Mike Simard

I seem to have broken my trans during a race and need a replacement.
If you have one to sell please pm me.

Mark Anderson

Sorry to hear that Mike. Do you know what broke? I do have a few later 5sp boxes plus some parts to possibly fix yours.

Mike Simard

I shifted into 4th and it started making a really horrible noise and shaking. I do know the TT shaft and clamp look good.
When moving the car around with the starter (to load on the trailer), it acts like there are missing teeth somewhere.
Oddly, this happened after backing off on my way back in after losing the brakes during qualifying. I shifted into 4th a little more gently than normal and all hell broke loose.
I just unloaded from the track now and will tear into it soon.
I'll call you next week. I wouldn't mind replacing the 2.20 box entirely, I don't like that tall gearing.

DougM

I have a G28/11 no LS that was pulled from a 65k mile car ready to be packed and shipped. It has the 2.20 gearing. I also have a G28/10 with the lower gearing and no LS which I was planning to use in my car. For the right price, I could stick with the 2.20.
PM me if interested.

123quattro

If it has issues in 1st using the starter I would look at the ring and pinion first.

mark kibort

see if 5th gear works. you probably did what i did with the holbert transmission after being 25 years old, and 10 years of hard racing. you probably stripped a couple of gears.
either way, it needs to come out an be replaced or rebuilt.

GregBBRD

I'd love to take apart the transmission and see what broke. (Working on "tougher" pieces.)

Mike: I've got an '85/'86 style box that I bought from Brendon as a core. Probably needs to be "freshened", but that can be done. No limited slip, but could install yours!

Mike Simard

Is this a normal amount of metal to come out with the drain plug?

Greg, whichever gear(s) those are from it was .941" wide. Does that sound like the input shaft? I don't like working on these, it's just not me, every other part of the car is me except this one. I'd love to have something that you went through, whatever it is I'd like to get away from that 2.20 once and for all.

I really miss the 79 box and its ratios, aside from it getting stuck in between gear changes and not have a reverse syncro, what's wrong with them? My old one never did break like this.

MK, what gears stripped on yours? Not the input shaft (5th)?

FredR

Mike,

Well that box is totally goosed by the look of it!

Have you power-boys looked into cryo treating the internals? I know Don Hanson did that on his big motor- no idea how effective it is but sounds a logical development. Doubtless GB knows much more about this

I know Don told me he deliberately tried to avoid too much low down torque to avoid ripping these boxes apart- how you do that with a motor like yours is beyond me [half throttle?]

Fred R

Mike Simard

Funny you should mention "half throttle".
I had to shed 200 WHP to be classed in NASA's ST1 where there's some great competition. To do that I set a throttle stop and they were limited to about 50% open. That made for a 444 chassis dyno reading.
By shifting "more gently" as I was heading back in, I was shifting around the torque peak which wasn't much lower in the restricted form.

Cryo is an unknown, it might do nothing or it might help a little. One sure way of doing good is shot peening.

IcemanG17

Mike
Shoot Sean an PM....he has a few later boxes lying around....

GregBBRD

I've been building race transmissions, since before I was in college. Sure, they were for drag racing VW's back then, but the lessons are still valid.

928 people, who think that these cars are made from special "unobtainium" material supplied directly from God (you know who you are), are not going to like this....but all gears have a life span. Some of the race gears I put into the 993-997 race cars have a life span as low as 24 hours....especially highly "loaded" shorter 3rd gears.

It's just life. As the output of the engine goes up, the lifespan of the gear gets shorter.

Gears load the base of the tooth harder and small stress cracks form, right at this point. Many "race" gears have the tops of the teeth cut at angles to "broaden" this contact point, but this only helps, and does not eliminate this highly stressed area.

Taking a used ring and pinion with an "established" perfect wear pattern and moving it to "another" gearbox and "reseting" it up perfectly will almost 100% of the time result in a slightly different "contact" pattern at the base of the tooth. There's always a spot where these two "base" contact patterns will "cross". Right at that "cross point" is where the crack will start. It's as certain as the sun coming up.

In the 928 gearbox, the lower "layshaft" runs on needle bearings. Almost no one (besides me) replaces these bearings, in a rebuild. The support shaft wears slightly, the layshafts wears, the bearings wear. The "original" contact patch that is worn at the base of the individual gear teeth is essentially "moved", by this wear, as time passes. Add in a bunch more "load" and a bit of combined wear from the bearing area and the "pattern" will end up "crossing" on an individual gear. The result is "ripping" the teeth off right at the base of the tooth, where the patterns cross. (On a straighter cut gears, like the gears made for each ratio, note that this "cross pattern" will usually be right at the very "edge" of the gear, not in the middle of the tooth, like on a ring and pinion.)

Outside of "fun" club racing, I would never start a 12 or 24 hour race with a transmission that had anything but a new ring and pinion, with new gears in the highly loaded "individual ratio" positions. The cases had to be perfect, without any "loose" pinion or differential bearings. Of paramount importance was for these gearboxes to be broken in carefully, prior to them getting high loads in race conditions. Throw a brand new ring and pinion into a transmission and turn it directly over to a "race" driver rarely results in anything good happening. Eddy current chassis dynos are perfect for this break in and are a must for "highly loaded" applications.

One needs to be realistic. Most everyone running a 928 vehicle is dealing with "junkyard" transmissions. People aren't buying new gears...hell, it is hard to get some people to buy a new bearing! Put the transmission together, replace any worn bearings, try and keep the "wear patterns" at the base of the teeth from shifting (never reset the pinion depth or backlash on a worn-in ring and pinion) and hope for the best.

Rob Edwards

So this is a 'best case' rebuild scenario (?) (Though I think we put a 034 09 shift spring in as well (?))

GregBBRD

Yes. A fifth gear bearing, which spins all the time the clutch is released, is a must. Hell, people replace a pilot bearing every time they do a clutch job....and it only spins when the clutch is released.

The price of a bearing is not a function of when it should be replaced! (Note that the only 5th gear bearing that is very expensive is the 5th gear bearing for the pre '85 transmissions. We now have the ability to machine these transmission cases to accept the '85-'86 bearings, which is slightly larger and much stronger....plus 1/10th the cost!)

Those two needle bearings for the layshaft are very important. Yes, they are submerged in gear oil, but they live a life of hell. The layshaft bearings for the "early transmissions" is not available from Porsche, but I've found them and have them in stock.

john gill

The layshaft is a very marginal design , it is only supported at each end of the layshaft with the 2 needle bearings , , this shaft must suffer from deflection during stress . It could be better supported if it had a centre bearing to alleviate the deflection , and an access for some additional oil drilling .

Did I see it somewhere that the g28/57 had the main shaft drilled for oiling ?