Having acquired a 4 speed non o/d gearbox, I was able to dismantle it. It was purely for interest and to "see how it works". It turned out to be in excellent condition, so I did not remove the various components from the shafts.. The key difference between this and an o/d box is the mainshaft, plus a few bits and pieces. O/D mainshafts are NLA (as are some parts) so it may prove more economical to source a recon box.

The workshop manual and Haynes are fairly good, but the following may help also. The "books" also give information as to clearances and checking for wear. Note that Haynes seems preferable to the Leyland WSM as the latter misses some important things out, such as how to deal with the pump cam on OD 3rd-motion shafts!

You will need to remove the "big nut" at the end on the mainshaft. It is probably worth doing this right at the start. Not needed for OD.

You need a piece of 6mm angle iron, about 2 foot long, with a suitable sized "bite" out of one end and two holes. The bite is large enough to fit the 1 5/16" AF socket and the holes line up with 2 adjacent bolts on the flange. I was lucky in that when bolted up, the angle iron rested against the leg of the bench, so I tied it to the leg and used a 2 ft breaker bar to undo the nut. It might be easier to leave the gearbox on the floor to allow it's own weight to help with undoing the nut:

Gearbox input shaft - release bearing inadvertently rotated through 180 degrees:

Front plate removed:

Check for shims in front plate:

What lies inside (behind the side cover): 'A' 3rd (left) and 2nd (right) gears on mainshaft; 'B' Laygear; 'C' reverse idler gear shaft; 'D1' driven gear on reverse idler; 'D2' driving gear on reverse idler; 'E' 4/3 gear selector fork; 'F' 2/1 gear selector fork; 'G' reverse gear selector fork:

I found removing the interlock plate (under the remote extension on top) impossible until I moved the gear selector lever out of the way by selecting third (I think) gear (Haynes says 4th gear which moves it out of the interlock, see here):

It's a fiddle and perseverance is needed! I used a socket on the end of an extension as a makeshift gear stick located in the hole. The plastic grommet had remained in place on the remote control shaft. See here for detail on the Interlocking Arm Assembly including removal and how it ensures only one gear can be selected at a time.

Now remove the overdrive - if fitted. Best done with the gearbox standing on its bell-housing end on wood blocks to protect the end of the 1st-motion shaft - remove the eight 7/16" AF nuts and lock-washers. Two of them cannot be fully unscrewed/removed until all the others have been slackened right off or removed, then lever carefully under one of the studs against the casing with a large flat-blade screwdriver to lift the OD away from the gearbox by a few mm and the other two can be removed:

The OD just lifts off the mainshaft - more oil will come out - which should leave the eccentric cam which drives the OD pump on the mainshaft. These splines and pilot bearing fit into the OD. Carefully wiggle the cam off the shaft, watching for the locking ball-bearing in a slot in the cam and a recess in the shaft. This is best done with the gearbox horizontal and the slot in the cam uppermost, otherwise the ball could fall out and get lost. Remove the circlip that is immediately behind the cam:

You now have the gearbox in a more manageable state - without the remote cover and the tail - just the mainshaft sticking out. Remove the selector rods and forks as described in Haynes. Check the clearances for the laygears.

'A' mainshaft gear driving the laygear and dog-clutch for 4th gear; 'B' laygear; 'C' laygear teeth to drive the reverse idler gear (reverse gear and shaft removed); 'D' 4/3 synchroniser and selector; 'E' 3rd gear on mainshaft; 'F' 2nd gear on mainshaft; 'G' 2/1 synchroniser and selector; 'H' 1st gear on mainshaft; 'I' reverse gear on mainshaft:

The reverse shaft has a location hole for bolt to prevent it spinning in the casing, hence the slot to turn it into the correct position.

