Porker928

Hi

I fitted a G28/55 transmission to my S2 Euro a couple of years ago to replace the damaged original G28/07. However the transmission came from a scrapyard and the millage was unconfirmed. Although the gears work fine there is an annoying rattle at high torque / low speed. I’ve spoken to a GT owner who has the same transmission with 150K miles on the clock and he said his has the exact same sound. So I’ve decided to have the transmission completely re-built to get rid of the sound and avoid any possible catastrophic damage due to the failure of a worn component.

So far I am replacing:

All seals
All Bearings
Most of the syncros
The output shaft and pinion
[The LSD has already been replaced]
[The 1st to 2nd guide spring has already been replaced]

I’m replacing the output shaft and pinion because a local transmission specialist, who has worked on 928 transmissions before, test drove my car and told me that it should be included in the re-build as it was worn. Although I don’t know how he determined this? However another leading UK 928 specialist told me that they have re-built many 928 transmissions and never had to replace the output shaft and pinion – so I don’t know who to believe.

I am writing this post as I need to know if it would be advisable to replace anything else? – I’m hoping that the main gear sets will be OK but I don’t know much about the shifter rods and forks - Does anyone know if these are liable to wear or are they pretty bullet proof?

Thanks.

SeanR

I'm wondering where they are going to get a new output shaft.

mark kibort

output shaft or are you talking 5th gear set (input shaft)?

Kevin in Atlanta

Subscribed...

I need the 5th gearset for a 1991 GT, too!

docmirror

I have a mild case of that whine, not rattle at low speed, high torque on my GT. Pretty sure you will find that there's nothing wrong in there, and when you're done spending $$$$$$$$$$ it'll be basically the same. I call it the 'hypoid strain' and leave it alone.

In 1971 I had a VW beetle that did the same thing on all of 54HP, and it worked for centuries without a problem. But - good luck, keep us informed.

Imo000

How about diagnosing the cause of the noise first AND THEN fixing it? Throwing parts at it may not do anything.

James Bailey

There have been a few GTS automatics which have had ring and pinion noise....

GregBBRD

You should not need a ring and pinion (output gear set). See immediately below.

Limited slip got replaced? Unless the person doing this understood that the bearing preload and the backlash needed to be reset when the differential is replaced, this could be a problem. If this was not done properly, there could be abnormal wear on the ring and pinion.

If it does need a ring and pinion, there are some very special tools and knowledge needed to set up a new ring and pinion. Make sure that the people doing this job have the proper tools and the knowledge to do this!

The shift forks and shafts should be good, the exception being the possible need to replace the 1st and reverse shift fork (928 303 115 41) on high mileage transmissions. The steel selector can wear this bronze fork badly. You will need to buy new roll pins (900 309 013 00 and 900 309 013 00.) Make sure the 900 309 013 00 gets installed in the shift selector lever and not in the shaft spring detent piece....PET has had this wrong, for all the years I can remember.

The shift sleeves and hubs generally do not need to be replaced, especially if the synchros are still good and it shifts without crunching.

Good idea to replace the bearings...people forget that the needle bearings on the lower shaft have a life span and need to be replaced. One of these bearings is NLA from Porsche, but still available from the manufacturer.

The main spring for the shift location (928 303 034 09) is a high failure rate part and I used to replace this spring on every transmission. It is NLA, unfortunately.

Buy a new nut for the pinion shaft (928 302 266 09)....it's a "one shot" thing. Buy a new pinch bolt for 5th gear (931 421 240 00.)

The one thing that you do need to know and make sure the person removing the transmission knows....make sure that the transmission gets pulled straight off of the torque tube shaft. Any angle other that straight can crack or break one of the three "leaves" on 5th gear. Unfortunately, this is somewhat common!

Kevin in Atlanta

Hmmm. Has anybody repaired cracked or broken 'leaves'?

GregBBRD

Not that I'm aware of. They are pretty hard and thin. Not sure how anyone could re-attach.

928cs

+1

I have observed that only really low kms 928 don't do this noise.
By low, I mean less than 50,000 kms.
The solution is to drive quicker!

Kevin in Atlanta

I was thinking about finding a used 5th gear set of another year and having a machinist (with the requisite skills) cut off the leaves with stem (since we are using foliage terminology) and graft it on to the '91 gear set.

Rob Edwards

RE: the low rpm/hi torque 'noise', if it's that graunchy noise made around 1500 rpm when just getting into the throttle from a standstill, every late-model 5 speed I've driven makes that noise.


5th gearsets for GTs were last available in 2012, but NLA the last time I checked in 2014.

They had plenty of GTS 5th gearsets, and they were cheap enough (~$700) that I put one on the shelf so that I'd never need one. I don't know what the difference between GT and GTS 5 gearsets is, but if putting a GTS 5th gear in a GT box it was a simple mod, I think Greg would already be doing that to repair GT gearboxes.


There is also the G28.13 (87-88 US 5-speed) 5th gear, which is theoretically still available for $1300ish)- Not sure why it couldn't be used for 5th gear in a GT box.

James Bailey

different lay shaft ratio...changes all the lower gear ratios..but it would "work" just alters the gears. 5th "gear" stays the same 1 to 1

GregBBRD

I don't see how doing this would be anywhere near as strong as it was originally. My initial reaction would be thst there would be carbon migration to the weld site, making this very brittle.

However, I don't know enough about the material or the process to be able to actually be any real help.

You need to talk with someone way smarter than me about this.