901 Transmission Rebuild

[Brett Voigt]

I have 2 weeks vacation coming up in May and I'm planning to do some work on my 69' 912. First is a suspension overhaul which I have a strong plan for. Afterwards I plan to rebuild the transmission. I already changed out the shifter bearings and there is still quite a bit of slop in the shifter. I think it's safe to say that over the 46 years of my cars life, it has some arthritis of the trans. I've gone through the tech article "Replacing the 1st gear on the 901," as well as the version "101 projects for your 911," and am in the process of buying parts for the 1st gear rebuild, i.e.
-Synchro ring
-1st gear dogteeth
-1st/Reverse slider
-transmission end cover gasket
but... was wondering what additional parts I need to replace the remaining gears?

[Matt Monson]

Buy Dr Evil's transmission rebuilding DVD.

[Brandon-FL]

Here's the speed reader version of the teardown....

The DVD might still be for sale on the 914world.com - The fastest growing online 914 community! forum and if it is not, you can PM me if you want to borrow my copy. Pretty informative, especially for a 901 "refresh" and I believe Dr. E can help with some of the parts and "special tools"

[Peter Graham]

Brett,

It's hard to say what you'll need until after the teardown, cleaning and inspection. Having just gone through this with one-each 4-speed and 5-speed transmissions to make a good 5-speed, it took pieces from both plus synchro rings, dog teeth, a gasket set, a pinion bearing, a 1st/reverse slider, diff carrier bearings and output flange stretch bolts. I may have left out several additional parts required.

The Dr. Evil method assumes that the transmission will go back together exactly how it came apart; this is not always a good assumption. There are several specialized tools needed to properly set pinion depth, pinion/ring gear backlash among other critical measurements.

If you don't plan to drive the transmission much, then have at it and learn. If you want a transmission that is reliable and trouble-free, do what I did: leave it to an experienced, professional Porsche 901/902 transmission builder. FWIW, I have driven my 912 over 45K miles since August 2012.

Peter

[Matt Monson]

Peter makes a good point about the ring and pinion. A '69 912 likely has a 6:29 in it, which is weak and frequently comes out with pitted teeth. Before you tear down the whole gearbox pull the sidecover and inspect the ring and pinion. If it needs replacement bolt the cover back on and send to a professional.

Most shops will not charge less to rebuild a gearbox delivered to them in pieces. In fact many will charge more because they have to spend more time sorting and organizing the components.

If the ring and pinion is good, proceed with your DIY plans.

[Bob Harriman]

902/6 and 902/16 are the only 912 trans with a 6:29 in it. Unless of course it has a 911 trans. I got under mine and found it had a 901/13 which is a 911 trans. Numbers matching isn't really any concern to me since the motor isn't any way. Go here for some good info.

901 Series Porsche Transmission

[1966912]

If you have a '69 912 you more than likely have a 902/16 or 902/06 transmission with A,GA,O,U,Z gears and a 6:29 R/P. You also have a magnesium case which is a lot lighter than the 901 transmissions. When you do the math a 902/16 comes out geared about the same but slightly shorter than a 902/1 or 901/02...You won't know until you tear it down how many syncro's and dog teeth you'll need but it's safe to say probably most of them except fifth gear which doesn't get as much wear as the rest...The 1st/Reverse slider is expensive but probably needed if your having issues with 1st gear. You cannot replace 1st or 2nd gears. They are a permanent part of the input shaft. If your 6:29 R/P is in bad shape I would highly recommend going to a 7:31 R/P to both take advantage of your current gear ratio's and the stronger R/P. Your gearbox is geared lower with the 6:29 and the 7:31 will make your transaxle way better on our lower profile tires. You can pick up a fourth and fifth gear from a 914 transaxle pretty cheap and Dr. Evil a.k.a. Mike Lesinak most likely has some he can sell you far cheaper than getting them on ebay or what have you. I rebuilt my first transaxle using his video and had no problems doing so. With that said you do need the special tools he makes to do so. These he can also sell you pretty cheaply. I'd email him and see if you can get a hold of his video on CD. Also, he can do your transaxle for you for around $300 plus parts and shipping. I shipped him my 902/16 so he could set the pinion depth on it with a 7:31 R/P. I LOVE my custom transaxle. it has A, GA, O, V, ZD gears and makes it much nicer on the highway and I'm geared taller than the average autocrosser who has an F gear.

Dr. Evil a.k.a. Mike Lesinak mike95125@yahoo.com

Jamie

[Matt Monson]

You can replace 1st. Just not 2nd.

[1966912]

I stand corrected. This is true. Gears don't wear out unless something horribly goes wrong in there. You don't 'grind gears' you grind dogteeth because your sycro's are worn. I see no reason to change your first gear. Thanks Matt it's been a few years since I rebuilt a box...memory fades as you get on in life...

