Hmmm that pic looks like something blew apart inside the differential. I've seen something similar when people rev up the engine and then do a "Neutral Drop" another words revving the engine and then throwing the transmission into Drive or Low ! If the U Joint holds and the tire has good traction . . . . . well the next weakest link is the differential gears! Years ago a friend was doing "Neutral Drops" and suddenly there was a loud bang and the car started drifting backwards . . . . even though it was still in drive! LOL LOL. When we looked underneath the car the inspection cover on the differential had outward dents in it . . . like a grenade had gone off inside it!!! Funny as all hell. When he / we took off the inspection cover multiple pieces of gear fell out in many sizes! He trashed the rear and went to a local salvage yard and picked up a new one. Lesson learned! LOL
So I'm sure this is not the case with the "Mini Cooper" but is there an explanation as to why it broke apart???
Billy
Since you asked, it is hard to explain. If you look at the picture, you will see that the spider gear pin ends are worn out to about half of its original diameter when the pin failed with a lot of clunking and loss of power .
But there are some contributing factors. From what I have learned during my years in theminiforum.co.uk, the oil type is probably the most important. Since the classic mini shares the engine oil with the "gear box" and differential, the oil type has to be a compromise that can do all three lubricating functions. Since the engine has no roller tappets, its adviced to use an oil with high content of zinc to reduce wear. Most oils do not have it any more because that affects the catalytic converter life. The transmissions and differentials need a viscous oil (engine oils tend to get thinner and thinner with the newer vehicles) to lubricate and handle the pressure between the parts in contact like gear teeth but at the same time be thin enough to lubricate bearings and bushings at high speed and provide cooling.
I have no idea what the previous owner had used but the engine and transmission inside looked fairly clean. I used Mobil 1 once. There is a story going around about somebody racing classic minis here in the US had started using Mobil 1 and then he started blowing differentials left and right.
There is another story that says that the later models of the mini built had the differential pin made out of chocolate.
A newly refurbished transmission (gear box) with a hardened differential pin from guess-works.com (the owner is John Guess) and Valvoline VR racing non synthetic 20W50 oil appears to have been the solution.
This is what the beast looks like after putting it together:
