901 transmission - we dodged a bullet, questions to the experts

[ischmitz]

Today we dodged a bullet when Mike and I decided to start putting together the 914/6 project.



I hooked up the transmission to the engine and noticed that both output shafts of the 901 appeared locked together. They behaved like there was a limited slip differential installed which is not the case…. Strange!!!

We pulled off the differential side cover. As soon as I loosened the 19mm special bolt that holds the output shaft the differential behaved like it should. Something was binding/locking the differential once the output shaft bolt got torqued. The nut inside the planet gears (for lack of better description) was kind of feeling weird. It somehow didn’t quite slot into the splined gear. So we decided to take the transmission apart to get the differential out of the differential housing. Fortunately, we had another 901 on the floor. Upon further inspection and comparison to second 901 it turned out that both nuts inside the differential cage were installed with the wrong end facing outwards!!! I could see where the spline pattern had eaten halfway into the pattern of the nut on the wrong side. This caused the nut to push against the square cage and lock the diff. The side that normally faces outwards has a slightly smaller diameter of these teeth so it fits inside the gear.

How can something like this happen? The guy who rebuilt the transmission (friend of ours) told us he didn’t take the differential apart at all and he has no reason to not tell the truth. I am perplexed. Can these nuts flip when the output shafts are off? Is this a "special mod" to lock the differential?

Any comments from the 901 gurus?

[Grady Clay]

ischmitz,

I didn’t think that was possible but I’m wrong again.

What is the prior history of the transmission?
What was done this past time apart?
Did you operate the transmission this time or did you stop when you discovered the two flanges were locked together?

Are you sure the differential hadn’t been apart this past time?

Please post some pictures of the damaged threaded pieces.

Exactly what transmissions are we talking about – type and transmission numbers please.

Best,
Grady

[Peter Zimmermann]

I'd post but I need pictures. At this point I don't have the foggiest idea what you're talking about...

[ischmitz]

Great, both resident experts are responding. Don't you love the PP-BBS....

To Pete: here is a picture of the items I was talking about. Both nuts (5) were installed the wrong way with the longer sleeve facing outwards !! The sleeves are supposed to face the center cube as shown in the drawing. Due to the nuts being in wrong they jammed the entire differential once I tighened the output flange srews.



There is a circlip on the splined part of the nut that limits how far it will penetrate the planet gears (7). The diameter and shape of the splines is larger on the (normally) inward facing side. On our part the planet gear (6) has eaten into the splined part of the nut. I will post a picture tonight when I get home.

To Grady: I don't have any history other than that the box was gone through by a buddy of ours. And he says he only cleaned the differential but never took it appart. This is why I was asking whether the nuts can turn without disassembling the diff. If not it obviously must have been appart at some point.

We didn't operate the transmission like that. I was fortunate to have caught that before we were going to stuff the motor/tranny into the car. I took the gear stack out, took the diff out and put everything back together the right way. Peeh.

Cheers,
Ingo

[Grady Clay]

Ingo,

Good that you ‘dodged the bullet’ and found this.

It seems that the transmission may have been operated like this. How else could the threaded piece sustained damage? I don’t think this happened turning the transmission by hand.

Did the damaged pieces generate metal chards?
What other parts could have been damaged?


A powerful 914-6 will certainly have wheel-spin under hard acceleration. This is going to be particularly hard on an open differential. Make sure that all the parts are in top condition. Again, what transmission are we talking about? There is a HUGE difference between a type 901 differential and a type 911 differential.

Best,
Grady

[Joe Bob]

Stock 901/914-6 transmission. The PO had been in the engine and after seeing what a mess that was we had the trans gone thru as well. Except for the specific 914/6 final drive we used scavanged parts for a 914-4 tranny, and new parts.

The diff was only cleaned and re-installed. Turning the output flanges by hand found the problem. We knew it didn't have an LSD and with both sides being locked on an open diff, it seemed odd. I'm glad Ingo spotted the problem before installation.

[Grady Clay]

So … it is an original 914-6 transmission. It is a better casting and stronger ‘Electron’ metal than the period 914. This has the larger type 911 differential, same as used in the type 911/01 from ’70-’71 911s (and early 915).

Good, it is appropriate to gear this appropriate for your intended use.

Does it have the shifting converted to ’72-> 914 ‘side shifter’? Has the nose-piece been changed? Consider a nylon ‘safety strap’ from the chassis to under the bell-housing.

Is the shift rod standard 914-6 from the engine forward and 914-side shifter to the rear. Is there a steering universal joint in between?

Are you still using the 215 mm ‘push-type’ clutch or has it been converted to the 225 mm ‘pull-type’ clutch? If so, did the pivot get repositioned? Is the arm modified to eliminate the pulley?

Are you using the original CV joints or has it been converted to the larger 108 mm CV joints from the period 911s? If not, you need to use the axle shaft (between the CV joints) from the 914-6. The big CV joints fit this axle spline.

What chassis reinforcements?

Good to see another 914-6 properly built.

Best,
Grady

[Joe Bob]

No mods to the shifter....I have the parts to do so but the tail shifter on the /6 is a better deisgn than the /4. New bushings......the diff appeared to be similar in design to the /4 we robbed parts from.

We redid the engine to "S" specs, no mods to the body except to repair the rust/battery acid damage to the suspension console on the passenger side.

[Joe Bob]

http://www.914world.com/bbs2/index.php?showtopic=85824&hl=

Link above is the build thread on 914 World.