I have just managed to successfully bench test an M41 gearbox and thought others might benefit from the experience.

History: I recently bought a 1971 142E that had been parked up since 1987. The main part I wanted was the overdrive gearbox. After buying it, I tracked down the original owner to see if they could tell me what the OD was like. He was a bit vague but was fairly certain that the OD didn't work and that the engine was seized when he sold it. I managed to get the engine running but would have had to fix the leaking water pump before I could road test the overdrive. I had read of someone bench testing one with an electric drill so I thought I'd give that a go.

The first step was to give the box a good clean. I covered the gear lever hole and used a can of degreaser and a brush and then hosed it all off. I fitted a steel plate to the two holes that the gearbox rear mount bolt to and clamped this plate into a large vise on a solid bench. I filled it with oil again (having drained it under the car to avoid tipping it out when removing the gearbox), then wired up a battery so that I switch on the solenoid.

The hard part was finding a way to drive the input shaft. My electric drill has a 13mm chuck so it wouldn't open it's mouth wide enough to clamp onto the input shaft bearing stub. My first attempt was to fit a piece of round bar to the electric drill and make up a coupling using a short piece of heater hose and two hose clamps. This allowed the gearbox to be motored over slowly but it would just slip when I tried to make it go a bit faster.

I figured it needed a more serious attachment so I machined up a steel socket with a 13mm shaft at one end to fit the drill and a 1" hole at the other end to fit over the input spline. I drilled two 1/8" holes about 1" apart on a line parallel with the axis of the coupling. A 3" nail with the head cut off and bent into the shape of a staple was then fitted to these two holes from the inside. This gave a very effective key that would engage on one of the spline grooves.

I then found that my 500W electric drill was not powerful enough to motor it over any faster than a crawl. I was planning to measure the output shaft speed with a speedometer - but this would rely on the input being driven fast enough. With the electric drill going so slow, I couldn't run it for long before overheating.

In the end I fitted my special driving socket to a carpenters hand brace. This worked great. I measured the output shaft speed by clamping on a piece of wire that would strike a piece of sheet metal each revolution. I had also fitted a pressure gauge to the plug hole near the solenoid so I could see that the hydraulic pressure climbed from 20psi to 500psi when engaged. It took about 30 seconds of manual cranking to get it up to pressure. To make sure the OD was working, I had my wife counting the output shaft revolutions while I counted the input. In 30 seconds I had cranked 64 revs and the output had done 80 revs. This is the ratio that would be expected with OD engaged (64/0.8=80). The OD could be engaged and dissengaged while cranking by switching on and off.

Of course such a test wont tell me that the OD is 100% OK. It might have faults that will only become apparent under higher speed and load. It does however give me some confidence that it might work when I have cut down my drive shaft and fitted it to the car.

I'd be interested to know if others have had similar experience and if there are any other tricks I should know about.