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MG MGB GT V8 Factory Originals Technical - mgb conversion

hello i want to put a v8 in my 79 mgb roadster,ive just purchased a 3.5 v8 with as 18 thousand miles on it but i dont know iff its suitable as its from a van.i am aware that the later mgb makes an easy there any cutting needed in the engine bay,the gear box is a 5speed so there will be some mods done the tunnel. i want to make the conversion as simple as it possible to leave the suspension and brakes on the car and just upgrade the discs and fit green pads, allso leave the back axsle on the car and fitting a mgc diff. this would leave me needing a drive shaft which fits from the sdi gear box to the origanal back axsle. does the v8 engine weigh alot more than the 1800 engine thanks daren


There's really quite a lot to be done. I suggest you read Roger Williams', book 'How to give your MGB V8 power'. It's mpublished by Veloce Press at 12.99 and worth every penny.

It will answer all your basic questions - and many you haven't thought of yet! It really is an excellent book.

Good luck

P L Hills


Most (if not all) of us will agree with Peter on reading Roger's book. There is a second edition out that covers all of edition 1 and includes more info on US conversions (ie: Ford 302's). Roger is working on a third edition if I'm not mistaken.

My conversion is much the same as you've described except my 3.5 came from an SD1. Something in the back of my mind tells me there's something about the van 5-speed that may not be suitable for your project but I'm sure someone here will have more info. I think the van gearing was different.

The only engine bay cutting I did was to fit RV8 headers. The gearbox tunnel "extension" (as described in Roger's book) has to be installed on the tunnel to clear the SD1 'box as it's taller than the MGB unit.

I'm running stock MGB suspension front and back except for lowered springs at both ends, MGC gears and cross-drilled/vented rotors with the green pads.

Drive-shaft was custom-made (about $600 CDN). The u-joints are fairly common so should be no problem getting one made where you are.

Lots of info on the weight of the V8. Most will agree it doesn't weigh anymore than the MGB 4 and may weigh slightly less. Look in the archives for details.

Hope this helps. Lots of good advice here and in the books. Good luck and have fun!

Simon Austin


From memory Roger's book suggests utilising replacement panels intended for the later model shells to modify the tunnel around the bell housing and footwell area. There were pictures also.

I currently have a RB heritage shell on a spit. If you want more photos illustrating the later model's firewall, transmission tunnel and plenum let me know and I can take and email the same.


"Roger is working on a third edition if I'm not mistaken."

You are correct, but he has finished it now and it's in the hands of the publisher. Roger is expecting it to be out this month if all goes well.

I have seen the draft, and it is indeed an excellent book, well worth the wait.
Dan Masters


Roger Parker has written regarding the various 5-speed gearboxes. It is included in his notes 'Fitting a V8 engine into an MGB'. If you search for that text-string in the 'Search' box of the MG Enthusiast's site home page it will show up Roger's note. It's a very valuable document.


P L Hills


In a 79 its virtually a bolt in conversion except for the gearbox tunnel which need raising in height to clear the rover box, you can get this made by any competent sheet metal worker mine cost 30.00. You then need someone to cut the tunnel and weld in the new extension. You will need a V8 radiator, engine mountings exhaust system amd prop shaft all available off the shelf from V8 conversions in Farnborough Kent Tel 01689 858716. I recommend RV8 style manifolds got mine from B Hive excellent quality, full stainless system 399.00.If your engine and box came from a sherpa van i believe the box will not have the right ratios. The engine may also be low compression 8.13/1
check by the dipstick hole, there will be an engine number and compression ratio stamped into the block if its 8.13/1 you will need to change the pistons 9.75/1 would be best if you want some real power!
I would certainly recommend getting the book it will save you a lot of time and give you all the information you want. You will certainly need to overhaul and upgrade the brakes as a minimum and make sure your suspension is in good order, there are lots of upgrades available just depends on the depth of your pocket.
Good luck,
K.R Jackson

thanks for that mr jackson ,im tyring to make the conversion as simple as possible. iff i can get hold of a origanal gear box then i will not need to cut out the tunnel, but are they only four speed and are they dated as in poor gear changing etc.then can i use the origanal back axle on the car and change the diff,then theres the problem modifying the drive shaft thanks daren


IMHO Don't bother with the original gearbox; the overdrive is fragile, and the gearbox not too strong.

Go with the Rover gearbox, but check the ratios, you can still use the original back axle, but MAY want to fit a 3.07:1 cwp.

Shortening the propellor shaft is no big deal, but have it rebalanced as part of the job.
Nigel J S Steward

"In a 79 its virtually a bolt in conversion except for the gearbox tunnel which need raising in height to clear the rover box"

.. with an RV8 crossmember the LT77 is .9cm too high only and catches on a seam weld - this can be dressed back .. and the tunnel modifications dispensed with.

With your eye on the budget - start with the Sherpa box and convert later to the Gentrag box - it's a straight swap and a worthwhile improvement.

the choice of exhaust system will determine power, cooling and running costs - follow Kevin's example

Cooling - the V8 rad and OE twin pushers will do all you need (with or without RV8 exhaust)

Choose your carb carefully - cruising consumption is much the same between them all but you may use petrol faster in traffic jams with some - petrol is headed for 6 a gallon ($10 USg)much faster than most realize. Looking ahead FI is the only practical long term solution.




