Rebuilding the D-Type Overdrive Part 5 

Thank you!

Great to hear! Did you get it back in the car yet? Hopefully all is well! I have another video on building a gearbox test stand. With the difficulty involved in pulling the gearbox, it's well worth it!

@@midwestmotoring I will be filling it with oil and bench testing it first. I am hoping to get away with using my battery drill on the input shaft to test it. Failing that I will need to set up a test stand too. I'm not putting this suckered back in untested 🙂

A battery drill may not have the power to rotate the gearbox, unfortunately, though I have done it with a corded drill. You'll also need a 1/2 inch chuck (most smaller drills aren't going to fit) and a high enough RPM (the overdrive won't engage below about 1200-1500 RPM). Good luck!

@@midwestmotoring my battery drill has a 13mm chuck and is a top of the line model with high torque. It worked well for me. It high gear it spun at 3250rpm, slowed a little once in gear but was enough to get the oil pressure up and test the overdrive. All works well 👌. Got it all back in the car and wired up, test drove it yesterday and it was perfect. Thanks again for the great video 🙂

Awesome! Another car on the road!

Thank you! It started off as a covid project but quickly got popular.

That's great to hear! You do need some patience, a couple special-ish tools, and a shop manual helps too. But anyone can do the work on these at home as you can see. If your overdrive/gearbox are separate and you're doing a conversion... you'll need a main shaft, as the overdrive and standard shafts are different parts. Anyway, good luck! I'd love to see photos or videos once the gearbox is in.

Absolutely agree. I also fit a relay wherever one wasn't originally fitted.

And the gearbox still works!

Generally, bearings and gaskets. It's pretty common to need a mainshaft as well, since it gets chewed up in these. I don't remember what I spent, but I bet it was about $300.

That's true, but only before mating the overdrive housing and the output shaft housings together. Once the nuts are tightened down on the 2 halves, the spring pressure forces the sliding member to lock against the annulus--so it's impossible to move the 2 sets of splines in relation to each other. I actually prefer using a mainshaft since it fully mimics the part coming out of the back of the gearbox, but if you don't have an extra like I do then a long screwdriver is the perfect tool!

Just to clarify, it's not easy to use the mainshaft to align the OD splines unless the gearbox is apart. Otherwise, you're basically installing the unit and may as well keep going. But yes--if you don't have a spare main shaft like I do, it makes sense to rebuild the overdrive, then disassemble the gearbox, then use the mainshaft to line things up, and then reassemble the gearbox and mate the 2 together. I believe that's what you were driving at--just wanted to help clarify for the readers. In fact, you should disassemble both, then order parts for everything to save on shipping. Then do the OD, use the mainshaft to line it up, and continue reassembly from there.

Noted! I think I actually have a GT6 brake servo around somewhere, though it may be corroded beyond repair. I'll take a look and add it to the list. You CAN buy replacement servos, but they don't fit the same. In my Mk3, I removed the servo and installed a brake master from a Mk2 GT6. It stops just as well, and I actually prefer the firmer pedal feel. The servo was meant to have a more "comfortable" pedal.

No. You should use an oil meant for manual transmissions, like Redline MT90 (a GL4), or a 20w50. I use MT90.

Hi William! Eventually yes, but differentials require some special tools that can't be easily bought at Harbor Freight. A case spreader is required to disassemble the unit, and a special torque gauge is required when reinstalling the flange at the front end (a specific preload is required on the bearings). Depending on whether you have a collapsible spacer in your diff or if the preload is set by shims, you might be able to just zip off the nut, pull the flange, replace the seal, and reinstall. If you do have a collapsible spacer, some will tell you that you should count the turns of the nut and reinstall it... but that doesn't come with any way to measure whether it was in the correct position in the first place. If you're replacing the seal, chances are pretty good there's some wear to be found and it may need adjustment. Anyway, I don't have a diff video in the works yet but don't count it out. A case spreader can be built, and the same is probably true for the torque gauge.

Thanks for the advice. I have a later TR6, so unfortunately that would be the collapsible spacer. I understand that pulling the seal out the front can be a big challenge, and to your point you can’t be positive the preload will be correct even if the nut is repositioned exactly where it was. The leaking isn’t too bad yet (yes I also cleared the cotter pin vent). While it involves some risk, I intend to replace that seal while I have the diff down to replace the mounting bushings. Thanks again!

If you have access to an inch-pound torque wrench (a bar or dial type, not the clicking one), you can disconnect the drive shafts and set the preload with everything apart. The preload spec is the torque required to keep the pinion turning--not to start it turning. So you kind of need to get it moving a little and then take your reading while it's in motion. There's an art to it, but it can be done.

Make sure there's enough fluid. Also check the solenoid is adjusted properly. Just because it's moving doesn't mean it's moving the right amount. Finally, you'll need a pressure gauge to see what's going on beyond that.

Clever solution. I usually just open the nuts holding the case together a little at a time until there's no pressure on the springs, but either way works. My only caution would be that the replacement for those nuts are usually some kind of lock nut. Be careful not to loosen and tighten them too much or it might lose some of it's locking ability. Also, be sure to load the springs evenly so as not to bend anything internally. Thanks for the tip!