I am checking interest in repro 904 main shafts. I think I can get them either made or made out of 901 shafts for much less than the $1800-2000 that they are commanding now.

If the price was right, yes. I would probably do 2

Paul

You might want to find out if this guy still has a connection. This is an old thread but he may save you some trouble if more can still be made.

http://forums.pelicanparts.com/porsche-911...here-950-a.html

Ottos Venice made a run of them and lowered the price due to flooding the market.

Carquip also has them, but I think he may have done a deal with Otto at the time...I forget how it went down.

Not sure how many he has left right now.

Rich

What is the advantage of 904 main shafts?

John

Interchangeable second gear.

On a "stock" 901 main shaft, 2nd gear is fixed, as it is a part of the shaft, so if you want any custom gearing, you have to work around the stock ratio for 2nd gear. You can swap gear ratios for 1, 3, 4, 5, but not 2nd. A 904 main shaft allows you to pick any 5 ratios for your gear box, as all 5 gears can be swapped for any ratios you'd like.

If you just want a taller second gear but won't need to change it again and again, its far cheaper to have a stock 901 mainshaft modified by welding on a third gear in place of 2nd, such as the "I" ratio.
I've done this several times.
In fact, the way I do it - I could easily cut off the stock second and build up splines for interchangeable second gears just like the 904 - far cheaper than buying a new mainshaft.

Its looking like the cost will be far less than the one offered on PP. What does it cost through you, Chris? No sense in reinventing the shaft if youa re doing it for a low price.

Since I've always welded the new gear on I have to double check the method of securing an interchangeable gear in the proper location, but I'm pretty sure I can mod a stock mainshaft to accept any 3rd (4th, 5th) gear in the 2nd location for $400.
I've already been planning to take a couple of the mainshafts I have laying around and do this so I can stock them instead of only doing it to order.

Nice! The only thing is that the swapable 2nd gears are internally splined differently than 3-5. If using 3-5 gears in 2nd gear, one's 2nd gear would be pretty colossal compared to stock F, GA, or H. I may not be understanding something with what you are doing.

There are certain ratios from 4 speed 901s, like HB and I, that are 2nd gear ratios, but sit in the the third gear position in a 5 speed. Remember that 1st in a 901 4 speed is the fixed mainshaft gear, so a HB or I second gear from a 4 speed has the larger splines. The 4 speed versions of these gearsets may also be easier and cheaper to get than real 904 gearsets. I like Chris's idea

Scott

Chris,

at $400-ish, I'm interested in buying a main shaft from you
I could provide a core if needed. I don't have the money today, but will get it and would love to know availability when you get things figured out

I'm running a 3rd gear moved to 2nd shaft currently. Mine is splined by making it out of a stock F-mainshaft exactly as your proposing. The current train of thought is that this mod is good for 200hp to the rear wheels in racing conditions. Its been done this way in colorado for approximately 10 years and held up to testing at 200hp

I'm currently running a 2nd gear of "J" with this mod

let me know if you make up some shafts in this manner
brant

I recall some posts on Pelican written by Grady Clay saying that the 904 2nd gear ratios on a 904 shaft were a little weak, and he preferred machining and welding like Chris does. I have an A mainshaft that I was looking to have modified, and may find a few more.
Would welding be cheaper than splining?

Best get your parts before you start. The gears ratios mentioned are scarce as hens
teeth and are priced accordingly. I've been watching for them for years and have found none used...new are made to order$$$. I finally went to the F mainshaft for first gear. You need a motor to pull it It does get rid of the cantilevered 1st, tho.

JP,
you need to step into the world of rolling starts.
then less motor is a requirement to use "F" as your first gear!

JP has heard me say this a thousand times...
but all kinds of things are possible I'm currently running F/J/S/O/V
5 useable gears to fit the needs of any track without changing the box

as long as you avoid standing starts its perfectly fine...
granted I'm shooting for 205hp to the wheels and will need to follow the 200hp rule also, but with a measley 2 liter that won't be a problem

Welding is cheaper than splining, however...
In my opinion, simply welding the gear in place isn't adequate to resist the applied torque with the ratios used in this gear position. I create 4 of the 8 splines on the shafts I weld a gear on. In order to make the gear removable four more splines need to be built up and ground, as well as machining of the shoulder that the gear presses against. All the spline grinding is done by hand.
I may have underestimated a little at $400 but I don't think its far off.
Welding a gear is about $300.

I'm still not 100% on the way the gear will be held in place. The intermediate plate bearing outer race will be taking the loads in one direction and its OD doesn't overlap the 3rd gear spline by much. If I was doing this with 904 2nd gears it would be no problem since, as the Dr mentioned, the splines are smaller in diameter. I would need a gear in hand to do it this way though.

Chris,
I know the senior manager at Insco in Groton MA. A good friend.
I would be happy to put you in touch.
They have the equipment to do the shafts and gears....thats all they do.

