Tuesday, June 27, 2023

Transmission One And Done

I've been working on acquiring an overdrive for Alice since I got her.  The overdrive is a highly sought after factory option, as it reduces RPM at speed and makes for a much more comfortable (and economical) driving experience.  I couldn't find an overdrive transmission, but I did find someone that would sell me the parts to convert my 4-speed to an overdrive.  I bought those last year and they were in a box until I finally managed to find time to get to work.  And with Alice's own 4 speed about to give up the ghost, it was time.

What's funny is that I ended up actually building two overdrives; I built one, and rebuilt another.  There was some mix-and-matching to get them done, but they both came together and I expect they'll work great.  So some of the pictures in this article don't exactly look like the same transmission, because they're not.  But the internals are the same.


Disassembly of the MGB 4-speed transmission is actually pretty simple and only requires a few tools.  The big parts are easy - remove the speedometer drive, the remote housing, the output flange, remove the rear extension.  These come off with only a few bolts.  Inside the rear extension is an assembly that keeps the linkage that engages the selector rods in place.  This can be a bit fiddly to remove, but have patience.  Once this assembly is out, the rear extension comes free.  The output flange has a very large nut, and an impact wrench (1 1/8" socket) is the simplest way to remove it.

Once these are off, you'll feel like you made a lot of progress.  Hold on to that feeling for a while.

The clutch fork and front cover are next.  Again, there's not much to it.  Remove the release bearing, then unbolt the clutch fork with a 9/16" wrench and remove it.  Remove years of grime and crud and then remove the front cover by undoing the 8 1/2" nuts that hold it in place and pulling it off the input shaft.

Now, it starts to get interesting...

Remove the side cover and gaze at the wonder that is a transmission's guts.  Then stop gazing and get back to work.

Remove the three 9/16" retaining bolts that hold the detent pistons and their springs in place, then remove the springs and pistons with a small magnet on a stick.  It you're lucky, they'll come right out.  I was not so lucky the first time, but they eventually came out.

Remove the selector rods by undoing the 7/16 retaining nuts and the 9mm (I'm sure it's Whitworth, but 9mm works) bolts, and withdraw the rods one at a time.  Once the rods are out, remove the selector forks, which are what move the synchro assemblies back and forth on the mainshaft as you move the selector rods back and forth.

The rest of the innards come out in a specific order:

First, remove the reverse gear by undoing its retaining bolt, withdrawing the shaft and then removing the gear from the housing.

Second, remove the shaft that holds the laygear in place.  The laygear is what transmits power from the input to the mainshaft.  Let the laygear drop down into the case - it needs to move out of the way for the next two parts.

Third, remove the input shaft (the part that connects to the engine).  This comes out with a little fiddling and as one assembly.

Fourth, remove the mainshaft.  This can be tough.  On one transmission, it took a half hour of careful tapping from the inside with a long punch against the main bearing's outer race.  This is one big bearing.  On the other, it almost fell out with only minor persuasion from a rubber mallet.  I was careful to not let the 3/4 synchro assembly fall off the shaft as I removed it.

Fifth, remove the laygear from the case.  This is fiddly as there are two thrust washers that keep things very closely aligned. 

Last, the mainshaft comes apart.  There is a retaining collar that screws into the front of the mainshaft, and it has a retainer that comes loose by tapping the spots where it's holding the collar out of the way.  Then the retaining collar can be unscrewed with some persuasion.  There's a special tool for this, but I used a Channel-lock pliers and put the mainshaft in a vise with padding to keep from damaging it.  After that, it's removing the gears one at a time, keeping them in order and proper orientation.

Folks, that's it for the big pieces.  You're left with an empty case.


Reconditioning is mostly a game of cleaning, but there are some parts that need to be checked and probably replaced.  First and foremost, the synchro "baulk rings".  They're called that because they help slow, or "baulk", the gears being disengaged and engaged by gripping a cone.  The slowing of the shaft allows the synchro assembly to line up two gears, disengage one and engage the other.  Because the assemblies are at rest relative to each other, the gears mesh without "crashing".  Once engaged, the input shaft turns the laygear to connect input to output, and the different input/output ratios make the output shaft turn at a different speed relative to the input shaft.  (That's basically the whole magic of a transmission.)

The baulk rings are designed to wear and usually last 60-70 thousand miles, more if you're a smooth and easy driver.  They have little tiny teeth that wear out, and when they're worn out they don't do their job well enough to slow things down.  That's why a worn transmission is hard to "put into gear" and makes grindy noises when you try.  The photo below shows a new baulk ring with an older one.  See the way the teeth are worn?

