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MG MGB GT V8 Factory Originals Technical - Gearbox Crossmember Mounts

Hi everybody,

Have got to the part where I put the engine and gearbox in...YYYYYAAAAYYYYYY!!!!!

Anyway, I have a 3.5L with Supra 5 speed.

What height should the gearbox output shaft be reference the crossmember?ie. Should I raise it as far as possible for clearance or is it not that important?

Right now, after cutting off the old mounts and sitting the gearbox mount on the crossmember it is 5.5 inches above the straight (upper) edge of the crossmember.

I realise this is a pain to check your own cars but any guidance as to the output shaft position would be greatly appreciated...regardless of gearbox type.

Thanks in advance.



The important thing is not the height with respect to the crossmember, but the angle of the mainshaft with respect to the differential. A line drawn through the center of the mainshaft should be parallel with a line drawn through the center of the pinion gear. For more info on this, see:


This is VERY important!
Dan Masters

Thanks Dan,

Those are great links!

The only problem is I haven't got all the bits fitted to the back of the car yet. ie fuel tank/fuel/spare tyre etc. I'm a bit worried that if I do all the measurements that they won't be valid unless the car is it at exactly the correct ride height. What do you think?


You got it right. Get all the bits you can on the car and then get it up in the air on it's wheels. some lifts use the wheels, some use the chassis. Ramps, blocks of wood (big single solid units not stacks of 2x4's) something strong and stable under the wheels so they carry full load. Also be aware that there are "shims" on the market that let you adjust the angle of your rear differential as well..

Larry Embrey

Also, think ahead about building shims into your tranny mount so that you can space it up or down in the future to help adjust for angle. To move the end of the tranny up, for example, shim between the mount and crossmember. To move the tranny down, you can shim between the body and crossmember, lowering the position of the crossmember, hence the tranny.

Best, Joe
Joe Ullman


What mods have you made to your rear axle? If you are using the stock rear setup, or if you have had your replacement made to the same specs, then all you have to do is measure the angle of a stock MGB engine installation and install your engine to the same angle. I don't know what it is on an MGB, but on a TR6, for example, it's 2.5 degrees (downward to the rear).

Otherwise, you should really have the rear at final ride height before setting the angles. However, if you are off a bit, you can always use shims as Larry suggested. These are (inexpensive) tapered shims to rotate the rear axle as needed to bring the angles in compliance. These shims go between the spring and the axle mounting pads, and are available in 1/2 degree increments, I believe, up to about 3-4 degrees.

Dan Masters

Now you've got me worried, but not much . I hope I'm right about this and not setting myself up as a clay pigeon but do please check and correct me on this if I'm wrong.
I'm at about the same stage as stainsy, supra gearbox and motor in the car and have just set up the cross member for the gearbox. Like him I'm worried about drive shaft /heal board clearance since the supra box is allot shorter than the MG box, so I've pushed the rear of the box as high as bonnet/hood and steering rack clearance will allow (not much, 25mm from the bottom of the crossmember to the bottom of the gearbox mount). I've also pushed the motor and box way back in order to get the gear lever in the right place (some "delicate" dressing back required by the way). I also (as per Roger Williams suggestion) aimed at getting the 14mm off set to the port side, this gets the gear lever exactly right and gives a little more room in the transmission tunnel for a larger drive shaft (the handbrake mechanism projects about 22mm from the right-hand side). In other words the universal joint at the back of the gearbox is at a different angle to the diff'!. I was really worried (after reading this thread) until I read Dans link and did some calculations,
If you figure that the motor and gear box (or the drive shaft if you look at it from the other end) is about 1 meter long, the 14mm left-hand offset corresponds to about 0.8 of a degree (According to Dans excellent reference this shouldn't matter, being within 1 degree). I then calculated what 3.25 (the max allowable @5000rpm) degrees corresponded to, 56mm.
That's a pretty large margin for error and limitations of the size of the transmission tunnel itself are going to keep us safe. In other words, don't worry about it
....I hope...


Forgot to mention, If the drive shaft contacts the heal board (I havn't had one made up yet)I plan to use chome bumper bounce straps to get the back axil up an inch or so


When you offset your engine, did you keep the centerline of the engine parallel with the center line of the car? You can move the engine around all you want - up,down, right, left - as long as the centerline of the engine is parallel with the centerline of the pinion shaft on the diff. If you have the axle installed correctly, the pinion will be parallel with the car centerline, so if the engine is parallel to the car, it will be parallel with the pinion also.

