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Filling the limited Slip Differential (LSD) with Oil

Updated: Jan 21, 2019

Fitment of the Limited Slip Differential (L.S.D.) was one of those tasks which occurred some weeks back and wasn't blogged. I may in time go back and detail the process although it's been documented by various builders in other blogs and to be honest I didn't really learn anything different, so there will be limited new content.

At the time I didn't fill the differential with oil as I then needed to fit the drive shafts and so forth, however having completed those tasks and given I was thinking about proceeding to trim the boot I thought it was about time the oil went in. Although you might be wondering what the heck has boot carpet and trim got to do with diff oil?

The trick to filling the diff is to use gravity, pour the oil from the boot which requires removing the boot floor, and so that is exactly what I did.

The boot floor is built from two sections, an aluminium section which sits over the fuel tank and plywood section which basically sits in front and over the diff. Both sections are attached to a metal bar which runs across the length of the boot and each are attached using some retaining screws. Removing the plywood section is as simple as removing the 4 screws which secure the wood.

Caterham 7 S3 420R boot floor
Removal of plywood boot floor section, 4 screws to remove.

After removal you can see straight down to the differential, brake lines and handbrake cable. Having removed the plywood I came to the conclusion it would have been beneficial to have done this during the differential and rear brake line install, the additional light below would have been worth it alone!

Caterham 7 S3 420R boot floor
Plywood removed, access to differential and brake lines.

Now that I had access to the differential I needed to construct something to support and hold the oil and a vessel through which the oil would flow. My solution was as follows;

  • x1 Large funnel

  • x1 Fuel hose off cut from the kit, around 5cm

  • x1 Jubile clip

  • x1 Gaffer tape

  • x1 Clear plastic hose, around 2m sourced from B&Q

  • x1 Camera tripod

Basically I inserted the funnel into one section of the fuel hose, then the plastic tubing into the other end. I used a combination of the jubilee clip and gaffer tape to seal. The construction was then supported and taped upright to the tripod which was carefully placed into the boot. The tubing was then fed down through the boot. The large 14mm hex plug was removed from the rear diff plate and the plastic tube inserted. Oil was then poured into the funnel and left to flow down into the differential.

My kit was supplied with the following differential oil for the LSD which I believe to be a BMW unit. The differential took the entire 1litre of oil.

Caterham 7 S3 420R LSD Oil
Mutual Gear Competition SAE75W140 Synthetic

Once complete the diff fill plug was reinstalled, the home apparatus removed and the boot floor reinstalled. Simple.

The diff fluid as always is a stinky thick oil and takes time to run through the pipe. I think in all it took about 2 hours to pass the entire 1 litre of fluid.

I've used an oil pump solution in the pasted on other cars and still managed to get oil everywhere, this solution was brilliant and resulted in zero mess. I would highly recommend this approach.

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