The gearbox is welded and fitted!
The MT75 4×4 gearbox has been TIG welded to a 1960’s BW35 auto bell-housing from a P5B Rover Coupe.
The welding was tricky but went very well in the end and from some angles it could be mistaken for a factory built box.
Welding cast aluminium is always going to be more challenging than new steel. As the aluminium ages, oil and dirt penetrate into the surfaces pores and simply cleaning it with a bit of engine degreaser isn’t going to work.
The aluminium has to be heated up so that the dirt and grease boils and bubbles out of the pores. As I found out,, cleaning the surface to be welded with a copper wire brush can leave impurities on the surface too.
Several attempts were made at this welding. A good weld could be achieved on both halves of the box, but as soon as the two were welded whilst butted together, the resulting weld was a black, brittle mess.
This is how the brother in law, finally got a good weld:
- Both halves were stripped back and cleaned with a stainless steel wire brush
- Both halves were pre heated with a oxy flame.
- The box and bell-housing were inverted whilst centred on the special plate I made.
The following set-up was used:
- 200 amp square wave TIG (water cooled torch)
- Green Pure Tungsten 2.4mm tip
- 3.2mm Silicon Ally Rods
The welding process actually put a lot less heat into the gearbox than expected. The heat dissipated throughout bell-housing and returned to ‘touchable’ very quickly. The box didn’t get any hotter than a good thrash around the block. Once the whole process was finished the special plate was lifted off with little effort, prooving that nothing had moved or distorted during welding.
The finished weld has been lightly sanded around the clutch lever area to allow the boot to sit tight and flush but apart from that, this is how it came back. 🙂
To get the box to fit, the bronze bush in the centre of the Rover’s flywheel had to be removed and replaced with a roller bearing. This is because the shaft on the LT77 gearbox normally fitted to the Rover V8 is about 2.5mm smaller than the Sierra 2.0lt Cosworth gearbox. The MT75 shaft would not fit inside the standard Rover bush / bearing.
Luckily, the special plate that was made for centring the bell-housing onto the gearbox must have been absolutely spot on. The MT75 gearbox simply slid onto the Rover V8 engine, with only a few taps of the rubber mallet. No different than any other off the shelf box.
Seeing the box finally in place really made me happy. Happy and relieved.
The whole assembly is a little close to one cross-member, but I knew this was going to be the case. Moving a cross-member 5mm isn’t the end of the world.
Now I know the angles that the box has to be lifted at to get it to fit into the tunnel, I am now free to add several small bracing tubes that will complete the central back-bone of this chassis. Although they weigh less than a kilogram, the strength they will give will be huge. I’ve added quite a bit of extra triangulation and bracing to the tunnel of this Haynes Roadster. In fact, the tunnel now provides a very significant proportion of the overall strength. Early FEA results showed flex had been more than halved.
The rear of the MT75 now sits on the standard MT75 gearbox mount and a 8mm cross bar. This means all the blocks of wood and axle stands could be removed from under the both the engine and gearbox. This is a massive step forward for me.
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