Rich

Rich,
I don't think a gear specialty machine shop is necessary.
All I need is a good precision machinist to do the preliminary cut to establish the minor diameter, and a final cut for the shoulder. The shop I've used locally for the previous shafts I've modded is perfect for this.
In fact I will be seeing them soon for a little ski shooting.
Maybe I should plan on bringing them several mainshafts to get the ball rolling.
Dave P,
I read Grady's article before I started my first shaft mod and decided to go one better by building up half of the splines before welding the gear in place.
I think this method should handle more than the 200hp limit Brant mentioned.

Edit: Hijack removed, Will send PM.

... ...Racer chris, Dr. Evil., Jp stein,R towel,Brant,and others....This is a very intresting Post I hope it continues into some detail ..such as what gears/ ratios would be suitable ...for...street use...autocrossing...and road racing/ D.E. track days......there are plenty of gear charts available (thanks Dr,evil) and others...but as J.P.Stein pointed out, FINDING some of the gears can be difficult/impossable. From experience I am not a fan of aftermarket gears. they are usually not properly HEAT TREATED to about 58/59 on the rockwell "c" scale. I had numerious "reproductions" break.(not 901 transmissions). I WOULD LIKE TO PURCHASE A LARGE SIZE (poster size) 901 914 transmission blueprint/ diagram, or something of quality/ detail that could be blown up. From what LITTLE I know about the existing possibilities for regearing , removing and welding on a different 2nd gear seems to be the most cost effective. It also seems to be a reliable way to go. However if a mainshaft were made / remachined which would allow interchangable 2nd / 3rd gears in 2nd gear position that may be a better option for some people. I think that both of you should be concerned about quality. welding on the mainshaft may aneal the shaft and gear. It may require rehardning to get back to O.E.M. specs. Making a new shaft will require a hardning process. both process will require quality control on strightness. I think what you guys want to do is great and needed.

I'm way ahead-a ya...
I make sure that I am not heating the gear teeth or the shaft core enough to soften them. As I mentioned before, the material is prone to hot cracking when the gear is welded on - so going slowly, keeping heat input to a minimum, and making sure the weld endpoints are properly filled is of utmost importance.
I did repair and assembly welding for machine shops for years so I have plenty of experience with hard-to-weld alloys, highly restrained weld joints and precision build-up for re-machining. This stuff isn't that difficult, it just takes time.

the 200hp rule is because of the active first gear with the "F" installed on it and hanging on the back end of the intermediate plate... Were using 1st gear in slow corners at full throttle. So the 200hp rule is about 1st gear. The 2nd gear portion of the re-splined main shaft is not the weak point.

Mine was/is a specialty box. ....tain't worth spit for other than AX and has modified to be prolly the only one in existance. I no longer own the car so it ain't my prerogative to tell you what is in there. IF IT WORKS we'll call it the Donahue box.

...J.P.STEIN...I think you are ...(not)...suggesting....that ...HEWLAND stright cut , dog tooth, gears, ...available in numerious ratios,..and a lot cheaper than some of the porsche special gears...will fit the 901 mainshaft.....Oh probably not, or maybe, or lets keep wondering.

No, it's a 914 box.....a CSOB special.

Last week I dropped off 4 mainshafts at my machinist (at their Christmas party) after grinding off the F gear teeth.
Today we discussed options for securing the removable gear and came up with a workable solution.

In order to retain the removable 2nd gear we plan to use a thin hardened washer between the gear and the intermediate plate bearing inner race. This will require moving the gear forward on the shaft slightly to create clearance for the washer. A small misalignment of the two gears in the 2nd gear pair shouldn't cause any problem.
The rear side of the retaining washer will be at the same position as the existing shoulder that the bearing race presses against so the overall position of the shaft will remain the same.

Now I can show a pic of the CSOB special trans. It's rather specialized.
10-12lbs of rotating mass has been eliminated.

That's RAD, JP.

Pretty cool

We ran it for a season with all the gears intact but locked out as a "proof of concept". As it now sits there were some more mods that we were quite a bit more work than we were willing to do if "things" didn't work out...like, is there enough motor to pull that F gear off the line in an expeditious manner? Will we find a course that we run out of 3rd gear? No problem....onward. It is still easy to return to a 5 speed.

Autocross is still a place that you can let your imagination run wild.....unlike too many other forms of motor sports. In this case we've stripped the car to the bare essentials needed to get it to the other end of the course.

Last week I got the 4 mainshafts back from my machinist.
Next step is to build up the splines.

Click to view attachment
Click to view attachment

Thats a stock 3rd gear in the picture.
The sleeve is a spacer from a 914 trans. It still needed to be machined to length when I took that picture a couple weeks ago.
Not shown is the thin washer that goes between the bearing and the gear.