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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8374
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Mon Sep 02, 2019 12:32 pm    Post subject: 3rd Gear Chatter, rebuild or swap gearbox internals? Reply with quote

Just trying to get some info before I strip my gearbox looking for why 3rd gear is making a chattering sound when engaged. All the other gears are silent and work perfectly, 3rd gear also works fine with no noticeable vibration through the gear stick but I have done a few thousand miles with it with this noise across Spain and Portugal but I'm looking to head abroad again at the end of the month so would like to try to rectify the problem before it does fail or gets worse. It sounds like a bearing rattling but I'm not sure if it is or a gear but I won't know until it's apart?

I have a spare gearbox that was rebuilt a few years ago but this has a cracked bellhousing/diff casing, I think my plan is to swap the gear clusters from my spare box to my current box which would save the cost of a rebuild.

How easy would this be to do, I'm hoping its just a case of unbolting the casings and swapping the gears over, is there anything I should watch out for as this will be the first time I have stripped the 996 gearbox down and I am trying to make it as painless as possible?

Is there any information to show what gear is what within the gearbox and would it be easier just to repair the gearbox I have, obviously this would need a press though to get individual gears and bearings off?




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Last edited by infrasilver on Fri Sep 06, 2019 10:11 pm; edited 1 time in total
 
  
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7209
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Mon Sep 02, 2019 1:10 pm    Post subject: Reply with quote

I'm going to upload a video in a bit about the gearbox internals.

MC
 
  
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8374
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Mon Sep 02, 2019 1:38 pm    Post subject: Reply with quote

MisterCorn wrote:
I'm going to upload a video in a bit about the gearbox internals.

MC


thumbsup Great timing then.
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TurboRob
Trainee


Joined: 18 Feb 2019
Posts: 82



PostPosted: Mon Sep 02, 2019 2:05 pm    Post subject: Reply with quote

I think swapping the good/rebuilt gear cluster in to your current casing is totally doable - I've had a spare gearbox apart recently and from what I can see none of the internal parts are machined to fit a specific casing.

EDIT - this is what I mean by cluster. Fully assembled mainshaft and layshaft on the spider.

[img]https://cimg1.ibsrv.net/gimg/rennlist.com-vbulletin/1297x975/img_6679_b61cf758186e58746af55d61ecf7b59505158014.jpg
[/img]

(Reverse gear missing from above, but you get the jist)
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8374
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Mon Sep 02, 2019 3:34 pm    Post subject: Reply with quote

This would be my preferred option as it is zero cost, but may want to rebuild this noisy gear cluster at a later date, again just to have as a spare ready to go in if required.

This does all depend on the condition of the gears I have in my other box, I know the input shaft was replaced and a couple of other things too but I would need to dig the invoice out to see what exactly.
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8374
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Tue Sep 03, 2019 8:13 pm    Post subject: Reply with quote

I managed to strip most of my spare gearbox down and inspect the gears today although it is quite fiddly getting a couple of the bolts off the retaining ring.

I noticed the previous time is was rebuilt it had three allen head bolts that are almost impossible to get at and they had previously been undone with a punch, I decided to do the same as I didn't want to round the head off the allen key hole as they were quite tight.



All the gears and syncros seem to be in perfect condition, no chips on the gears or pitting and no teeth missing.



I just need to work out how to allow the gear cluster to come away from the front housing, I haven't done any research on how this comes away yet so only got as far as I could work out what can be removed easily.

Also I compared my previous input shaft against the one in the gearbox and you can see the pitting the old one had on (I think) 2nd gear?


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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7209
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Tue Sep 03, 2019 8:17 pm    Post subject: Reply with quote

The small gear is used for reverse at the top and first lower down. The larger gear is second.
I believe you need to undo the retaining plate, then warm up the housing and lever it out. It is mentioned in the links I sent.

