Ring Gear Swap Procedure?

Wrecked944 · 08-03-2002, 12:42 PM

Damn, damn, damn...I am trying to remove the 8:31 ring gear from my stock 911SC differential so I can install my new, sexy 7:31 ring gear. And Herr Bentley has totally let me down. The section on differentials is pathetic. Checked Haynes, even 101 Projects, printed off the Pelican exploded diagrams. No help. So I broke down and ordered a complete set of factory manuals. But that was last week and still nothing has arrived. Grrrr...so does anyone with the manuals know how to 1) Remove the stock ring gear currently blocked by the speedo ring (I assume I must somehow remove the speedo ring) and 2) What the approved procedure is (including ring gear bolt torque specs) for installing a new ring gear to an SC differential?

Damn, I already owe you guys more beers than the entire annual consumption at OktoberFest...

Jim Sims · 01:20 PM

Janus, When you get your factory shop manuals you'll read that: 1) the pinion shaft and ring gear are matched, 2) that they have been run together to determine the correct position of the pinion gear for minimum noise and that this distance is marked on the end face of the pinion gear, 3) to obtain this position shims are used where the pinion or output shaft bearings are bolted up to the transmission (differential or final drive) housing, 4) due to inherent manufacturing tolerances in the tranmission housing the position of the face of the pinion gear has to be measured to determine the shim thickness using a dial indicator in a rig that mounts in the transmission housing and emulates the final drive; this rig is rather special in that it allows the transmission housing side cover to be put back in place and still allow one to read the face of the dial indicator, 5) there are also shims under side bearings of the final drive that have to be varied to adjust the ring gear plus the differential drag measured with a torque wrench and these same shims varied to adjust position and drag to within tolerances. To do this swap properly you'll need this tooling or good substitutes; if you're going ahead with the swap I'd find a specialist with the tooling to do the measurements and select the shims. The speedometer magnet disk is under the tapered rolled bearing. The tapered roller bearing is removed using a two arm puller reacting against the end of the differential through a pusher (to avoid damaging the end of the differential). The arms of the puller go through two enlarged holes in the magnet disk and must pull on the inner race of the tapered roller bearing (not the cage and rollers!). The arms and tips of the arms of the puller may have to be custom ground or machined to fit. Also, the keepers or locks onth ring gear mounting screws are one-time use parts and should be replaced if you swap the ring gear. Not what you were hoping to read I'm sure. Are you sure you want to change the ring gear and pinion? Good luck, Jim

Wrecked944 · 08-03-2002, 02:08 PM

Wow, thanks again. Jim, where did you get all of your insane amounts of tranny knowledge? Just one rebuild?

In theory (famous last words, right?), I should be in good shape regarding the shims. I have taken my 7:31 ring and pinion shaft from a complete 1974 donor tranny. So I have both the ring gear, pinion shaft and original shim pack to go with the matched set.

That speedo disk is troubling, though. Crud. Not sure how I'm gonna get him off without massive casualties...

Jim Sims · 07:51 PM

It isn't enough to just move the pinion shaft shim with the other parts as that shim was selected to match the pinion shaft, ring gear and the transmission housing (differential or final drive housing). If you move everything to another transmission housing the pinion shaft position will have to be rechecked and perhaps a different shim used. Plus there are shims under the differential tapered roller bearings that again are selected based upon the fit/tolerances/drag of the ring gear, differential, the tapered roller bearings (including their races) and the transmission housing. You may get lucky if you just move everything but I bet instead you'll end up with a noisy differential which will be more prone to wear. Cheers, Jim

Green 912 · 05:05 PM

Jim Sims speaks the truth. The ring and pinion have to be set up to the case. I do my own trans work but leave that one part up to the pros. The tool required to set the distance is way expen$ive (over 1K) and the procedure is a pain. Take it to a GOOD Porsche trany shop and let them set it up for you.

Or just get some smaller diameter wheels and tires...

Wrecked944 · 05:36 PM

Well, not to start any controversy, but I was operating on the advice of John Walker who wrote in a previous thread:

swapping the R+Ps is just a bolt-in. you just use the shimming that goes with the set you want to use. install the whole thing, diff and all. if you swap the early, thinner ring gear to the electric speedo diff carrier, you will need to find a 2mm spacer to install between the ring gear and diff. try one of the posi or gear suppliers advertized in excellence. the SC is electric speedo, the early one is not. that's the main problem.

Of course, John's definition of "bolt-in" may be different from mine. I can't even put a tape in the tape deck without skinning a knuckle. I have the required 2mm spacer (plus ring gear bolts and retaining clips) from PowerHausII and (obviously) more balls than brains. The gears and everything are already pressed onto the 7:31 pinion shaft. So I'm in pretty deep on this one. Fortunately, I am a supremely stubborn (or stupid?) SOB and will never accept defeat of any kind. Damn I wish my manuals would arrive !!

"Perhaps my life's purpose is to act as a warning to others?"