Aside from the baulk rings, the bearings that support the laygear and between the input and output shafts can wear, but it's not too likely.  I replaced them because they aren't expensive, and I don't place on going back in there anytime soon.

The laygear shaft and thrust washers are known to wear, so inspect them.  For the thrust washers, this is where you check the 'end float' (the amount of space between the washer and the housing) when the laygear is installed.  If out of tolerance, replace them.

A full gasket set is required, of course.  The front cover seal should be replaced even if it feels supple.  The rear seal would also be replaced, but since I am converting to an overdrive I won't be using this part again.

Be sure to inspect each gear to ensure they're usable.  Chipped teeth, while unlikely, make the entire endeavor a waste of time.  You need these gears for the new mainshaft you're going to build.


There are a few parts that change when you convert to an overdrive.  The mainshaft is different at the back because it fits into the overdrive unit.  The remote housing becomes an adapter to hold the overdrive unit.  The speedo drive is part of the overdrive unit.

The main part of the conversion is the swapping of gears from the non-OD mainshaft to the overdrive mainshaft.  Having removed them and kept them in sequence, this is pretty simple.  Just put things on the new mainshaft in the same order they came off, and lube everything a little bit as you do so.  Once the gears are in place, refit the retaining collar and its retainer.  Again, no special tool meant a Channel-lock was carefully employed.  Tap the retainer back into place to keep the retaining collar from turning.


The Haynes manual loves to state that "installation is the reverse of removal."  This is a bit cheeky, but basically true.  The parts go back in in the order they came out.

First, the laygear and its thrust washers.  This rests in the bottom of the case like before so you can get the mainshaft and first motion shaft into place.  You will have fitted the new bearings before you put the laygear into position.

Second, the mainshaft.  This is a bit delicate as you don't want to damage the rather large gears that barely fit into the housing.  There is a key like a half moon which lines up with a matching pin, and that sets the orientation of the bearing outer race.  Once aligned, the outer race can be gently tapped with a rubber mallet around its edge.  The bearing will fit slowly at first, then start moving more freely until finally at the end it pops into place flush with the back of the housing.

Third, the input shaft.  Don't forget the caged needle bearing that goes between the input shaft and the mainshaft.  The input shaft should fit into place with little effort.

Fourth, install the layshaft.  This is a bit fiddly, and I tip the case so the layshaft will fall into place under the influence of gravity.  I use a long extension to help fiddle the laygear and thrust washers around until the layshaft slips into place.  Once installed the layshaft has a specific orientation that the front cover will maintain as it has a slot for the protrusion on the end of the layshaft.

Fifth, install the reverse gear and its retaining bolt.

Once all those parts are in place, it's a good idea to see if things turn freely.  The gears are in 'neutral', so you should be able to turn the input and output shafts separately and without significant effort.

With the guts of the transmission back in place, refit the selector forks and rods and the detent pistons and springs.  The selector forks and rods have a specific orientation and only go together one way, so you can't really mess it up.  But I take lots of pictures both for articles like these and to help my poor short term memory.

Once again, stop and admire your work for a moment.  You earned it.

Finally, refit the front cover.  This requires a little bit of math.  (I never said there would be no math.)  You measure the depth of the front cover where the input shaft bearing fits, add a little bit for the cover gasket, and subtract the height of the input shaft bearing where it protrudes from the case. This tells you how thick any shims between the cover and the bearing have to be.  This is really hard unless you have a good micrometer, but I only have an okay micrometer.  I managed.

It is usually okay to just refit the cover with the shims that were there before (replacing them if they're damaged, which happens often).  However, note that you really should measure this.  If there is a gap, the input shaft bearing can move back and forth and cause premature failure of the bearing.  One trick is to get some modeling clay and make a reverse cast of the inside of the cover, which is easier to measure.

Fit the cover and any shims, and tighten the cover down slowly and evenly.  Once fully in place, make sure the input shaft spins freely with the selector rods in neutral.  If it doesn't, then you have too much shimming.

On one of the two transmissions, I actually needed to make a double gasket to create a little more gap to allow things to spin freely.  This transmission had something similar done before.  On the other, the shims were needed.


Now that the main portion of the transmission has been reassembled, the fun part can begin.  The overdrive unit was after all the whole point of the exercise.