Next to drift the layshaft out. You need a long drift and a piece of coat-hanger wire. As you drift out the shaft push the wire (not shown) down the centre to keep the thrust washers roughly in place. Allow/assist the laygears to move to the bottom of the casing:

I managed to remove the mainshaft whilst the box was horizontal-it slid out easily- note the cut-out in the bearing housing. This is the cutout in the bearing housing and corresponding 'lump' in the case, not the much larger cut-out on the inside of the case for the reverse idler gear when disengaged. They don't all slide out that easily! You can use the large cut out for a drift but that only gets it out so far, for the last bit I had to use two prybars under the flange on the outside of the bearing housing:

It might be necessary to put the box bellhousing on the floor (on blocks (to protect the 1st-motion shaft if still fitted)) and tap the rear of the casing to free the mainshaft. In this case ensure the laygears are out of the way, as being vertical, they can move across and jam the mainshaft. (The only way I can see this working is if you are pulling up on the main-shaft - which is heavy. If it comes out easily that way then it would have come out just as easily with the box horizontal. Inspect the mainshaft clearances.

When removing the main-shaft the 3/4 selector and the needle roller bearing between it and the 1st motion shaft will probably come off and be left lying in the case.

Output/mainshaft with the '4/3' synchro assembly moving left for 4th and right for 3rd, ditto the '2/1' assembly, and near-equal 1st and reverse gears. 'A' is the needle roller that engages in the end of the 1st-motion shaft to support that end of the mainshaft. 'B' is the worm gear for the non-OD speedo drive:

Take care not to damage the plastic speedo gear (Note this only applies to the non-OD gearbox, it's in the overdrive unit otherwise):

More shims on the (non-OD) mainshaft:

To dismantle the mainshaft special tool 18G1024 which is basically a hook spanner is used in holes round the outside of nut 22H772 at the 4th gear end of the shaft. However Haynes describes using a hammer and drift. Note that lock-tabs on locking-washer 22H773 need to be knocked back first. The 3/4 selector/synchro hub slides off over the nut and lock-washer:

The above is basically how I found the V8 gearbox when it had developed getting on for 5mm of play between the gears instead of 3-8 thou! Nut partially unscrewed, and the lock-washer almost certainly reused once if not twice, not staked into any of the three recesses in the nut.

The thrust washer is sandwiched between the 3rd and 2nd gears, inset to the 2nd speed gear but slightly proud of it, to give the required clearance between the gears (3-8 thou). The nut and lock washer, 3/4 selector, splined sleeve and 3rd gear need to be removed to access it. The radial grooves (both sides) ensure lubrication of all the components, possibly through centrifugal force applying a pump action:

Two types of lock-washer as pictured by (but not available from ...) Brown & Gammons (left) and Rimmers (right), the one on the left seems to be the original design. The bent over parts face away from the nut fitting in cut-outs in the 3rd gear sleeve which is keyed to the shaft (the gear itself is free to rotate on the sleeve). The nut has three angled recesses, the edge of the washer is tapped into these, the angle of the one on the right making it easier to start but probably limiting how many stakes can be made to two or possibly even one. With the original on the left unless one of the recesses lines up with a tab you will get all three recesses staked:

It would have been possible to make one, but I did find that by turning it round on the shaft by 180 degrees I did get two undamaged sections of the washer beside recesses (the third one was adjacent to one of the tabs locking into the shaft). That and thread lock would probably have been enough.

I said 'not available' but on the advice of Roger Parker and after trying at least half a dozen other places, as a last resort (based on past experience ...) I tried MGB Hive and they had two in stock! I left the other one for somebody else ...:

Being flat against a flat face on the shaft the staking is difficult to get started, really it needs something like a narrow flat-blade screwdriver with the end ground to a chisel-edge. But I managed to get it into all three recesses in the nut:

To remove the input shaft you need to use a (softish) drift on the outer bearing face from inside the box. It takes time and patience as the angle is not easy! Not really needed unless you are going to work on it, but it can make reassembly easier. The manuals show this step early on, carefully recovering the needle roller bearing between the 1st- and 3rd-motion shafts. With the main-shaft already out a long drift can be used through the casing square on to the bearing outer and round most of its circumference. It is actually easier this way round, going 'by the book' i.e. with the mainshaft still in the angle is indeed awkward with limited places to drift against. There seems no benefit in getting the 1st-motion shaft out first as you still have to drift the main-shaft out in exactly the same way as if the 1st-motion shaft were still in place:

It's now apart! To re-assemble........

