Four-Speed Transmission
I bought this transmission to use in a seminar given by Mike Lesniak, AKA Dr. Evil, the uncontested authority in rebuilding early Porsche transmissions. Unfortunately, I got sick the day of the seminar and couldn't attend, so I was left with a good but imperfect box taking up garage space. Between Mike's video and other information I've scrounged up, I've been able to rebuild several of these things; the time has now come to deal with this one. As it happened, the box didn't need a full-up rebuild, but I did pull it apart and got everything right. It should be a fine transmission at this point.
It's worth mentioning that many Porsche transmissions sold as "rebuilt" are not really what you might expect. Replacing all the synchros, bearings, sliders, and seals in a four-speed costs about $2500 and, in a five-speed, $3500 (2021 costs). If all those parts were replaced, a transmission rebuild would have to cost $4000-$5000. Most owners would not be willing to pay that much. With such high costs, most rebuilders retain all usable parts and often replace worn parts with good used ones. In fact, many boxes are sold as "rebuilt" when only the synchro bands have been replaced. In my opinion, that's not good enough, but it is the common and inevitable result of the high parts prices.
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Here's the transmission. I've already started disassembling it.
The important markings on the transmission are shown below. The first picture contains a date code, 41/66, showing that the case was cast in the 41st week of 1966. The other number is the Porsche part number for the case, which many people wrongly assume to be the transmission's part number. The next number, 902/0 stamped in the lower rib, shows that this is an early, aluminum-case four speed, with the standard gear set, A-H-Q-X. The last number, also on the lower rib, is the serial number.
First thing to come out was the differential. The inside of the case actually was pretty clean. Still, I gave the "pumpkin" a good cleaning, to remove old oil.
The diff looked good. The numbers stamped on its edge identify the pinion and ring-gear combination (they are always made as a pair) and some setup info.
The reverse-gear slider is located under the tail cone. It also looked very good. The reverse idler was also in good shape and was reusable, despite a little roughness from "oopsing" reverse gear.
The tail cone was stained internally, but a little solvent cleaned it. Same story for the intermediate plate and its components.
The internals are attached to the intermediate plate. Once the tail cone is off, the whole thing simply is lifted out.
A first look at the synchros. They were almost like new. The bands had no polishing or grooves and the teeth were sharp and undamaged.
It was clear that I didn't need new synchros or, as it happened, sliders. I did choose to switch the 4th gear band with the 1st gear and the 2nd with the third. This should give the bands a little more life, as the higher gears wear less than the lower ones. I also switched the sliders. I left the synchro hubs ("dog teeth") in place as they showed no visible wear.
The gear assembly went back together uneventfully. I made a special tool, shown in the second picture, to lock the output shaft while torquing the bolt that holds it all together. I also made a special offset socket so I could torque the nut on the main shaft. Don't freak out; I did support the intermediate plate while torquing the nut! Also, the teeth of the vise-grip pliers fit the spline perfectly, so they won't do any damage.
I discovered soon afterward that it all goes together more easily if you wait to tighten the main-shaft nut until the output shaft assembly is installed.
At this point, the gear assembly was back together. You can see the bands, teeth, and one of the sliders. The teeth on the synchro hubs look blunt in some pictures, but that is because the tops are beveled. They all were quite sharp.
It is critically important to adjust the slider positions correctly. That requires the special fixture shown below. The fixture is also useful for holding things in position during other stages of assembly.
Now, the fun part: setting up the differential. Usually we assume that the original setup is still good and try to retain it. In this case, however, it was clear that the differential setup wasn't right. I'm not sure what caused it to be off; often, that indicates a bad bearing, but both bearings looked good. In any case, it had to be fixed.
Porsche made a special tool, the P258, to set the pinion depth. Used ones are occasionally available, but, if you can find one, it will be stunningly expensive. Fortunately, it is possible to set up the gears using gear-marking compound. Just paint the stuff (which is actually a colored grease) onto the teeth of the ring gear, put it all together, and turn the output shaft a few times. The pattern made by gear contact is then evident.
Once you have a pattern, it is a simple matter to correct it for good contact, at least if you have a special line to the Oracle at Delphi. The Oracle will tell you what spacers to insert under the ring-gear bearings, and you can fine-adjust the pinion position with the thickness of the gasket under the intermediate plate. This is somewhat easier than in other cars, because changing the spacers in this transmission is relatively simple. However, the adjustment for the ring-gear preload and position interact somewhat with the adjustment of the pinion depth. Working systematically, though, it isn't too difficult to get it all dialed in, but it is a little tedious. The first picture shows the contact pattern on the drive side of the gear, and the second picture, which is unfortunately a little hard to see, shows the coast side.
These are about as good as it gets. The mesh is probably not perfect, but it often can be virtually impossible to get ideal mesh with a used gear set. Even small amounts of wear can prevent that.
With the differential removed, it is easier to see the markings.
I made a few new spacers from brass shim stock, since those spacers seem no longer to be available. Stainless steel is usually used, but brass is easier to work with. I've used brass spacers in differentials before, and they have always worked well. I made a wood fixture to hold a square piece of the shim stock in my milling machine, and I ground a special tool that fit into the fly cutter and cut out a circle. I trimmed the outer circumference by hand.
Just for fun, here are some other tools I made for working on Porsche transmissions. I got the idea for these from Mike's video. The top item is a shaft-locking tool for five-speed transmissions; it is made from a worn-out first-reverse slider and a length of 3/4-inch square steel tubing. On the bottom right is a similar tool for four-speeds, shown in use earlier. To its left is an offset socket, also shown in use above, for torquing the main-shaft nut; the last item is a super-long socket for the same nut. It is made from a 32 mm deep socket, cut in half and welded to the ends of a steel pipe.
Now to finish it. First, I installed the seals and gaskets. The case has four important seals: the two output flanges, the input shaft, and the gear-selector rod. All were replaced, of course. I had determined that the thickness of the intermediate-plate gasket should be 12 mils, but the closest I could get was a 5 mil and an 8 mil, for a total of 13 mils. They are just paper, though, and can be expected to compress a little.
It's worth noting that my gasket set included only 8-mil-thick gaskets! It seems inevitably necessary to make some by hand. I did that by scanning a gasket, printing it on various kinds of paper, and cutting it out with a craft knife and scissors.
The intermediate plate and gear assembly were installed, along with the reverse-gear slider and shift rod. The reverse-gear idler is not shown in the pictures, as it must be installed with the tail cone. The thickness of the gasket on the tail-cone side of the intermediate plate is not critical.
Replacing the seal and o-ring in the speedometer drive is important; they are a common source of leaks. The seal is mounted in the brass piece. In the photo, it has been replaced but is hard to see.
Finally, it's all back together. This will be a nice box for someone.
