How to identify 901 gearsets

[304065]

I've been looking for a backup 70-71 "911" gearbox or two recently.

Other than the number on the case, how can I tell what gearsets have been installed?

I guess the only way to do this is to pull the end cover and pull the intermediate plate, removing the gear stack and count the teeth.

Now what?

I understand that the paper gaskets between the intermediate plate and the trans case aren't merely gaskets, they are also shims of the correct compressed thickness, which positions the pinion correctly with respect to the ring.

But the relationship between those gears is a delicate one, right? When somebody sets up, for example, a new LSD, it's necessary to make sure the ring and pinion are in perfect mesh, with the right backlash, correct?

But you don't have to do that if you remove the gear stack and put it back with the right shims?

Grady, didn't you recently mention that a gear stack change was possible at the track? What would be involved in a procedure like that? Just removing the stack?

I'm sure the answer is simple to those who have done it before. . .

[silverc4s]

John,
i am sure you will get more detailed answers to your questions, but after all these years, i would only trust actually eyeballing those gearsets. true Porsche gears are always serialized matched sets and have the "letter code" etched on as well, as I remember...

[jpnovak]

If it has not been altered, these should be the correct ratios. Thanks to Warren for this chart.



If the box was modified you will have to open it up and count. the letter of the gearing will be etched into the side of each pair, along with the matched serial number as Bill noted.

btw, you will have to reshim and set the pinion depth if you change anthing inside the gearstack or differential with the exception of a new slider or syncros. There is a method that was posted using machinist blue dye and checking the mesh pattern. I am not sure what the correct mesh pattern looks like.

[jpnovak]

here is the cross reference for the gear letter designations.

[Grady Clay]

John,

This is a subject I could go on about forever. Anyone should feel free to correct anything I say because it has been a long time and my not-so-sharp memory. I won't be offended.

There are three types of gears that were in the 901/911/914/914-6/904/906,910 transmissions:
Ground gears
Shaved gears
910 gears (the number escapes me at the moment – type 822?)

All of the gears prior to ’70 are ground gears. Most of the standard street ratios have 901 or 911 part numbers. The “optional” ratios usually have 904 part numbers. Each pair has a matching etched serial number, 2-4 digits, hand applied. Most have the Roman Numeral “I” on the input shaft gear and “II” on the pinion shaft gear. Many have the part number and/or date of manufacture stamped in them. This is the mainstay of 901 gears.

In ’70 and later the 914 (not -6) had shaved gears with many of the same ratios as 901s. In practice I found no difference. They weren’t serial number matched – in fact most weren’t marked at all. Every used pair I encountered, I serialized with matching numbers because they had been successfully running together. In ’70-’71 the type 911 transmission had only ground gears.

The 910 gears were very unusual as they were like the ground gears but were undercut, cross-drilled and radially drilled between the teeth for oiling. The ratios split the 901/904 ratios to almost double the number of possible ratios. While I had a complete set, I never used them because I recognized their uniqueness. They are probably fragile with a 910 and not appropriate with current 2.0s or larger displacement.

Now there are are aftermarket "other" gears of verying quality.

One (of many) subtle changes was in late ’70 when the second gear was widened by a small amount necessitating a change in the clamping plate and bolts in the intermediate plate.


John, if you are careful and check the ring & pinion settings occasionally, you can change ratios between track sessions. The critical issues are that the original set-up must be correct and you must attempt to maintain it, you must have all the right tools at hand, have all the gears organized, have every part needed (replacement parts and if something gets lost) and a clean place to work. I had all the spare gears with bearings, proper dogs and syncros ready to go, perfect used sliding sleeves to exchange, every other part that might be found defective and all the consumables (gaskets, seals, lock tabs, roll pins, etc.) There are many simple tools (pin punch, chisel, etc.) you only use once in this process but they must be right there and dedicated to the process. To do this efficiently you must practice and never have to look for a tool or part. Planning pays.

There are many “tricks” I can share if you are willing. There are no secrets here.


The art of keeping a 901/911 operational is driving. One hamburger shift can really screw things up. But of course that is true for any transmission – 915, 930, even a G50. "An ounce of prevention is worth a pound of cure."

Best,
Grady

[louielargo]

This is great information.

Regards, Louie