MG Bellhousing Repair

Vintage MG preselector gearbox bellhousing
Mounting tube clamps
Broken tube clamp
bellhousing with steel tube inserted awaiting repair sleeves to be soldered
Repair sleeves
New sleeves soldered on and clamp bolt holes and features remachined

I was asked by Alan, my customer, who restores vintage MG cars (made around the mid 1930s) to see if I could develop a repair strategy for this bellhousing casting.

The bellhousing is the component, usually a casting, that connects the engine to the gearbox. Usuallly the clutch and engine flywheel are to be found inside the bellhousing. However, this particular car is being restored to the racing specification and is going to use a preselector gearbox. This type of gearbox has no clutch pedal as a manual drive car would have. Gear changes are made by just directly selecting the gear you want using a lever on the side of the gearbox! The gearbox inside has alot in common with a modern automatic car gearbox. This type of gearbox, known as an ENV 75, allows faster gear changes during racing.

The bellhousing was specific to MG, but the ENV 75 gearbox was used on Crossley, Lagonda, Alvis and Riley. Each car maker used a gearbox bellhousing suitably designed to mate with their own engines.

 

Back to the plot

The history of the bellhousing spoke for itself. You will notice that the bellhousing has a cast in tube with a clamp at either end. This takes a steel tube which mounts the engine gearbox bellhousing etc when they are assembled. The steel tube is attached at either side of the chassis. The photos show how the tube clamps had cracked, one piece had been lost altogether. Some welding had been carried out in the past but this had not succeeded in solving the problem. 

 

How to repair the bellhousing

Firstly, aluminium is difficult to weld as, although its melting point is about 650ºC, its oxide, which forms a thin layer on the surface, melts at just above 2000ºC! In practise this means the metal melts but the oxide gets in the way of making a joint. This oxide is the reason why aluminium was once more valuable than gold as it was so difficult to extract from its ore. Compare welding aluminium to welding steels which melt at about 1500ºC, the oxide having a similar melting temperature. Although the melting temperature is very much higher, steels are really easy to weld together. 

Alan and I  looked at a number of repair strategies, steel clamps, bolting the steel tube directly to the casting etc. I had had some success repairing aluminium with a soldering technique. I suggested soldering on repair sleeves to replace the cracked and missing cast in clamps.  The casting was prepared by cutting away the broken clamp and skimming the outside of the cast in tube to accept the new sleeves. The pictures show the sleeves and the eventual result after the clamp features had been remachined. 

The repair 'looks the part' and strengthens the cast material right where it is needed.