Solving the Differential Equation

The car is down to the bare frame and I’ve needed to make some fundamental decisions about the rear suspension setup before I send it out to be sandblasted. This process had been brought to a screeching halt because I had not been able to positively identify the vintage of the differential. When I bought the project car I knew it came with a roto-flex setup, but I did not know if that had been scabbed onto a Spitfire differential or if it was on a GT6 differential. Having figured out that the chassis and frame were likely a matching set from the factory, if the differential was also from a Spitfire I had been inclined to toss (read: sell) the roto-flex system and go back to the swing-arm suspension with a camber compensator. Even though that was not the best design in the world, it would make the car more true to its original setup. In addition, that would save me the trouble of making sure I had correct brackets on the frame for a GT6 rear suspension.

Having pulled the differential apart and confirmed it was a 3.27:1 gearing, I was confident it was indeed a GT6 rear end based on the research of Paul Tegler ( But when I started looking at the parts catalogs to order parts for the rebuild, I saw there were two different versions of that rear end (–Triumph-GT6-Rear-Axle-Differential–m-689). That put me back to trying to find a positive ID for this part.

There were some pretty obvious numbers and letters cast into the case, but those lead me nowhere. I had tried to find any other identifying marks on the case, but was not making any progress. I tried to remove the pinion nut to see if there may be some markings between the case and plate bracket, but did not have the right socket so I let that idea go in frustration. I went back online to see what I could find, and located this diagram from Rimmer Brothers:


I looked back at the differential and saw absolutely nothing in the location referenced in this diagram. I scrubbed that area for a couple minutes with a brass wire brush and the flat of a screwdriver, and managed to see one faint straight line that didn’t look like a natural corrosion line, so I added some apple cider vinegar to my toolkit and went to work with elbow grease.  I painted vinegar on that area, scrubbed with the brass wire, scraped with the screwdriver and repeated for about 30 minutes. Slowly but surely a couple more faint lines started appearing, so I then used some dentist-like pick tools and ran them over the faint lines, repeating the process over and over and over.

After all that work, I found some faint stamp lines that had not been obliterated from 40 plus years of being under a car and exposed to the elements:


The stampings are still tough to make out from a photograph, but they say “KC1348”. I could not find a fifth digit, despite even more scrubbing. Based on this stamp, this differential came from a 1966-68 Mark I GT6. But roto-flex was not used on the Mark I cars, so I truly have a hodge-podge of pieces from different cars of different years.

I am going to submit this to the wise folks at and get their input on what rear suspension setup is best for the long haul and make a decision from there. I don’t like getting bogged down, but I also want to have a solid setup when I plop the chassis back on the frame.


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