Porsche 930 fabrication help

[Dave Deerson]

Hey guys, I'm not sure where to turn for this, so given the wide experience and demographics of this forum I will put it out here:

I converted my 930 gearbox to dog change and have been sorting the many little teething issues that arise whenever you make something do something that it was not intended or designed to do.For the most part,when working correctly, this shifting is out of this world! However there is one last issue that has been throwing me for a loop. I think I have found the cause, so now I need to correct it.
The problem is that when shifting as quickly and aggressively as you do with a dog box, the shift "dongle" or lever will come out of the shift rail fork and the trans will then be stuck in gear until you get back to the pits, pop the pivot cover off and pop the rail back into neutral. This has only been happening when shifting from 2 to 3 (crossgate and up),but 3 to 2 is fine. This makes me think that the physical kinomatics of my arm put different stresses between pushing out and away and pulling back and to me.
The cause of this ,after comparing the shift mechanisms of Hewlands to the Porsche ,I feel are the size of the notch in the shift rail fork. The original Porsche Syncro needed approx. 5/8in from N to engaged, a Hewland(and now this Albins conversion) needs approx 1/4in from N to engaged. The Porsche rail fork has a notch almost 5/8 where the Hewland has one at near 1/4in. What does this all mean? With the 5/8in notch traveled 5/8 to engage, the notch of the other rail is "blocked" or covered. If you move the 5/8in notch rail only 1/4in of travel than there is an open "window" that the lever could pass through given the right conditions of movement. Also there are detents at the 1/4in engaged point,but the rail can move 1/8in or more as the detent moves up it's "ramp",but still return the rail to engaged if the lever falls out,producing the condition of being stuck in 2nd, but the lever winds up in the 3/4 rail fork.
So, here's my question:::
Is there anyone here or know of someone, that has the ability to copy the shift rail forks for 1/2 and 3/4 in a 930 box but with a 1/4in groove instead? I would think that these would have to be done in steel and hardened as this is a heavy wear surface. I would fabricate the lever/dongle with a corresponding smaller tip on my end here. I also have contacted Albins on this but as of this afternoon,have not heard back.The rail forks are shown in this pre-conversion pic:

 

[Russ Noble]

Dave, I have just taken my 'dongle' out and had a look at it. It is quite a bit smaller than the yoke that it engages in.

What is happening because the dongle/yoke clearance is excessive is that the 1/2 shaft is not being pushed far enough back to engage the neutral detent before the 'dongle' slips across into the 3/4 yoke .

What I would do, build the end of the dongle up with brass to take out the extra clearance so the dongle can't move across into the 3/4 yoke until the 1/2 yoke has been pulled fully rearwards into the neutral position and is properly secured by the neutral detent. This may make the change action slightly notchier, but that will happen which ever way you go.

It doesnt matter what size the dongle and the yoke are, if there is no slop there will be no problem. 5/8" or 1/4" will work the same if the clearances are the same. This is something I will have to watch out for when I eventually do mine. Thanks for the heads up that could save me a bit of aggro in the future

 

[Dave Deerson]

Russ, visualize it this way.
make 2 1in by 1in squares of cardboard(to represent the shift yokes) draw in a channel 5/8in wide in both.place them on paper with 3 lines 1/4in apart(to represent neutral and engaged forward and back in the Albis dog conversion). As you slide one yoke 1/4in to the engaged point, notice how much of a "window" there is for the dongle to either get hung up on or possibly pass through. Imagine what goes on as we shift "in anger" accelerating from 2nd to 3rd( one of the highest load points) and I can see how given the right amount of side thrust the dongle may squeeze through and get down into the 3/4 yoke before the 1/2 gets a chance to get back to N.
Now there are a few ways that this can be corrected, so I'd like the advice of someone like a JacMac( who has probably delt with metallurgy questions like this before). I can:
1. Just weld some 3/16 stock to the faces of the shift yokes and reduce the dongle to suit. However,I am worried about sliding wear as the original parts are locally induction hardened. So, can I use maybe some harder material such as 4130 and then have it hard chromed? Or perhaps braze something harder(welding might mess up the new piece's surface hardening).
2. Do the above, but then use this as a prototype and have it replicated in a better material and heat treated properly. I feel that even though this is potentially the most expensive option, the added cost are outweighed by the frustration I have endure this season with lost track sessions and complete weekends.
I guess the main issue is this: Because this is a track car and not a street car(seeing less mileage) is the "hardness of these parts THAT critical? I have only delt with hard chroming and have never had a part that I made heat treated before. But I guess there is a first time for everything. Is this some thing that can/is expensive if done in small batches(3 pieces 2 yokes and the dongle)??Or even difficult to find a reasonably local outfit to do the work?

 

[Russ Noble]

Dave,

I understood how you were looking at it, I just think you are making it way more complicated than it is.

I'm a lazy bugger, I don't like doing things the hard way when I think there is an easy solution. So I'd take out the excess clearance and make sure the front face of the dongle is squared off so it cant begin to move across into the 3/4 slot until everything is lined up.

A relatively simple and inexpensive solution that I'm sure will work. If it doesn't you havent really lost anything except maybe another DNF.

Good luck which ever way you go.

 

[Lim]

I am worried about sliding wear as the original parts are locally induction hardened. So, can I use maybe some harder material such as 4130 and then have it hard chromed? Or perhaps braze something harder(welding might mess up the new piece's surface hardening).


How about En39B Carburising steel .after heat treated you get 0.5mm surface hardened up to 62 HRC.

Lim

 

[zollis]

Hi Dave ,

I might be able to machine them in my machine shop . If you can send me a drawing with the dimensions and tolerances , I can have a look . The hardening process I will leave up to you to do . The postage from NZ might take a bit of time , depend on how much of a hurry you are in .

Z.C.