Borg & Warner 4-Speed Manual Rebuild

[Forrest Gump]

After the Nationals I pulled the engine and gearbox out. The gearbox needs a rebuild because there’s a problem selecting second gear. I’ve got a new alloy sump for the engine, so it was easiest to remove both engine and gearbox together. I‘m also going to fit a Hurst shifter.



I had the gearbox rebuilt professionally in 2011. It was good for a while, then it started to pop out of second gear when I lifted off at higher revs. I lived with that. Then second gear synchromesh started to play up, particularly at higher revs and also got worse when then everything was hot. I‘ve been double de-clutching to get around that problem. This is all a bit of a pain and was affecting driving enjoyment so time to investigate…….

 

[Forrest Gump]

The gap between second gear’s brass syncro ring and the gear itself looks tight. This gap on the other gears is much bigger.

Closer examination shows indeed this gap is virtually nil, in fact the face of the synchro ring has significant rub marks (see pic below) on it where it’s been coming into contact with the face of the gear. That shouldn’t happen - it suggests it’s not locking up on the cone properly to match the gear speeds and hence gear selection becomes difficult. Finding second gear was always more difficult when everything was hot which puzzled me, but my theory now having seen this is that as the brass synchro ring gets hot and expands, it is able to move further up the cone and contacts the gear face. Once that happens it’s not locking on the cone. Probably starts to act like a washer and blocks the slider. The synchro teeth on the gear and in the slider have taken a bashing (I was expecting that). They’ve lost some definition.

The grooved inner face of the synchro ring is worn (*see pic below) increasing the diameter and allowing it to move too far up the cone. I’m not sure what would have caused this wear, but I’m fairly sure this is root of the gear selection problem?

 

[JonnyC4]

Looks like another great article for the magazine, good luck with the works.

 

[Forrest Gump]

Looks like another great article for the magazine, good luck with the works.

Cheers Jonny, you’re the second person to suggest that (no guesses!) I will certainly do a write-up providing I can work out how to get all these parts back in

 

[johng]

Great stuff Andy.
How easy/difficult was it getting the engine out?
What size lift did you use?
Is that block of wood in the first photo, that is barely under the tyre, your wheel chock?

 

[Forrest Gump]

Great stuff Andy.
How easy/difficult was it getting the engine out?
What size lift did you use?
Is that block of wood in the first photo, that is barely under the tyre, your wheel chock?

On my car it’s pretty easy removing the engine because I haven’t got all the usual clutter in the engine bay. The load leveller is the the key so I can tip the engine back and help it out. I didn’t even need to take the radiator out.
Don‘t you worry about my dodgy wheel chock John - the car is not jacked-up! It was just to hold the car back while I gave the hoist a tug.
The lift is a Clark Strong arm. With everything extended it is just clear of the nose of the car.

 

[JonnyC4]

Cheers Jonny, you’re the second person to suggest that (no guesses!) I will certainly do a write-up providing I can work out how to get all these parts back in

Excellent, always good to have a technical article in the magazine.

 

[Roscobbc]

On my car it’s pretty easy removing the engine because I haven’t got all the usual clutter in the engine bay. The load leveller is the the key so I can tip the engine back and help it out. I didn’t even need to take the radiator out.
Don‘t you worry about my dodgy wheel chock John - the car is not jacked-up! It was just to hold the car back while I gave the hoist a tug.
The lift is a Clark Strong arm. With everything extended it is just clear of the nose of the car.
View attachment 23578

Now try doing the same thing with a big block swinging around on the end the crane......lol!

 

[Oneball]

I’m my experience poor material is the reason for premature wear of the synchro ring. Get your new ones from somewhere reputable.

The other option is they’re very old. The staining on them doesn’t look like a new part to me. How many miles has it done?

 

[Forrest Gump]

I’m my experience poor material is the reason for premature wear of the synchro ring. Get your new ones from somewhere reputable.

The other option is they’re very old. The staining on them doesn’t look like a new part to me. How many miles has it done?