Two schools of thought here. Refitting the 1st motion shaft would normally be done after the main-shaft if - in the time-honoured manner - you reverse the removal instructions in the manuals. I pondered this for some time. There are warnings that the main-shaft should be pressed in as getting it cocked will jam, and I wondered if putting the 1st-motion shaft in first would help to align it. But with the rear bearing housing being deeper than the front bearing putting the main-shaft in first is easier as there is no risk of the nose of the main-shaft damaging or displacing the needle rollers. Putting the 1st-motion shaft in afterwards you can see the needle rollers through the side access, the two shafts engage just before the 1st-motion shaft bearing reaches the front of the case, and there are no splines to engage. However the 3/4 selector has to go on the main-shaft before you start inserting the main-shaft bearing into the case as it will not pass through the hole at the front. Neither will it pass the laygear to be fitted to the main-shaft unless that shaft is angled slightly. Make sure the baulk rings are fitted to both sides of the 3/4 selector! The needle-roller bearing is best kept in the 1st motion shaft. Inserting the shafts with the box horizontal means everything stays in place - short of violent hammering. to

Standing the gearbox on its bell-housing means that the 3/4 selector and its baulk rings will slide out of position unless retained and may jam against the laygear which would need to be held out of the way, unless the 1st-motion shaft is fitted first then the 3/4 selector with both baulk rings positioned on the end of that shaft. Which as well as not being able to see if the needle-roller bearing is correctly aligned with both shafts, splines have to be engaged. I fitted both shafts with the casing on its side - main-shaft first then 1st-motion shaft, tapping on alternate sides of the bearing housing and bearing with the alloy end of my wire-wheel spinner hammer and they went in surprisingly easily - easier than either came out! Doing it that way with the side opening on top means the laygear is more likely to lie in the bottom out of the way. Double-check the 4th gear baulk ring and needle-roller bearing are in position before you start!

I put the box on blocks on the floor to drift the input shaft assembly back in. (Standing the box vertically on its narrow end must be entail some risk of toppling unless restrained in some way.) Again a softish drift is called for but as it refused to go in, I finally resorted to a 4 ft long steel bar and the wheel spinner hammer, but keep it on the outer bearing casing. Just tap round and round till it goes in! Check the needle roller bearing is either on the mainshaft or the inner end of the input shaft-doubtful! Rest the bell-housing end on blocks on the floor and ease the mainshaft in (note cut-out in bearing housing). It will need jiggling as 2 sets of splines must engage (one pair). An assistant is useful in ensuring the laygears are kept out of the way. Once the mainshaft is in, move the laygear assembly till it "engages", find a piece of 20mm electrical conduit or similar (plastic water pipe won't damage the bearings), and insert this over the coat-hanger wire to reposition the laygears and thrust washers more centrally. This still needs the laygear and thrust washers to be positioned correctly before it will pass through. The factory tool is pointed, but if you cut a chamfer on the pipe and rotate that it should pick them up. But because it only picks up one end and clearances are very small then with the other end not lifted an equal amount it will jam. I had to use a cable-tie round the middle of the shaft to lift/pull it into position. The Leyland manuals show circular thrust washers at both ends of the laygear but my V8 box has an elongated spacer at the back with a fork that engages with a projection in the case, make sure this is in the correct position as with wire though in place of the shaft it becomes disengaged:

The front has a similar arrangement but with a single tab that locates in a slot, they prevent the thrust washers spinning against the case and wearing it away. Haynes shows these for the 4-synch, items 27 and 28 (the Leyland Parts Catalogue does list them for 18V847 and later). It's not possible to see if 27 is in the correct position without removing the laygear. Hopefully as long as the laygear always has one of the shaft, drift, wire, or tube though it and the casing it will be!:

