Changing to the 3.9 rear gear from an early MGB is a great solution, and over a year ago I purchased the proper rear gears on eBay. But it took until now to get Cor Engelen and Troy Nace to help me install it. The work began yesterday afternoon, when Troy and I removed the rear end from the car, leaving a pretty gaping hole:
The rear was moved to the workbench, awaiting Cor's arrival today. He has done this conversion multiple times, and his process differs from the extensively documented processes on the web. It's easier! Today we got started by disassembling everything:
In that photo, you can barely see the gear remaining in the housing, but it had to come out too, to be replaced with the one from the MGB. That led to a process of "assemble and try," repeated multiple times, until the clearance was exactly right. How fussy is it? Well, we had two possible shims that could be used alone or together, as well as using no shim. One shim measured 0.00095" and the other 0.00143". We tried various combinations and none was correct. So then, we carefully removed material by sanding a large spacer, removing minuscule amounts of material until it fit perfectly.
That gear meshes with something called a "ring gear," and we installed it in the other side of the housing:
Then began the process of making them mesh properly, and that's where Cor's method differs from the others on the web. It is brilliant in its simplicity. Cor realized that the unconsidered variable in the process was the gasket that lives between the two halves of the housing. Instead of machining spacers to ridiculously precise tolerances, you can control mesh by varying the thickness of the gasket.
We began with no gasket at all, and it was too tight. Then we used a stock gasket and it was still too tight. That gasket measured 0.035 mm in thickness - Cor is Dutch, and works in the metric system. In one day, he made me wish my machine tools were all metric!
We went shopping, and at Autozone found some gasket material in 0.06 mm. Seven dollars later, it was at Grant Street and Cor was making a custom gasket. The seeming explosion on the workbench is a clue - this project used a ton of different tools, and made a pretty big mess. Both gear oil and brake fluid were present, and leaked at every opportunity. The torch in the foreground was to heat components to expand them slightly before pressing them into place.
Thankfully, that gasket was the perfect thickness. We proceeded to reassemble the diff and return it to the car. Straightforward wrenching, but we even had to take time to use Cor's British Standard taps and dies to clean up some bad threads. Then we got everything back together:
Once completed, it was time for a test drive, and even expecting big results, I was dumbfounded. This has totally transformed Beck TD! In city traffic, the gear ratios are perfect. Third gear is great for up to, say, 35 MPH, and at the common dense-traffic speed of 45 MPH, I'm only going about 2300 RPM. Much more relaxing than the previous 3000+ RPM! Up on the highway, a comfortable 55 MPH (enough to keep from being run down) is only 2800 RPM. For the first time, I can drive Beck on Route 30 without being tense the entire time. My personal comfort level with that old engine will let me touch 4000 RPM briefly, and that is now 78 MPH - enough to get out of the way of a thundering 18-wheeler.
That's twice in two weeks that Cor and Troy have helped me transform Beck TD into a safer, more modern car. Thanks, guys!
Does this use the MGB spider gears and cage or retain the T parts?
ReplyDeleteThe only parts used from the MGB are the ring gear and input shaft with gear, plus maybe some of the shims, etc. The key is to have the early "banjo" style MGB pumpkin - that's the only one that will work. I believe an MGA gearset will work also.
ReplyDeleteIf you want to run wire wheels, then you can swap in an entire MGB rear axle assembly, but steel wheels won't work because MGB's are four lugs, and TD's have five. I have read that there is a modern four-lug wheel, something like a Nissan spare, that will work and looks very similar to the stock TD steel wheel when used with a TD hub cap.