If you got the 14mm offset by angling the motor (which I doubt), you may get some vibration, but how much I don't know. I would rely on the recomendations on the two web sites as to how much error is allowable.
Dan Masters

Thanks Dan,
I have angled the motor (no room for engine bracket offset)but I am within the allowable tollerance from your reference(<1deg). If the thing vibrates madly I'll now know what the problem is and I can re-alighn to centre , In the meantime I'll give it a shot because of the mentioned advantages. I'll post a thread in about a month or so if it works (or not) because although it isn't essential to have the gear lever in it's right place, it isn't a bad thing either and space for a larger drive shaft isn't to be sneezed at.

Stainsy,(or anyone with a supra box)
What did you do about that big centre bolt of the supra box mount? It looks to me like it is a safty measure to stop the mounting being torn. I'm guessing that it should not be done up tight but a gap should be left between it's head/washer and the crossmember.


Thanks everybody for all responses.

Peter, I think you're right about the main centre bolt. When I was doing the crossmember I did the following:

1. Cut off the original mounts from the crossmember.

2. Drill four holes for the metric 8mm bolts that secure the mount to the crossmember.

3. Drill a large hole through the centre for that large bolt.

4. Because the crossmember is not flat, the 8mm bolts will not sit flat on the crossmember and the inspectors will just love that if they see it. What I did was fabricate and weld a half box sort of arrangement that essentially flattens the bottom of the crossmember. It is 120 mm long, as wide as the crossmember and about 6mm deep. This means that the large bolt head and the 8mm bolts sit flat.

5. If I have to raise the gearbox then I will get a longer large bolt and shim the gearbox mount.




Great minds think alike re:crossmember)I also made up something similiar for the inside for the mount to sit on. I cut it out of a piece of 4mm, 3 inch square tubing. Lots of effort, in retrospect it would have been easier to use a piece of channel.
Stainsy Question re: the supra gear lever. Its much to big & bulky I think and I would like to cut it down however the construction seems to be an outer chromed sleeve, a rubber bond and an inner stick. The question is how long is that inner part?, can I remove the outer and just go with the inner, or is it too short?


The inner part of the gear lever on my W58 box is exactly 160mm long measured from the top of the retaining plate that holds the lever in the top of the The plate with the four bolts in it.

It also reduces from top to bottom. The top 30mm is 10mm diameter. The next 50mm is 12mm diameter. The rest at the bottom is 13mm diameter.

Whether this is a suitable length for you I don't know but I myself am going to try threading the top part and put an alloy gear knob on it.

It is a real pain to get the outer off so be careful not to damage the inner.

Hope this helps. Am going through shocking divorce at the moment, will be back chatting to you all in a couple of weeks.

Kind regards,



Is a Supra box the best choice? What's the benefit? I am starting on my second car and I was just gonna use another TR8 .833 5-speed box, since I've got all kinds of miles on mine and it seems perfectly OK. I think my motor is a little over 250bhp. Whatever. I just don't really like the feel of the T5 or T56 boxes too much, though I know they're reliable and you can probably improve the feel with a few add-on parts.

Actually, I think if I could figure a way to sort the bellhousing and so on, I'd put a BMW 5-speed box on there ... same pattern as the Rover BTW. (Of course that begs the question of why not just shove a Bimmer OHC engine in there! ... I feel like we would really improve the state of our MGBV8 lot if we could figure out a way to get an OHC powerplant in there.)

Thanks Stainzy, don't let her get the car!
I'd probably use a T5 box if I was in the US but they are very expensive here. A local guy, Dello, makes a bell housing, folk race etcfor about 450 Australian for the suprabox/Rover V8 (don't be in a hurry though).He can also provide a clutch and slave but they're not so hot.Better to BYO. Aside from delivery time & Clutch etc its a good deal and he's very helpful.He can also provide a gearbox, choice of ratios and gearlever position (we want the longest, 21 inch)
My supra box is a W58 with an over drive ratio of 0.78. They are cheap and strong and many will tell you you can fit them with no dressing of the transmition tunnel, not entirely true, in order to get the gear lever in exactly the right place you do need to dress up the area under the heater and between 3 and 5 oclock LHS, but not very much. About 5- to 10 minutes worth of bashing away.
I've obviously yet to take this for a drive but they are supposed to be very smooth and way better (and cheaper)than the rover variety
Heaps in the achives about them.

This thread was discussed between 01/10/2002 and 11/10/2002

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