1971 911T Transmission Conversion

[John Phillips]

I am replacing the 901 transmission in my 1971 911T with a 915 transmission. The 71 has a one piece transmission mount/carrier as compared to later models that have a carrier with replaceable transmission mounts.

The 71 mount does not mate to the 915. Will a later model carrier (which will bolt to the 915) bolt into the 71 or will I need to manufacture a new carrier?

[Grady Clay]

John,

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

Yes, you can use the 915 mount on the 915 transmission and it will fit the car. Test fit it to the chassis and confirm you have about 6+ full turns of thread engagement. The mounting bolts usually have a tapered lead on the end of the bolt.

The 915 will interfere with the chassis between the rear seats on a ’69-‘71. It will take some pounding with P1 (BFH) to get sufficient clearance. This is particularly at the top of the nose piece. You will need to re-route the two evaporative emissions hoses and remove (or pound flat) one of the brackets holding the hoses.

You will need to extend the back-up light switch wires.

You will need the ’72-> shift housing (5-R is opposite to the 1-R lockout).

What Type 915/xx will determine the clutch cable you need to use. If you want to use the “Omega” over-center spring at the transmission, you will need to change some things at the pedal assembly.

Before you start the project, make sure the new transmission has the correct axle flanges. You will want those for 108 mm CV joints (the flange measures about 110 mm OD) and uses two 10 mm roll pins and four M10 bolts. Remember to inspect the bolts and use new Schnorr washers. Re-torque the CV joint bolts soon after you drive your 911 and regularly for a while.

It is VERY important that the transmission casting not be able to contact the chassis under any circumstances. If it does, it will wear the casting and chassis , cause noises and stress components not normally stressed. In general it needs about 1 cm minimum clearance.

Best,
Grady

BTW, what are you doing with your old type 911 transmission? I need one for pictures. Email contact: gradyclayathotmaildotcom. I’m collecting info to brow-beat Wayne into authoring a Pelican transmission book.
G.

Grady - sent you an email re early transmission.

I'm doing a 915 transplant into a '70. Used an air chisel with hammer bit to massage the tunnel. If this isn't enough, I'll get out the BFH.

[911pcars]

Nice dimple Mark. Did you hold the offending 915 gearbox nose piece up to the install position to confirm? I remember my BFH marks were at around 10am as well (perhaps due to errant strokes of the ball peen or because of closed eyes upon impact). At any rate, I created adequate clearance, even where it wasn't needed.

Sherwood

[gulf908]

John,
Don't do it !
A correctly set up 901 is much nicer to use than the recalcitrant 915 box.
Unless that is,you plan to use that 3.6 twin turbo you have !!
Ask me how I know after 3 901s and 2 915s !

Cheers,
Dennis.

Originally posted by 911pcars
Nice dimple Mark. Did you hold the offending 915 gearbox nose piece up to the install position to confirm? I remember my BFH marks were at around 10am as well (perhaps due to errant strokes of the ball peen or because of closed eyes upon impact). At any rate, I created adequate clearance, even where it wasn't needed.

Sherwood

Sherwood,
I sat the 2 boxes next to each other to get a visual on the differences. It appeared the protrusion on the 915 was between the crossmember bolts. I'll tweak on installation day.

[911pcars]

Mark,
I removed the nose piece off of the gearbox. Along with the gearbox mount, I could see pretty easily where add'l space is needed. You don't want to do this trial and error process using the entire gearbox.

FWIW,
Sherwood

[randywebb]

time to buy a bunch of kids play clay -- stick it on the tranny - slice to give you the amt. of clearance & est. what you need by how much the tranny can move from a big road bump

works just like doing valve clearances, but you will have too much clay to keep it in an old STP "The Racer's Edge" can...

[John Phillips]

Thanks everyone for very, very helpful feedback.

john

[viejopatron]

This has been a helpful thread for me. I haven't bought clay yet, however I did mount the nose housing to the cross-member for fit-checks.

I'm using the bolts (Item 27) from the 901 installation. Are they longer than the bolts (Lower view Item 8) used to install a 915? When I did a fit-check, I noticed a gap between the top of the rubber mount and the body pad. Comparing the IPB Katalogs, it appears the early bolts MAY be longer. Can it be true?

... and is the relative elevation (nose housing to body) impacted?

[rothaus]

Interesting ! This is great info.

I also plan to put in a 915 in my '71 911T.

I still need to get all parts together. Will the 2.2T clutch fit to the 915 transmission ? and how about the release bearing. Those are the 2 items I'm concerned about.

I just try to get all the parts, if they are different, before disassembly and than wait with the car apart.

Thanks for any information.

[RWebb]

Originally Posted by gulf908

John,
Don't do it !
A correctly set up 901 is much nicer to use than the recalcitrant 915 box.
Unless that is,you plan to use that 3.6 twin turbo you have !!
Ask me how I know after 3 901s and 2 915s !

Cheers,
Dennis.

BINGO - you are giving up the outstanding road racing shift pattern and dulling the feel

but if you do, swap shift tubes also

[rothaus]

The conversion to my AX car will be a 2-part conversion.

Part 1. Converting to the 915 with Quaife and ring and pinion 7:31

Part 2: When I find a good 3.2L I will add it.

However, at this time I try to get all the parts I need for the 915, so I hope I can do it on one weekend, and miss no races.



Cheers
Engelbert

[Carrerax]

Originally Posted by Grady Clay

John,



What Type 915/xx will determine the clutch cable you need to use. If you want to use the “Omega” over-center spring at the transmission, you will need to change some things at the pedal assembly.


G.

Grady, I am also doing a late 915 (1982) into a 1970 911t with a 3.2ss. Mine had the old 70-71 type cable. The new trans has the omega spring. What needs to be done to the pedal box or do I need to replace it and what cable should I use? Thanks in advance!!! Mark

[joetiii]

what year did Porsche add the Omega spring?

[911pcars]

My guesstimate, around '77-'78.

Sherwood

[Zeke]

I think my '77 had one.