Replace on the bench. Clean the layshaft and check for any burrs or damage. Attach a bungee cord or rubber bands to the end of the electrical conduit at the tail, sufficient for it to be taut enough to keep the end of the conduit against the end of the shaft as it is drifted in. Alternatively an assistant can keep pressure on the tube. Drift the layshaft in from the bell-housing end taking care that the orientation of the cut-out matches that of the front cover (you can use mole grips to rotate it once it is inserted). I was able to hold the tube in place with one hand while I tapped the layshaft in from the bell-housing end.:

Slide the selector rods in through the forks. Note that the rods have holes in which the bolts bolt into. A delicate touch whilst finger tightening these bolts and moving the rod very slightly will ensure the bolts are in the right place. I got it wrong and caused the "fingers" (that protrude out of the tail) to jam (these are three sections that interact with the interlock plate and selector lever):

Once I rotated the shaft a tad the bolt in the fork (remove it and peer through the hole ...) engaged properly and the "fingers" slid past each other. Now slide the tail on having first replaced the gasket, and bolt up. Oddly one of the shafts protrudes into the bell-housing even when in neutral let alone when the gear lever is pulled back for that gear, leading you to think you have the shafts the wrong way round!:

Fear not, there is a recess in the cover to accommodate it 😊

Bolt the remote casing back on (I bolted it up without a gasket initially, to confirm the main-shaft bearing housing was fully seated and no gap between the two casings). Squirt some oil on to the cogs and up the oilway on the ends of the layshaft, and twiddle the input shaft. I was storing mine so just used enough to keep moisture at bay. If replacing in the car I would add more to allow for initial lubrication.

Replace all four paper gaskets (the rear casing gasket is different between OD and non-OD). Replace the OD gasket with the fibre type from some specialists rather than the paper one from the usual suspects. Replace the front cover oil-seal. Annoyingly the MGB covers have a back to the recess which means you can't drive it out with a socket whereas covers for other BL models have a through-hole! The V8 type is even worse as the release bearing guide tube means that from the outside you can't get a drift anywhere near the back of the part of the seal that contains the metal insert. If you have a hooked tool you may be able to hook it out from the inside which would need the cover to be held firmly but carefully, or some other implement with a right-angle screwdriver-type blade which could be used from the outside. I drifted the back face out from the inside with a wedging action using a large screwdriver that prevents damage to the sides of the recess, resting the cover on the end of the tube and its three 'flat' faces to work round it a bit at a time. In all cases it helps if the previous installer hasn't fully seated the seal in the recess!:

Drift the new seal in having greased it and the recess. A 1 3/16" socket was almost the perfect size - just a little snug, 29mm would probably have been spot-on:

Don't bottom it in the recess, only until the lip is just about flush with the face of the recess:

Grease the lip and the 1st-motion shaft before fitting with the new gasket.

About the last things to be done will be to replace the remote housing (after the OD if relevant) and final fitting of the the side cover (so you can confirm everything is moving as it should (and only as it should ...)) before reinstallation to the car. It's not worth replacing the interlock arm assembly until immediately before the remote housing as it can get dislodged. Engage 3rd (I think) (Haynes says 4th which allows the selector lever to be clear of the interlock) gear to allow the interlock to be inserted. See here for information on removal and refitting. Feed it in rounded end (not the finger end) first as far as possible in a diagonal direction. Then rotate it and move it slightly left to allow the flat plate to sit in the recesses. Again a fiddle-at first it seems impossible and when it finally slips in, you wonder how you did it! With 4th gear selected it should slot straight in. However! Fitting the remote housing becomes more of a fiddle than fitting the interlock because as well as having to slot the plastic bush on the remote arm into the socket on the selector arm the cover also has to be positioned so that the locating pin in the gearbox casing (arrowed below) slots into the hole in the remote housing. Until that happens the housing will not sit flat on the casing and it's all too easy for the plastic bush to get cocked in the socket, and because the selector arm is free to move the interlock can get dislodged. Really with the interlock in you need to select 1st gear by manually moving the selector arm, and that should keep both in position: