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MG MGB GT V8 Factory Originals Technical - V8 conversion - gearbox options

Hello all,
Looking to pick brains / receive wisdom etc.
have a sebring bodied GT ('started as a '78) looking to fit up with a 3.5 hc rover v8 I've been hoarding for a while and looking at the gearbox options.
Have the option of a 5 speed getrag box from an S3 jag which can be had cheaply. Does anyone know of (or have!!) a suitable bellhousing/ adaptor plate that will connect the two?
The bulkhead of the car already has been remodelled to go down the LT77 box route but without spending a fortune on a rebuilt LT77 I don't want to risk the pennies on a fleabay purchase.
Understand getrags are farly bulletproof whereas LT77's spit their dummies out now and again.
ALso will need to fit up with a suitable rear axle. Assuming the stock MGB one won't take kindly to the increased torque of the V8, what other options would be open (bearing in mind budget considerations and V8 diff's being mega)
Any help/advice gratefully received.
Andrew Tyson

I use a Rover R380 box, which is the later development of the LT77 and was fitted to most RV8s, as well as Land Rovers, Sherpa vans, etc. It is OK although not the slickest change in the world. Don't even think of a used Rover box - it must be reconditioned. TM Transmissions in Birmingham can supply good recon boxes. I think some of the Americans have used the Getrag box, but they are outside my knowledge.

An MGB axle certainly can cope with the torque of a Rover 3.5 - the issue is the ratio. The Rover V8 is not a revvy engine but has loads of low down torque. So you need a really high ratio to make the car nice to use, hence MGs switch to 3.07:1 for the V8 BGT. I have Hoyle IRS which uses a Ford Granada diff with a ratio of 3.14, and even that is a bit too low.
Mike Howlett

Hi Andy
Here in Ozz we tend to use the Toyota Supra alloy 5 speed box with a Dellow Automotive ( bell housing.
I have a MG 3.07 rear axle in my '77 CB conversion.
Peter Malkin

Thanks guys.
My instinct on the LT77 might have been good then!
I'm right up the road from Rimmer bros but then you're looking at mega pennies for a box even though they scrapped 90% of their india haul.
Had looked at the R380 option (but there again, it seems, so were many others!!)
Had also looked at the supra box as saw someone selling a decent w58 unit. Getting or sourcing the bellhousing was not cheap though.
Will keep my nose to the keyboard and keep searching.
Thanks for the info.
Andrew Tyson

Andy no conversion is cheap - but lt77s can be often sourced from local scrappies - especially around the Fens!

If you are really lucky you might find one of the LT77 "interim" boxes. These were produced just before the introduction of the R380, with beefed up innards. Couldn't tell you numbers etc., but someone on the Web will know.
My theory about the frail nature of the LT77 is down to the use of ATF. When first introduced, filled with proper gear oil, they were very troublesome untill hot. Leyland filled them with ATF to get over this, but ATF is nowhere near as good a gearbox lubricant and, I think, shortened their lifespan. Fortunately there are great synthetic gear oils which transform the operation of the box and look after them better. Redline MTL and Castrol SMX-S 75w-85 are just two.
Allan Reeling

Rimmers found a LT77S which has worked for me with synthetic gear oil (Allan Reeling's advice). By repute the getrag box is better but both need a small mod to the tunnel. I used a TR8 bellhousing (also Rimmer). However, if starting again, I would follow the Toyota route - they have small strong boxes which need no mods to the tunnel.
If you go for a secondhand LT77 - the 2300 SDI was less demanding on its box .
R Walker

Bite the bullet - call/email Dellow in Oz and order the bellhousing, Supra gear box, clutch fork/throw out bearing ( and clutch assembly), install and drive it. Dollars spent will be forgotten in next to no time. The guy to give you the nitty gritty (gear box model et al) is Peter Sherman who frequents this forum.
Do it and enjoy the car.
Peter M
Peter Malkin

Don't forget Supra boxes tend to buzz the teeth off the input shaft and cluster with a bit of torque put through them, but having said that I suppose a 3.5 won't do much damage - I'm a big fan of the P76/Valiant 4spd single rail box. Good ratios and bullet proof in a B, but getting a tiddle scarce nowadays - Willy

What about a T5 ???
William Revit

Hi Willy
Why would you opt for a 4 speed box? I'm on my second V8 - the first a roadster I converted in the 90s and my current GT. Both vehicles have/had uprated 3.5s without any problems with the Supra box. I must say that I'm not a racer but I also don't spare the pressure applied by my right foot either.
Not familiar with the T5.
Peter Sherman - where are you?
Peter Malkin

We've sort of found that by using a tall diff and the P76 4spd box being a close ratio unit with from memory about a 2.6;1 first gear, you end up with first, second, third and fourth roughly the same ratios overall as a Supra box's second third fourth and fifth with the lower geared rear end. The end result is like a Supra box with no first gear. Probably not the best for shuffling round in the city traffic but for going places in a hurry it's all good.

T5's are the 5spd boxes used in Falcons since the mid 80's through till a couple of years ago when they started sticking 6speeds in them
Cheers Willy

I'll get you a pic of what happens to Supra gearsets
William Revit

Found it

William Revit

Reagrding lubrication of Rover LT77 and R380 boxes, I use Difflock's Evolution 1 lubricant. This was specifically developed for these boxes and is supposedly superior to other oils. I've used it so far for three years and 10,000 miles with no issues.
Mike Howlett


Where can I get a W57/8 Toyota box , the UK has zero kicking around.
S w Holmes

Hi Willy/Sw
"Ouch" - thanks for the photo Willy.

Sw I'm away from my office for a few days but the gearbox, bellhousing etc is available from Dellow Automotive in Sydney Australia. Jeff Dellow is great to deal with. Just Google them up.
Peter Malkin


You could also try:

But it is expensive and a T5 is common. I remember that was involve in such kit.


Jean G.
Jean Guy Catford


I had the LT 77 for more than 25 years in my GTV8 and the only weak point were the rubber bearings of the shifter extension.
I used Castrol synthetic gear oil on mine.
The engine was a modified SD1 with Holley carb, so called RV8 headers and a 3.3 rear.
Due to my age, I sold the car early this summer after 30 years of delightful driving.
O.K., I once had a problem, when a ground wire of the fuel pump dropped of but the LT 77 was always very reliable.


R.S. Ralph Siebenhaar

This thread was discussed between 15/08/2011 and 28/11/2011

MG MGB GT V8 Factory Originals Technical index

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