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MG MGB Technical - Gearbox to Engine

I have a 3 Syncro gearbox in the car which will have to be changed sometime, I also have a spare 4 Syncro box (both with overdrive)If I recon the 4 Syncro box can that be fitted onto the engine OK.????
I have a wide tunnel body but don't know what engine I have (no engine number)Is the crossmember in a different place.????

Your comments please.


Graham
GJ Barker

I'm not sure about the crossmember, but you'll need to change the backplate, flywheel and starter motor AFAIK. Probably the propshaft, too.
Dave O'Neill 2

The tunnel isn't an issue no matter which way you are going, it was widened for the automatic box.

The crossmember itself didn't change, it was common to all models including V8 and auto, but the mountings did change between Mk1 and Mk2 i.e. 3-synch to 4-synch.

There are two pairs of crossmember mounting holes on the chassis rails of both my 73 and 75 V8, but once you have the front mounts lined up and the crossmember attached to the gearbox that will tell you which pair to use.

The propshaft for the 3-synch with OD and banjo axle is the same as both variants of the 4-synch (which always had the Salisbury axle). But the 3-synch with OD and Salisbury axle uses a prop-shaft that is nearly an inch longer, at 32", so you will need a 31.125" propshaft, which was used on the 3-synch without OD but with Salisbury axle as well as the other two combinations.
Paul Hunt

spigot bush ????
William Revit

Hi Will, I had heard about the spigot Bush being different but when you look at the Clutch,Flywheel,Backplate and Starter Motor, is it worth doing my 3 Syncro box up instead.???
Anyone want to swop a 4 Syncro O/D box for a 3 Syncro O/D box.????

Thanks for all the answers.

Graham
GJ Barker

4-syncro 'boxes are stronger than 3-syncros and about a quarter of the price on the secondhand market.
Dave O'Neill 2

It's a relatively simple conversion and well worth it for the reasons Dave notes above.

Spigot bush: Only changed between 3 main brg and 5 main brg engines. If your engine is 5 bearing, the spigot bush is the same. If you have a 3 bearing engine then it's a bit harder, you'll need to machine the first motion shaft to fit.

Back plate: An alternative is to keep your existing backplate, flywheel and starter and simply cut a hole in the belhousing to accomodate the inertia drive of the starter. All the holes line up between bellhousing and backplate except the ones above & below the starter and by the slave cylinder. The top starter bolt can be drilled & tapped into the new bellhousing, the bottom starter bolt can be bolted to the backplate either by going to 7/16 or by using a helicoil; the bolt by the slave cylinder is best addressed by building up with allow weld at that point and drilling & tapping. However most people leave it out altogether with no ill effects.

Propshaft: the propshaft combinations sound confusing, but the simple thing is that all gearboxes were the same length overall except the 3 sync O/D one. If your current gearbox is one of the latter, then you'll need a 1" longer propshaft. Given the cost difference in the gearboxes, you still come out ahead.

Tunnel: The only tricky bit, as the gear lever comes out a couple of inches further back than the 3 sync O/D, and 4 inches further back than the non-O/D version. Remove the cover from the top of the tunnel, cut it across the narrow bit and extend it by 2.5". And obviously the hole in the tunnel likewise. It's a bit of extra work but simpler in the long run to fit new captive nuts to the tunnel for the new positions of the rear bolt holes.
And then you need to modify the transmission tunnel carpet.

Things to be aware of:

The speedo cable has a tighter bend because of the smaller tunnel, so curve it carefully from the gearbox exit.

There is no provision on the 4 synchro box for the fore/aft engine restraint that the earlier boxes had, so you'll need to make one up, attaching to a bracket at the base of the bellhousing as per the rubber bumper cars.

Gear lever: Not the same. The bottom toggle is linger and limits the action with the result it won't go into gear properly. Use the 4 syncro one.

Clutch arm: also a bit different in length. Use the 4 synchro one or the release bearing will be a little off centre and have a shorter life.
Paul Walbran

I'm no expert on such things, but if the prop shaft is 1 inch to short, couldn't you use a 1 inch spacer at one end of the shaft with longer bolts?

Just a thought.

Tony
Tony Oliver

If Graham has the Salisbury axle then he will need a shorter prop-shaft, not longer, 31.125" as opposed to 32".
Paul Hunt

Paul, if the diff isn't being changed, it is a constant. The only thing which might change the propshaft length is the overall length of the gearbox if the original was 3 sync overdrive. This box has the output flange an inch further forward than all the other boxes, which were all the same length.
Paul Walbran

Prop-shaft length is dependant on diff and gearbox. Change one - either end - and you might well have to change the propshaft, depending on what the original and the replacement are. It just so happens that the 3 synch with OD has the same spacing to the banjo axle as the 3 synch without OD has to the Salisbury axle, which is the same as both versions of the 4-synch box have to the Salisbury axle. The 3-sync without OD and banjo axle have a shorter propshaft than that, and the 3-synch with OD and Salisbury axle have a longer one.
Paul Hunt

This thread was discussed between 09/08/2014 and 14/08/2014

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