Question about broken mainshaft for 915

[gulf911]

Could not find the part here on Pelican, anyone have an idea on cost or best place to buy? I have the early (73) and would like to get the updated one. I would prefer the mainshaft and gear as a set, but would buying just the shaft be ok as well?

Thanks for the help,
Dan

[gulf911]

Anyone?

[Grady Clay]

Dan,

I don’t think I have ever seen a 915 input shaft broken unless the entire transmission was broken in half. Tell us more about what happened here.

The first gear (Gear I) is machined onto the input shaft. The mating gear (Gear II) is the free gear on the pinion shaft. The two gears are matched to give the proper backlash clearance (determined by the tooth radius of both gears).

I suppose you can mis-match the serial numbered gears so long as you are in spec for backlash. I never have.

You should be easily able to buy a matched pair.

Best,
Grady

[gulf911]

Thank you very much for your reply Grady!

Some of the splines are broken? The newer input shaft has larger/more splines? So I am looking to replace/update the Shaft (1st Gear) and the mating gear. Just having a hard time finding the part. I did see one place that had one, but they wanted $2550.00 which seemed rediculous. Anyway, thanks for your help.

[gulf911]

Wanted to show you the pic of damage.

[john walker's workshop]

happens a lot. i probably have a good used matched set available. early or late.

[Grady Clay]

John,

So you can twist off 2nd gear?

I guess I haven’t been involved enough recently. That is certainly is an operator issue. Is drag racing involved? Someone prior?

As Porsche 911 engines got bigger and torqurier, I suspect there was more opportunity for abuse. Part of the issue is the tires have more grip than when a 915 was new. Keep in mind a 915 is an extension of the transmission for a 908 3.0 (actually the 907 6-speed with a 2.2 771 eight.)
"
"
(C) Ludvigsen.

These gears and design probably can be torque over-stresses with a CIS or DME 3.0/3.2. There is a reason for the 930 transmission (with Type 912 influence).

Best,
Grady

[john walker's workshop]

it isn't twisted off, just the end faces on the splines that support 2nd gear chip off. the later ones have a slip on collar under the gear.

[Mahler9th]

It is my understanding that that type of damage happens pretty often. Mine got damaged after 2 seasons of racing. My application is with a 3.8, but I am not sure whether that made any difference. I bought a good used shaft/first gear from Partsheaven. Of course the later style. I understand that you can also have a collar machined and re-use an earlier one with the broken splines, but I did not want to do that.

In my case, the failure occurred in late 2003 at Sears Point, when the car jumped out of gear midway through turn 7. I couldn't select a gear.

One thing that may be important is to check carefully, or have someone knowledgable check carefully, all of the other parts in the transmission for resulting issues. My somewhat uninformed visual inspection identified zero other bad parts, so I just put in a few fresh synchros. About a year later I had a series of failures that were likely due to parts that were damaged that I could not identify.

If the transmission is used for track driving or racing, or if you just want to make some improvements, any of the parts sold by Wevo ought to be considered. I have their maincase, bearing retainer plate, shifter and shift joint. All are great parts.

Little things make a difference, and I am thankful to have received help nailing down my issues over time. The key thing is to check everything else carefully-- if the main shaft stack moved, especially in a track driving scenario, their may be collateral damage.

[gulf911]

Thanks Mike, I do have a 915 guru looking at it as we speak. I have a 73 911 RSlook w/3.2 and this took almost 6 seasons to occur so i guess I am pretty lucky. Does anyone have the porsche part number for the original and the updated shaft and gear?

Thanks for all the help all!

Dan

[Grady Clay]

John, guys; thank you

What makes this Forum so great is I learn something important every day. When I err, it is corrected and not perpetrated as some “fact” to the loss of “Porsche”.

I had always assumed the change was for manufacturing cost.

What forces or conditions could cause this failure?

Best,
Grady

[gulf911]

I thought I would post what I found. The updated 1st speed gear set is 915.302.921.14 and is $3054.82 list from Porsche... The early style I believe is 915.302.901.05. If anyone can confirm these numbers for me that would be great.

Thanks all,
Dan

[Mahler9th]

I am not sure of the part numbers, but I know there are many, many of the updated parts around in good used condition. I live near Partsheaven, and they have dozens of 915s. Maybe a couple hundred. They have tons of these parts.

If you go used, it may come with the updated first gear slider and asymmetrical teeth. That would be good.

If you go used, you may want to have the parts crack checked. We have a place near San Jose that'll do a whole basket of parts for about $100.

Places I'd look for damage are synchro rings and the clips that hold them in place. Also, take a good look at the guide fork on the cover to see if there are any marks. Also look closely at all dog teeth to make sure they are still on with no gaps and are still in good shape. I'd also take a good look at the spendy bearings. I broke the big one on the pinion shaft a couple of years ago-- that was ugly.

[Joe Bob]

I have complete 915 that is good condition....located in CA. $800 complete plus shipping. Out of a 78.

[gulf911]

sorry for the belated reply..

Thanks to all for your help, it looks as though my mechanic has all the parts needed at the moment. The additional items will be an updated slider 1st to 2nd, and 3rd to 4th and the gt plate that holds the bearings, along with any needed dog teeth etc.

Anyway thanks again!

Dan