MC
 
  
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TurboRob
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Joined: 18 Feb 2019
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PostPosted: Tue Sep 03, 2019 8:52 pm    Post subject: Reply with quote

Boiling hot kettle of water on/around the housing where the end bearing is retained and it just dropped out for me
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8374
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Tue Sep 03, 2019 8:56 pm    Post subject: Reply with quote

MisterCorn wrote:
The small gear is used for reverse at the top and first lower down. The larger gear is second.
I believe you need to undo the retaining plate, then warm up the housing and lever it out. It is mentioned in the links I sent.

MC


Just reading through some of that info now, I have all the retaining ring bolts out but I haven't punched the pins out from the selector rods. I was hoping it would all come out as one assembly as this would make life a hell of a lot easier just swapping the whole lot over but it seems I'm not able to do that. I can then hopefully heat the housing up to remove it all and then get on with removing my currently installed gearbox.
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8374
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Fri Sep 06, 2019 9:26 pm    Post subject: Reply with quote

Got the spare box all stripped of its gubbins now and all the bearings feel smooth so nothing needs changing at all luckily so now I just need to roll the car in remove the box and swap internals over, easier said than done.


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sportsandclassic
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Joined: 21 Apr 2008
Posts: 273
Location: Nether Alderley Cheshire


PostPosted: Mon Sep 09, 2019 7:05 am    Post subject: Reply with quote

Good job so far - How do you plan to shim the bearings back up ? The pinion shaft bearing and input shaft bearings have shims to set clamping force on the outer races and set the pinion shaft height ?
There is a tool to do this but they have to be removed.

Also you are now swapping a pinion shaft into another gearbox with a different differential.. ? They are matched when new and have matching numbers with installation heights and backlash figures.

The differentials all need pre-loading and then the backlash setting.

This really is a specialised job as quite a lot of specialist tools are required in order to set the gearbox up properly.
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7209
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Mon Sep 09, 2019 7:14 am    Post subject: Reply with quote

sportsandclassic wrote:
Good job so far - How do you plan to shim the bearings back up ? The pinion shaft bearing and input shaft bearings have shims to set clamping force on the outer races and set the pinion shaft height ?
There is a tool to do this but they have to be removed.

Also you are now swapping a pinion shaft into another gearbox with a different differential.. ? They are matched when new and have matching numbers with installation heights and backlash figures.

The differentials all need pre-loading and then the backlash setting.

This really is a specialised job as quite a lot of specialist tools are required in order to set the gearbox up properly.


I have read about the different shims for this, and seen photos of cut-away cases being used so that the clearance can be measured with the case fitted. What I haven't found yet though is the required clearance figures. In fact there generally seems to be very little information on rebuilding these gearboxes.

MC
 
  
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8374
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Mon Sep 09, 2019 2:44 pm    Post subject: Reply with quote

sportsandclassic wrote:
Good job so far - How do you plan to shim the bearings back up ? The pinion shaft bearing and input shaft bearings have shims to set clamping force on the outer races and set the pinion shaft height ?
There is a tool to do this but they have to be removed.

Also you are now swapping a pinion shaft into another gearbox with a different differential.. ? They are matched when new and have matching numbers with installation heights and backlash figures.

The differentials all need pre-loading and then the backlash setting.

This really is a specialised job as quite a lot of specialist tools are required in order to set the gearbox up properly.


I'm not sure of the clearance figures, it would be interesting to know how they are measured, I can get the shims (plus the ones I already have) but getting the clearance right will be difficult.

The end shims are sort on conical, I'm guessing to take up any slack, is the how they should be or should they be perfectly flat?

How does it set the height of the pinion, I'm not sure how that would be affected by shimming the end of the bearing, or are you taking about something else needing shimming? I also heard about the open casing idea when talking to MC recently to be able to visually measure clearances, but I don't get how that casing would then be the correct dimension before putting it back in the box you are rebuilding?

I know about the diff being set up and others have had problems when fitting a new diff with it making noise as it was loose, I assume more research will be required on this?

