Having figured out the vintage of the chassis, frame, engine and transmission, the last mystery was the differential. I had not been able to get a good calculation of the gear ratio by spinning the pinion, so I cleaned it up and brought it inside to have a look at its insides.
Once I had the case apart, I inspected the gears and everything looked fine, although there was a smell of burned/old gear oil that I hadn’t smelled since I was working on my Capri in high school. I counted the ring and pinion teeth, which had 36 and 11, respectively. Looking back at the reference guides, this could only have come from a GT6. I still was unable to find any meaningful stamps on the differential to tell me what year of GT6 it came from, but it confirmed that the differential, along with the half-shafts, did not come from a Spitfire. With that information, I think I’m going to bite the bullet and go with what I have. This will require the addition of a frame bracket to accomodate this suspension system, but I can order those and weld them on with not too much trouble.
Here is a picture of the differential after being opened up:
Considering the smell of the inside of the differential, I think it has never been rebuilt and I’m assuming it’s 40 years old or more. As a result, I am going to take the time to completely rebuild the differential with new bearings, seals and shims. It needed new seals at a minimum based on the buildup of gear oil on the case.
Over lunch today I picked up some steel stock to repair the notch near the transmission mount on the frame. I’m going to have a friend shave it down from 0.1875″ to 0.150″ to match the thickness of the rest of the frame and then go to work on tacking it into place and cutting it down to fit the correct contour. Using a piece of folder paper, I traced out the right side of the frame and matched it up to the left side so I know what the new piece needs to look like. Here are a couple pictures of the cleaned up frame area and tracing of the paper:
I’m going to get this piece welded in and then add the suspension mounts for the GT6 rear end. Once that’s all done, I will send it out for sandblasting and then prime and paint it myself. I have decided not to go with powdercoating for reasons that will probably get explained later.
The process of figuring out where these parts came from has been a pain, but at least I know what I’m dealing with. The combination of the bigger displacement engine, four-speed transmission and relatively low rear-end gearing means this should not have neck-snapping acceleration, but should be a blast on the highway.