The LH type overdrive is an interesting piece of kit.  When not engaged, it transmits power by a 1:1 ratio between the input and output shafts.  (Gear ratios are measured by input:output, so X turns of the input shaft equates to Y turns of the output shaft.)  The action of a solenoid engages a little hydraulic pump that builds pressure.  This pressure forces springs forward to engage a cone clutch, which sends the input power through a planet-and-sun gear set and changes the input/output ratio to something less than 1:1 (like 0.82:1).  The output shaft spins more slowly as a result.  This 'over-unity' ratio is the 'over' part of 'overdrive'.  It's pretty simple and bulletproof if not abused.

The first part of installing the overdrive is installing the remote adapter housing.  This housing is shorter to make up for the overdrive unit's length.  (In fact, the non-OD and overdrive units are the same length, and for the later overdrive units are a direct swap.)  The remote adapter housing has the linkage to control the selector rods.  Fit a new gasket and bolt the housing into place with the linkage engaged to the selector rods (but still in neutral).

I ran into an interesting problem when I first assembled one transmission.  The mainshaft and laygear didn't line up quite right.  I spent quite a bit of time fussing about this, and finally called my new friend Dick Moritz, He the Keeper of Overdrive Secrets.  He informed me that sometimes, the large double-bearing on the mainshaft gets distorted a bit when the pressure on it relaxes during disassembly.  His answer was to bolt the remote housing down evenly and watch the gears move into place.  And golly, he was right.  The other transmission didn't have this little issue and I fitted the remote housing without incident.

See the before and after I'm talking about...

The next part is one that you cannot forget, or nothing will happen when you engage the overdrive.  The overdrive's mainshaft has a small eccentric cam held in place with a 3/16" ball bearing.  This is what drives the pump inside the overdrive unit.  Fit the cam and bearing, being very careful not to let that ball bearing fall into the case.  I pack cloths around the mainshaft to prevent that from happening.  Once fitted, remove the cloths.

Fitting the overdrive to the remote housing can be difficult, unless you know "the trick".  Fortunately for you, I am going to share the secret passed on to me by He the Keeper of Overdrive Secrets...

First, line up the teeth inside the overdrive by using a long screwdriver to force them in a counter-clockwise direction.

Second, turn the output shaft until the big part of the cam is facing the flat part of the case.

Now, put the transmission into a gear (any will do) and use a locking pliers on the input shaft to keep it from turning at all.  This is the part of "the trick" that makes it work.

Next, remove the filter cover and filter from the bottom of the overdrive unit and thread a small nylon rope through and around the pump drive.  You want to be able to pull the pump drive back about 1/4" so it won't run into the cam, which you thoughtfully oriented so the smallest part is where the pump will be.  Make sure you can pull back on the pump drive a few times before proceeding.  It shouldn't make that much effort.

It's time to offer up the overdrive unit.  Fit a new gasket between the remote housing the the overdrive, and slide the overdrive onto the output shaft.  It will go a little ways, then stop.  That's okay.

Here's the rest of "the trick".  Pull the nylon rope that pulls on the pump drive and turn the output flange counter-clockwise ever so slightly.  After maybe 1/8 of a revolution, the whole overdrive unit will drop into place with a 'thunk' and rest against the remote housing.  Remove the nylon rope and refit the filter and cover.  Bolt the overdrive into place with the 8 nuts and lock washers (four have a lower profile because it's really fiddly to get them into place).  Remove the locking pliers and take the transmission out of gear, and make sure the input and output turn freely and independently of each other.



The last bit of assembly is to fit the remote shifter unit with a new gasket.  The little retaining assembly for the linkage goes in first, and that can be tough to fit.  Line up the linkage in the selector rods and then you should be able to slide the retainer into place.  Have patience - it came out, so it will go back in.  Once in place, fit the gasket, line up the ball in the housing's linkage with the socket in the selector's linkage, and bolt the housing into place.  Don't forget about the little plastic cup that goes on the ball.

Fit the side cover with its own new gasket.  Refit the clutch fork and a new boot and release bearing, and you're done.

See, that wasn't so hard!

Rebuilding a gearbox is really all about diligence and patience.  There's nothing magical going on in there.  Fitting the overdrive is also seen as a black art, but now that you know "the trick" it's not a big deal.  I did all of this with a few wrenches, pliers and sockets, a couple of screwdrivers, a bench vise and an impact wrench and large socket (the one "special" tool I used and would have had to buy if I didn't have it).

Now that the transmission's done, it has to go into the car.  But that's another story...

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