901/911 trans

[gamin]

What special tools are needed to set up the differential on a 901/911 transmission?
And if needed, where can they be acquired?
Also, any special tools needed for changing a gear set?

[kevingross]

Gary, you need to set up two things. First is the bearing preload. A beam-style torque wrench, or something like it that can measure turning torque is needed.

The second is ring-to-pinion backlash / free play. You can make up your own tooling for this, with the understanding that you want it to lock to the differential spigot and not its nut behind the side gear. Or you can buy tools from the VW521 set from porsche.snapon.com, depending on which version of the 901 and its differential you have.

[gamin]

Many thanks. Can I assume from your answer that no special tools are needed for changing a gear set?

[Matt Monson]

Changing a gear set is pretty straightforward and other than a tool to lock the mainshaft from spinning, nothing special is required.

Setting a ring and pinion requires a pretty expensive tool and specialized knowledge, and generally is more cost effective to leave to a professional.

[kevingross]

Originally Posted by Matt Monson

Changing a gear set is pretty straightforward and other than a tool to lock the mainshaft from spinning, nothing special is required.

Setting a ring and pinion requires a pretty expensive tool and specialized knowledge, and generally is more cost effective to leave to a professional.

Sorry, I missed the gear set part of the question. Matt is correct, of course. I do prefer to use the P 260 plate when setting the forks, altho I know people who believe it's unnecessary. The tool Matt mentions for locking the mainshaft is helpful, but you can substitute the center steel section of an old clutch disc, clamp it in a vice as an alternative. And you will really need (or strongly benefit from) a hydraulic press: not crazy pressure, a 10 ton model will be plenty.

[targa72e]

Lots of good information on rebuilding 901 transmissions that can be found on Pelican and 914world. Since the 914 also uses the 901 transmission most of the information is the same. There is a poster on 914world called Dr Evil who has made a video that covers disassembly/rebuilding and tools required that can be made or purchased (he also does rebuilds) but does not cover differential set up. There are also several threads on 914 world (as well as pelican) that cover the same. good luck.

john

[Walt Fricke]

Matt and Kevin have this stuff down pat. But I'm not sure whether you (and they) are referring to setting the ring and pinion depth (which is necessary when you change those and tricky, with a quite expensive tool pretty much required), or just changing the differential (like installing a limited slip).

For diff changes, what you are doing is trying to place the ring gear mounting flange in exactly the same side to side position as it used to be. That way the ring and pinion gear teeth are where they used to be side to side (replacing a diff doesn't change the fore and aft dimension).

Side to side is set by the shims on the bearings on both sides. To start with you have to establish the pre-load, which is determined by the total width of the shim stack, and this will change only if the new diff is dimensionally different. You do this without the ring gear attached. This is where an inch ounce (or metric equivalent) torque measuring device is needed, and is something otherwise of no use to most of us. You can fake it - an experienced tranny repairer might be able to come close just by feel. You can use a weight on the end of a string to approximate it.

You might not have to change any shims for this, which would be good, because changing is a PITA without a specialized tool for dealing with pulling the bearings (though you can use a standard two jaw puller and some cleverness). On the advice of an experienced mechanic, I took a used bearing and honed its ID enough that I could get it off and on with my fingers. This seems to work if you are using the same brand of bearing. In addition, it isn't guaranteed that you can get what you need by moving the shims you have around. Shops have a bunch of the various thicknesses of shims on hand to deal with this.

You can approximate the preload by leaving off the O ring from the cover plate, and measuring how proud of the case the cover stands. There is a dimension for this (0.15mm). Normally used as an approximation to get a start.

And once you establish the shim stack total, you aren't out of the woods. Next involves putting the ring gear back on and checking the lash between ring and pinion teeth. If you are super lucky, perhaps none of this has changed. But you have to check. Here is where you can come up with your own tool. You are measuring the number of degrees of lash, basically. The manual shows how the factory did it, and it turns out to be a distance of movement at a radius. But you can calculate what that movement is at other distances.



But this, too, can be time consuming, since any swapping of shims requires pulling the bearings.

You wouldn't start this process without a more specific guide, like the factory manual or some similarly illustrated document. But you can do it without the expensive tools. And you can see why many prefer to have it done for them. No other aspect of ordinary transmission work other than setting the pinion depth requires this kind of precision measurement.