[Brett Voigt]

Thanks for the info. I'm looking for the Dr. Evil video. Also, I bought the 912 as a personal project, so the only thing I'm planning to send parts out for is paint. Any ideas on total time for rebuild of the trans (adding in need to buy new parts)? Also, any idea to the max cost of rebuild?

[Matt Monson]

If you need to replace the ring and pinion it will cost you $5000 in tools.

A regular rebuild is 8-12 hours for an average DIY mechanic.

[1966912]

Okay, I've rebuilt ten boxes in the past few years. If you want to clean the case of all the grease and oil this will take a whole day by hand. If you have access to a parts cleaner than you can be tearing down the stack while in the cleaner. Tearing down the stack and disassembling the gears to replace dog teeth and syncro's will take 1/2 day. If you have the parts ready to go you could rebuild the stack and replace the bushings and get it back together that same day but I'd plan on a third day to finish. It is not something you want to rush. Getting the gears apart is a hassle without the right tool and can take some time at first. You'll get used to it but I tended to bleed doing this part of the job at first... If you want to reset the pinion depth you'll have to take it or send it to someone that owns that special tool like Mike or a shop like Hi-Tec Automotive in San Rafael or similar. Otherwise you use just measure the gasket in between the case and the intermediate plate with a micrometer or calipers and use a new set of gaskets with the same thickness as before. Getting the gears back together and putting in all the gaskets takes some time and should not be rushed. I'm saying three days with no parts cleaner...

Jamie

[Ed T]

There is some over-broad info above. The ring and pinion ratios vary in 1969. I finally got a 1969 factory service release and sent it to the folks at the 912 registry, and they put it in their table at : 912 Registry | Transmission Type Table

Take a look there. In 1969 the factory changed from aluminum transmissions (902/04 and 905/05) to magnesium (902/06). I don't remember what the 902/16 or 902/55 cases are, but there are many kinds of 1969 transmission, so be careful!

[Matt Monson]

OP has already implied that he's got a 5spd since he's talking about the 1st gear rebuild in Wayne's book. So that rules out the 902/04 (4spd) and a 905 would be sporto. The 902/55 is a special order motorsports gearbox. I've only seen 1 or 2 in a 912 ever. That's called the vowel box and the holy grail.

That leaves the 902/05 and 902/16 for ROW vehicles and 902/06 for US/CDN cars. Odds suggest he does have the short r/p in it. Nobody said it was a sure thing. We just told him to watch out for it.

Do you have the original factory book you are pulling this from? I'm not familiar with the 902/04. I'm familiar with the 902/14. Is that a typo? I'm assuming that 905/05 is a typo because that's also one I don't know.

[1966912]

A 902/06 and a 902/16 are the same basically. Same gears, same magnesium case. I know of many 902/16's in US cars so not sure about saying they are for ROW cars either. Paula Golus, the administrator for the 912BBS, my former boss and I all have one. Mine came from a California car that was a late '69 912....BTW, if you put the 6:29 in a 902/1 or 902/02 five speed you end up with essentially an A,E,I,O,U geared box...cheaper than getting the elusive 'vowel box'...

[1966912]

Chart showing 902/1 with 6:29 R/P swap in red line...

[Matt Monson]

I was basing my comments off the linked chart on the 912bbs. Here's a page from the factory spec book:

Obviously the chart has a lot more than this sheet, and thus why I asked about the source document.

[Matt Monson]

Originally Posted by 1966912

Chart showing 902/1 with 6:29 R/P swap in red line...

Two problems with that.
1. The 902/55 has the vowel gears AND the 6:29 r/p. It's the same hill climb set up they used in the famous Berg Spyder

2. Your shorter r/p doesn't close up the rpm drops between the gears. The gap between an E and an I is much smaller than the gap between an F and an M, and so on through the stack. So while changing the final drive will increase your mechanical advantage, and therefore your acceleration, the closer ratio option is generally the better approach. That's why the factory did it that way.

[1966912]

Matt, you must mean the 902/50 which has a 7:31 R/P. Where do you presume that a 902/55 exists? I cannot find such a transaxle in any of my literature. The 911 workshop manual #1 has this as a 902/50 with a 7:31 R/P just like the sheet above,



If there does exist a 902/55 with A,E,I,O,U gears and it has a 6:29 R/P than it will be geared MUCH lower than the actual 902/50...too low. Do you have data showing this box exists?

[Matt Monson]

It's in PET. Illustration 398-00R. Part number 902.302.012.55 for ordering the complete gearbox.

The 902/04 and 902/05 that Ed references are there as well.


It's intended for one thing, hill climbs. There's also the 901/51 if you want to talk super short boxes. That's also a hill climb box. 7:31 w/ AA,E,I,M,O.