Differential Oil Change

I am grateful to David Thrower for his detailed technical advice on this task. He also designed the tool to fit the drain plug on the RM differential.

It easy to ignore the differential but the recent experience of an RM owner faced with a bill of well over £2,000 to remove, repair and refit a differential that had broken teeth on a crown wheel is a salutary reminder to check the oil level from time to time and change it if it is old. Many new owners will have no idea how long it is since the differential oil was changed and since draining and replacing the oil only costs about £35 it is hard not to justify the investment.

You will need a hexagonal headed tool to fit the brass drain plug at the bottom of the differential. David Thrower has very kindly offered to make his amazingly useful tool available and if you would like to borrow it, please email him. The drain plug should be wired in place and you will need to cut and remove the wire with piers.

With this tool and a good deal of leverage and elbow grease the drain plug can be loosened. Drain the old oil into an old washing up bowl. It will take a good while to stop dripping. Carefully sieve the old oil to make sure there are no worrying bits of broken metal in it before pouring into a container to dispose of thoughtfully. I drained 2.2 litres out and was relieved to identify no foreign bodies. Feel round inside the bottom of the differential before replacing the draining plug and tightening it. Don’t over tighten or you will strip the brass thread. Re-wire the drain plug carefully to prevent it coming loose.

Now unscrew the brass filler cap nut and clean off the old thick greasy oil from the walls of the filler pipe. Refill with EP80/90 GL-5 differential gear oil. (Easy to get, as it is the same as used in car differentials and costs about £7 a litre). The litre containers have a flexible spout making it easier to pour it in. It is very much easier if you warm the oil first. My diff took 4.6 litres to fill it up to the point when the oil starts to overflow from the filler pipe.

Replace the brass filler nut and tighten. You should check the level again after a short while and then at regular intervals.

If you do have to repair the differential Queensbridge PSV Limited are very helpful and have the necessary skill and experience. They are also expert at RM gearbox repairs.

Please email me with any corrections and additions to this article.

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