The gearbox was rebuilt in 2011, so the rings would have been new then. The other three sync rings all look okay. Mileage on it since is less than 10K probably.
At that rebuild 2nd gear was replaced (that has a date etched on it to verify) and the 1-2 slider was replaced. The slider is a torque lock type, so puzzling and disappointing that it still jumped out of gear. I was expecting to find a standard slider.

 

[Oneball]

12 years is longer than I thought, that’d explain the colour. 2nd gear is usually the one that I find goes first in the Mini.

 

[Forrest Gump]

New gearbox parts have arrived.
- Rebuild kit with all the gaskets, bearings, shims etc and four new brass synchro rings
- New first and second gears
- New torque lock sliders for 1-2 and 3-4, and new selector forks
- New slider hub for 1-2
- New Speedo drive gear and parts

 

[teamzr1]

Since going through all this work
Are you going to replace pressure plate, clutch, pilot, throw out bearings and U-joints of driveshaft ends ?
How's the shifter linkage, sloppy feel ?
Setup for long or short throw shifts ?

 

[Forrest Gump]

Since going through all this work
Are you going to replace pressure plate, clutch, pilot, throw out bearings and U-joints of driveshaft ends ?
How's the shifter linkage, sloppy feel ?
Setup for long or short throw shifts ?

The clutch is good, it’s only got 1.5k miles on since new. U-joints are fine too.
I am installing a Hurst Competition Plus shifter.
The old stock shifter wasn’t great.

 

[Roscobbc]

Hurst shifter comes with all new shifter rods - does it come with new shifter arms too Andy?

 

[Forrest Gump]

Hurst shifter comes with all new shifter rods - does it come with new shifter arms too Andy?

Yes, I thought they were included in the not-inexpensive installation kit, but I did just go and check.
The two forward gear selectors are slightly different lengths. Presumably there‘s a reason for that?
All these parts are filthy black from their treatment.

 

[Roscobbc]

Presumably mild steel and case hardened? - I know the selector arms on the Hurst kit when I installed mine, were (are) case hardened
Problem being that you can't then cauk and close-up the openings fractionally with a centre punch if they are a lttle sloppy on the gearbox.

 

[teamzr1]

Yes, I thought they were included in the not-inexpensive installation kit, but I did just go and check.
The two forward gear selectors are slightly different lengths. Presumably there‘s a reason for that?
All these parts are filthy black from their treatment.
View attachment 23657

I suspect the arms are different heights so that the shift rods do not clash into each other at certain shift angles ?

 

[Forrest Gump]

I suspect the arms are different heights so that the shift rods do not clash into each other at certain shift angles ?

That would make sense. The arms on the shifter itself have the same difference, so I will have to make sure I get them fitted the right way. The existing stock shifter might be the same, that’s still dangling under the car at the moment. At least with the hurst I can get it set-up while the gearbox is still on the bench.

 

[Forrest Gump]

I had to pause work on my gearbox due to a very busy spell at work and a couple of other things that have kept me off this. I was able to get back on it at the weekend though….

Sliders and hubs assembled. I kept the old hub for 3-4 gears and just put a new slider on it. For 1-2 gears I bought a new hub and slider. I’m not taking any chances with used parts for 1st/2nd gear.



This is the inner face of the slider. It’s an upgraded “torque-lock” type that is supposed to be less prone to jumping out of gear. You can see the splines have a slight back-cut / taper.

The main shaft is re-assembled. I’m using a new 1st and 2nd gear and new synchro rings for all gears of course.



The cluster gear runs on four rows of loose needle roller bearings. This is a bit of a game loading all 120+ of these inside the gears. Sticky assembly grease is the way!



There’s numerous spacers and spring clips holding gears and bearings in place. The gears can then go back into the gearbox case. The blue part on the right is a new speedometer drive gear, the old one was very worn.


The yoke bearing sleeve and seal are replaced in the tail shaft housing.



Then the tail shaft is bolted back to the gear case. This is a fiddle too (most parts of this job are fiddly) because you have to get the reverse gear selector to engage with the gear, while trying avoid touching the wet sealant on the gasket at this joint. It feels like I’m winning now this bit is back on.