You may not need to do anything to the axle/diff', unless you really want to. Changing diff's and axles is expensive so why not try it with the orrigional diff first and see?. My cars a '75 (now with a 3.9V8) and the back axle/diff' is as orrigional, the same as in your car, certainly strong enough. However I'm using a toyota gearbox which has a 0.78 5th gear. This box will fit without any tunnel mods, by the way, the catch is you need to spend 400 or 500 australian dollars on a bellhousing kit from Dellow.
My car ended up lighter, mostly because of the alloy box. I up graded the brakes, but only because the engineer said so (we need an engineers certificate here in Australia). I honestly think the MG brakes in good condition with good pads are sufficient and next time I will get a brake fade test done prior to spending all that money!. I did get bigger 15inch wheels and 195 by 60 tyres and this did make a big improvement in control/roadholding. It was a bit TOO exiting at times with the old tyres.
You're going to just love that car when you're finished!

Looking forward to your new book.

I saw somewhere online a conversion for MGB, from for a nissan 280z motor. can someone help, I can't find it again. it looked easier and less painful. Thanks for any help.
Thorn Tim

The V8 is easier because it's been done thousands of times, you can buy all te bits (if you want to). The car ends up more Not less valuable, its easy to resister

I tend to agree stick to the V8 vs Nissan 6.
However I'm told the Nissan 260Z gearbox was one of the best 5 speed boxes around. In competition circles it was a sought after box due to its close ratios (in contrast the earlier 240Z and later 280Z box's 1st and 2nd gears were too low). Has anyone heard of a conversion using this box?

There are 2 places in the US that offer conversion kits for the gearbox used in the Nissan L-series engined Z car with an B series ebgine. The ratios of the box you e refering to are similar to the 'intermediate' ratio box used on the MGC.

The 280ZX Turbo used a T-5 with a 3.35 first gear similiar to the ratios used on the Ford Mustang and Chevrolet Camaro.

Not sure this addesses' you're question- not sure exactly what you're question is. Most installers in the US use a T-5 in their conversions, or an LT77 depending on the source of their engine.

Not aware of anyone adapting the box you describe to the V8- don't know it could handle the torque.

If someone in the US wants a closer ratio T5 they usually use the 2.95 first gear cluster.
greg fast

Yes, the 260Z had a 2.9 or 2.95 1st ratio.

My source rallied a fuel injected 260Z (a US homologation). I believe this car in its full race tune produced around 270HP to 280HP (not sure about torque). The gearbox was never a concern.

It would appear to be equal to the Supra box.

'start with the Sherpa box and convert later to the Gentrag box'

No problem with the Getrag, but have you looked at the ratios of 1st 2nd and 3rd of the LT77 series (SD1) gearboxes used in the Sherpa range? They are intended to allow an overloaded Sherpa van to be able to start from stand still on a steep hill and accelerate. That makes the ratios very poor for a low revving V8, so consider this as a stop gap gearbox only. There are in fact 4 different ratios for 1st and 2nd gears, 2 for third, 1 (direct 1 to 1) for 4th and 3 for 5th, all depending on application.

Gearboxes with codes of 29A and lower relate to saloon car applications, 30's series are Sherpa and generally 40's and 50's are 4WD applications. Then there are the specials where 70's are found. The much much better R380 series gearbox follows the same numbering pattern.

Roger Parker

I'm in the process of trying to fit a Rover v8 with a toyota w58 gearbox. I have read that you can make up a bellhousing using a auto bellhousing and making up a adaptor plate to fit the gearbox then welding the two together. Has anyone actually gone all the way with this idea and any advice would be appreciated.Mine is a 73 mgb ,was wondering if I will need to do tunnel mods.
Colin Walton

Sounds interesting but you would have to really get the adaptor plate exactly right. By the time you mess around with this I think you will spend more than the $400 odd that Dellow wants, and it mightn't work. It might be a whole lot cheaper (and much much easier)to buy one off Dellow Automotive in Sydney. I paid just over $400 for the bell housing and fork spigot etc. You really don't need his clutch gear (the supra slave is much too small) and the tail shaft yolk you should be able to do your self. With the speedo cable you would probably do better to go to a speedo cable place and get a ratio adaptor done at the same time. My W58 supra box worked out at almost exactly 0.70 ie when I am doing 100Km/hr the speedo shows 70.
I used a 1 inch diameter range rover slave which I bolted to a couple of handy protrusions on the bell housing with a small simple adaptor plate. The Range rover slave has the same thread pattern as the MG Hydrolic hose. It also gives the right amount of travel for minimum effort. If you don't already have a fly wheel you will need to have half an inch (alot!)machined off a range rover one, which means you must have the spring/oressure plate holes redrilled, which means you can use any spring plate you like (that will fit inside the bell housing, not the range rover one alas)

Not directly related to the topic above but a question on your Heritage shell : was it a local supply or did you purchase it dirctly from the UK?
I am looking at doing something similar myself but I was not aware of any sources fro the Heritage shells here in Victoria.


Cheers , Pete.
Peter Thomas

He got insanely lucky and bought it off a guy somewhere for a very good price. He's even got it set up on a rotisery so you can tip it upside down with no effort at all. Sooo jealous.

Peter, That makes two of us !!!

Thanks for the information.
The reason is given the extra power and / or increased cornering and handling most modifications generate perhaps the Heritage shell would be a better choice given the newer steel and so on.

Just a thought.

Cheers , Pete.
Peter Thomas

This thread was discussed between 03/04/2005 and 24/04/2005

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