Rover V8 to MT75 Gearbox adaptor

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I’m one stage further with the Rover V8 to Ford MT75 4×4 5 speed gearbox conversion.

I was always worried about the accuracy of this conversion but I’m now happy for it to go off to be TIG welded.

Having tried once before with a wooden jig (duh!), I decided to make something a little more solid and less prone to flex or distortion. During TIG welding the whole casing will get incredibly hot and everything that can twist or buckle will.

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This all started with finding a BW35 bell-housing from a 1967 Rover P5B. I could have used a T5 4×4 gearbox and a bell-housing from a LT77 Manual SD1 but I’m using a J.E. Motors Rover V8 and there was some question as to whether the T5 was up to the job. I currently have an MT75 box that I cut in half and a rather rare bell-housing that I also cut holes in.

This is how I made my steel jig for holding everything square and solid during welding.

MT75 to Rover V8 conversion.

The brass bush in the end of the crank is designed for an LT77 gearbox. I will remove this and fit a roller bearing similar to one that Ford use.

Firstly I crudely cut a piece of 3mm steel plate the the right shape and put a rough hole in the centre big enough to clear the centre of the crank.

Rover V8 to MT75 gearbox adaptor

To get the drill holes in the correct places, I place the bell-housing on top, then sprayed red paint through the holes. I then centre punched the centre of the resulting red dots and drilled them to the correct sizes. The two important holes are the dowel pegs. I didn’t drill these out all the way. I drilled slightly smaller, then hand filed the holes to the correct position and size. This was probably paranoia as all the other holes turned out bang on.

Rover V8 to MT75 4x4 gearbox adapter

With the plate secured to the engine, I then very lightly turned out a tube to fit tightly over the round boss on the end f the crank shaft. This was placed over the boss and the plate scribed with the correct hole size for the plates’ centre.

Rover V8 to MT75 4x4 gearbox adaptor

Once the plates’ central hole had been carefully filed out to the correct size, the tube was pushed as far as it could go over the crank boss and tack welded into place.

I then moved to the bench and with the tube 100% square in all directions, I MIG welded the tube 360°. The tube is pretty solid with very little flex, but this is while cold, so I will add some bracing soon to ensure as little flex as possible. I don’t want this to twist or distort with the heat from the TIG.

Rover V8 to MT75 gearbox adaptor

Luckily, by some fluke, the boss on the centre of the Rovers’ crank shaft is the same diameter as the base of the shaft coming out of the MT75. This means the tube sits very snuggly over both. When slid over both, the plate should in theory be level in all directions and have very little flex. I left plenty of spare on the length of the tube, so now it needs cutting down.

Rover V8 to MT75 gearbox adaptor

The plate now sits snuggly on the bottom of the shaft and against the bell-housing. I was actually extremely surprised that I had somehow, incredibly managed to file the cut-off MT75 virtually perfectly flat. It was one of those smug moments. Nobody was around at the time, so I’m ‘blog bragging’.

Rover V8 to MT75 gearbox adaptor

The MT75 output shaft doesn’t touch the tube all the way up, as a portion is splined. It have solid contact with about 3 inches of the shaft, which should be enough. Even without bolts, the bell-housing now only moves with the add of a mallet. However, I made an extra plug on the lathe to sit in the top of the tube. I welded a piece of scrap to it so I could grip it whilst pulling it out. Taking pliers to the outside of the plug would rough the surface and I’d never get it in and out. My original intention was to weld it in place, but the plate might have been troublesome to get off. There isn’t much room for distortion, so even a small amount could seize this plate into place, the more bits it is, the easier it might be to get off – hopefully?!

Rover V8 to MT75 gearbox conversion

I could have made custom pegs to sit in the mounting holes, but I cheated and used the shafts of and handful of drill bits to get the correct rotation of the plate before I bolted it down tight. You’ll have to trust me when I tell you that you could now pick up the gearbox using just the bell-housing but this adaptor plate is so snug, It shouldn’t be a challenge. A bit of light oil cures this, but this oil will boil off during the TIG welding process.

All that is left is to fill up a few holes in the bell-housing. I don’t want the clutch to slip every time I go through a puddle. Shoveling mud out of the bell-housing is not going to be on my maintenance routine. It is not obvious in the picture below, but there are actually 5 little extra pieces of aluminium to be welding in place. Once this is done, with the clutch boot in place, it should be much more resistant to the elements.

Rover V8 adaptor plate

This is not your conventional Rover V8 adaptor plate as it will end up in the scrap bin once ever thing is welded together, but I reckon this is the best way of doing it (I think).

The next challenge is TIG welding cast Aluminium. Watch this space.

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