock.
The second part of the repair is to remove the smooth glazed surface from the synchro-cones. Put the synchro cone-clutches together and feel how much friction there is when you turn them against each other. Then apply some fine valve-grinding paste and work them against each other for a few minutes. Then thoroughly clean away every trace of paste. Test the cones for friction and you should find that the friction has increased significantly.
Seals and bearings
My bearings were in good order and I refitted them. They are all ball-race bearings with open cages for oil lubrication. Note that the primary and main bearings have a groove each for the external circlips. Their dimensions are as follows:
The front seal on my gearbox was probably incorrect in size and type. The shaft is metric (40.0mm dia.), although the housing is 60.4mm dia. So, I used a seal with OD=60.0mm, ID=40.0mm and thickness-10.0mm, and used sealant to keep it secure in its holder. This was helped by using modern nitrile seals with the 'rubber' also covering the metal case, for a firmer fit. The rear seal was imperial in size: OD=68.4mm (2 11/16"), ID=44.4mm (1.75"), thickness=12.7mm (0.5").
Lubricating all the bits means obtaining some monograde engine oil, normally SAE 30 unless you are in a sweltering hot climate that requires SAE 40.
This gearbox is lightly stressed when used on the AC, and so wear and damage is unlikely to be an issue, as long as it has been looked after. A few parts might be available from classic Jaguar suppliers, such as thrust washers and layshaft spindle, if there is some wear detected.
The main challenge here, is that the layshaft cannot be fitted onto its spindle until the top row of gears and shafts are in place. Worse still, the layshaft has all those needle-rollers inside, plus various thrust washers to hold in place and thread the spindle through.
My solution to the above challenge was to make a dummy spindle for the layshaft from aluminium tubing. The actual spindle is a metric size, 25.0mm diameter. I suspected the tubing I ordered might turn out to be imperial, and it was. 1 inch or 25.4mm. Using sluminium meant I could file it down. I took it down to about 24.8mm to make it an easy sliding fit. I cut its length to just squeeze into the gearbox casing, but long enough to hold the thrust washers. Grease should be used to hold the needle-rollers and the washers in place in/on the layshaft. I inserted the dummy spindle and did a test run to install the layshaft, sliding the spindle through, pushing out the dummy.
Place the layshaft assembly, with its dummy spindle, at the bottom of the gearbox casing, aligning the front washer's peg towards the top. Slide the synchro units onto the main shaft. Remember to fit the ball and peg for the interlock in the 2nd gear synchro (the peg sits against the shaft, with the ball on top of it). Insert the assembled main-shaft through the top of the box and through the rear hole. Next, the main bearing goes onto the shaft. A drift applied to the inner race of the bearing, tapped with a soft mallet, should do it. Then gently tap the bearing into the casing.
Next, refit the primary shaft. If the bearing still has its external circlip fitted, you will need to make and fit the front paper gasket before this shaft goes in (and don't forget that there's a spacer ring between the circlip and the casing). And also, don't forget the needle-roller cage bearing to fit over the end of the main-shaft.
Now it's time to lift the layshaft up and get its spindle in. Lever up the layshaft with a large screw-driver or rod, and then carefully insert the spindle, making sure that the dummy spindle keeps in contact with it until the spindle is supporting all the fiddly bits. Hopefully, all the gears will turn freely. If the main bearing is not fully against the shaft shoulder, then you can get it to slide into place by temporarily reassembling the entire rear end and using the drive-flange nut to force the bearing along. Before testing the gear selection, bolt on the rear extension casing and the front seal housing. These will stop the bearings (and shafts) moving.
New paper gaskets can be made from thin (1/64" or 0.4mm) oil resistant gasket paper. Don't forget to fit the red fibre disc over the front end of the layshaft spindle. The front gasket needs to be fitted before the primary bearing's spacer and circlip are refitted, followed by the seal housing. I cleaned metal surfaces with methylated spirits before applying gasket sealant. Pay special attention to the lower regions where leaks may occur. In fact, I would recommend cleaning and applying sealant to the rear ends of the layshaft and reverse spindles. The last thing I want is an old British car that drops oil on the drive - giving Jay Leno more ammunition for British car jokes :)
On the clutch release mechanism, the carbon thrust washer on my AC had plenty of wear left in it. Presumably a new one can be mounted if required, or you might track down a complete replacement (I've seen them come up on ebay).
The rest of the reassembly is straight forward, with new paper gaskets for the top cover and the speedometer gear-casing. The large nut for the rear drive-flange was not very tight, when I originally removed it. Fit a new split-pin.