901 tranny gear question

[R&TSWB]

Hi, i am thinking of rebuilding the 901 tranny in the 65 track car with lower third, fourth and fifth gears.
I would like to know if i can use the third gear as fourth and the fourth as fifth and just add a new third gear? thanks for any info and advise regards rommy

[Scott Clarke]

I think that will work. Do you know what ratios you have?
-Scott

[jpnovak]

Yes, it can be done. you just need to carefully match the ratios. Here are a list of gear ratios for the 901/902/911 type transmissions in the following thread.

Gear Ratios for a type 911 transmission?

A typical 3rd could be an O gear. This was used as 4th on airport gears and 5th on hillclimb gears. Will work fine. The problem will be locating the rare short 3rd and a mainshaft that has a suitable 2nd. They are rare and quite expensive.

Sounds like a fun time though.

[Grady Clay]

Welcome to the Forum. You will find a lot of help here.

In 901/904/906/911/914/914-6 transmissions there are:
First gear ratios;
A = 11:34 = 3.091:1,
AA = 12:34 = 2.833,
B = 14:37 = 2.643,
C = 15:36 = 2.400,
D = 16:35 = 2.188.
Note that you must use the appropriate reverse idler gear for the Gear I of 1st gear.

Second gear ratios are;
E = 17:34 = 2.000,
F = 18:34 = 1.889,
G = 18:33 = 1.833,
GA = 18:32 = 1.778,
GB = 16:30 = 1.765,
H = 19:32 = 1.684,
HB = 19:31 = 1.632,
HA = 20:32 = 1.600,
I = 20:31 = 1.550.
The F and GA ratios can be cut on the input shaft (they are on most street 911s) but all are available to fit the 904 input shaft. All the other 2nd gear ratios only fit the 904 input shaft.

Third gear ratios are;
I = 20:31 = 1.550,
J = 21:31 = 1.476,
K = 21:30 = 1.429,
L = 22:30 = 1.364,
M = 22:29 = 1.318,
N = 23:29 = 1.261,
O = 23:28 = 1.217,
P = 23:27 = 1.174,
Q = 24:27 = 1.125.
The 3rg gear is cut the opposite direction from the 4-5 gears.

Fourth and fifth gears are interchangeable and are;
M = 22:29 = 1.318,
N = 23:29 = 1.261,
O = 23:28 = 1.217,
P = 23:27 = 1.174,
Q = 24:27 = 1.125.
R = 25:27 = 1.080,
S = 25:26 = 1.040,
T = 26:26 = 1.000,
U = 26:25 = 0.9615,
V = 27:25 = 0.9259,
W = 27:24 = 0.8889,
X = 28:24 = 0.8571,
Y = 28:23 = 0.8214,
Z = 29:23 = 0.7931,
ZA = 29:22 = 0.7586.
ZD = 31:22 = 0.7097.
We usually refer to these as 4-5 gears.

I’ll come back and edit this list and add part numbers where I can.

Gears are a matched pair and must always be replaced as a pair. When you take a gear pair out, wire them together.

The issue comes when you use a 4-5 gear in 3rd gear application or a 3rd gear in a 4-5 gear application. First, the ratio is reversed. An M 3rd gear is 22:29 =1.318:1 but used as a 4-5 gear it becomes 29:22 = 0.7586, the same ratio as a ZD gear. Because the 3rd gear is cut the opposite direction from the 4-5 gears if you use a 3rd gear in a 4-5 application or a 4-5 gear in a 3rd application it adds axial load on the pinion intermediate plate ball bearing. This bearing is a common failure point. As you can see from the two threads below, there is some controversy as to the suitability of doing this.

”Identifying 901/911 Gears versus 915 Gears”

”Flipping 901 Gears”

There are 3rd gears and 4-5 gears from 4-speeds and 905 Sportomatics that are available at less cost than the special ratio “904” gears. Note that the 4-speed gears are the same as the 5-speed minus one. The 2nd gear in a 4-speed is the same as a 3rd in a 5-speed. The 3-4 gears in a 4-speed are the same as 4-5 gears in a 5-speed.

Best,
Grady

[R&TSWB]

Hi everyone, its getting really exciting to hear that its possible to move the 3rd to fourth, fourth to fifth and insert a new set for the third.

My tranny case says 901/0 which means then that the gears are as ffs: AA,GA,O,U,Y

We are planning to insert a J gear set for third which will make the final gear stacking as ffs, AA,GA, J,O,U. something like the airport gearing in the sports purpose manual with a slightly longer 5th gear.

The J gear set is available as a new item made by a co. in colorado.

When we get this tranny built and run in the track, we'll get back to post some news good or bad. In the meanwhile, regards to all. thanks again rommy

[JWest]

Originally posted by R&TSWB
My tranny case says 901/0 which means then that the gears are as ffs: AA,GA,O,U,Y

We are planning to insert a J gear set for third which will make the final gear stacking as ffs, AA,GA, J,O,U. something like the airport gearing in the sports purpose manual with a slightly longer 5th gear.

As Grady has posted, you can't just move "O" 3rd gear to the 4th position, as the fixed and shifting gear are reversed. The only way to move 3rd to 4th is to reverse the gears, but this flips the ratio from 23:28 = 1.217 to a tall "Y" 28:23 = 0.8214.

[R&TSWB]

yes we did take note of the 3rd being reversed if we moved it to fourth, so the plan was to flip the fifth Y gear for third and set aside the third O gear. Hope fully this should work out right. thanks, regards rommy

[R&TSWB]

Hi grady, do you think it will be possible to use the Y 28:23 fifth gear and "flip" it for the fourth making it O 23:28 ratio, considering that the fourth and fith are "interchangable"?
thanks for your help regards, rommy

[YTNUKLR]

This can get confusing, cant it! You can flip 4-5 gears with 3rd gear, but you can't flip a 4th with a 5th. If you have a "Y" 4-5 gear, it can be used in 4th or 5th, or flipped and used as an "O" 3rd, that's it.

[Grady Clay]

Scott is correct. The 4th and 5th are interchangeable; that is you can use a “Y” 4th as a “Y” 5th. You cannot flip it and make an “O” 4th. You can flip it in the 3rd gear position and it becomes an “O” 3rd ratio with the above linked caveat.

I’m supposed to post the part numbers of the gears. I guess I’ll have to get with the program.

Best,
Grady

Scott, where in Bezerkeley are you? I have a house up on N. Queens Rd. gradyclayathotmaildotcom.

[randywebb]

I don't suppose anyone has the type 911 gear ratios already loaded into a spreadsheet by any chance???

[YTNUKLR]

Randy, I do, email me and I'll send it to you
scottkinder@berkeley.edu

The ratios for the 911/01 5-speed are A, GA, O, V, ZA.

Cheers
Scott