Would the best way around this be to change just the damaged parts of my original box, would that be the best option, then as it should all go back the same as it came out?
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sportsandclassic
Approved Trader


Joined: 21 Apr 2008
Posts: 273
Location: Nether Alderley Cheshire


PostPosted: Mon Sep 09, 2019 7:03 pm    Post subject: Reply with quote

MisterCorn wrote:
sportsandclassic wrote:
Good job so far - How do you plan to shim the bearings back up ? The pinion shaft bearing and input shaft bearings have shims to set clamping force on the outer races and set the pinion shaft height ?
There is a tool to do this but they have to be removed.

Also you are now swapping a pinion shaft into another gearbox with a different differential.. ? They are matched when new and have matching numbers with installation heights and backlash figures.

The differentials all need pre-loading and then the backlash setting.

This really is a specialised job as quite a lot of specialist tools are required in order to set the gearbox up properly.


I have read about the different shims for this, and seen photos of cut-away cases being used so that the clearance can be measured with the case fitted. What I haven't found yet though is the required clearance figures. In fact there generally seems to be very little information on rebuilding these gearboxes.

MC


I am not sure that’s what we are talking about, we have casings with the side cut away in order to check engagement depths of shift sleeves. Nothing to do with pinion heights or pre-loads.
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7209
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Mon Sep 09, 2019 7:15 pm    Post subject: Reply with quote

Ah yes, that was it. Shimming the shafts to check the engagement. Apparently incorrect setting of these was thought to be a cause of the 2nd gear popout on early gearboxes.

Is there a good document for required clearances for setting up the bearings?

MC
 
  
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sportsandclassic
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Joined: 21 Apr 2008
Posts: 273
Location: Nether Alderley Cheshire


PostPosted: Mon Sep 09, 2019 8:00 pm    Post subject: Reply with quote

MisterCorn wrote:
Ah yes, that was it. Shimming the shafts to check the engagement. Apparently incorrect setting of these was thought to be a cause of the 2nd gear popout on early gearboxes.

Is there a good document for required clearances for setting up the bearings?

MC


It’s all maths!

Measuring the gearbox bore where the bearing is housed and then measuring the bearing. There is a formula to work it out but with out the tooling it’s pointless.
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Dammit
Indianapolis


Joined: 23 Sep 2016
Posts: 2275



PostPosted: Mon Sep 09, 2019 8:11 pm    Post subject: Reply with quote

Is the tooling to do the measuring, or fitting the shims? Or something else?
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sportsandclassic
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Joined: 21 Apr 2008
Posts: 273
Location: Nether Alderley Cheshire


PostPosted: Tue Sep 10, 2019 10:14 am    Post subject: Reply with quote

Dammit wrote:
Is the tooling to do the measuring, or fitting the shims? Or something else?


Morning,

The tooling is there to do the measuring of the gearbox internals.
You need to measure pinion engagement height with the special tools.
You need to set pre-loading on all bearings, some are done with tools to work out shims, others with dial test indicators. Some need pre-tension to hold the bearings down others need pre-loading on taper roller bearings.
You also then have to adjust everything - again the tooling is required to verify and correct it.
Also needed is to set the crown wheel back lash, this is scribed on the crown wheel by the setup operator when they are new, this is the optimum running position.
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Dammit
Indianapolis


Joined: 23 Sep 2016
Posts: 2275



PostPosted: Tue Sep 10, 2019 2:26 pm    Post subject: Reply with quote

Fascinating - thanks. Did you have to make the measuring tools yourself, or did you order "G96 rebuild kit" from Getrag?
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sportsandclassic
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PostPosted: Tue Sep 10, 2019 2:36 pm    Post subject: Reply with quote

Dammit wrote:
Fascinating - thanks. Did you have to make the measuring tools yourself, or did you order "G96 rebuild kit" from Getrag?


Bought them over many years of doing gearboxes.

Can not do the job properly without them - we do crown wheel and pinion installations on high powered Porsche gearboxes and I for one don't want any issues with running clearances or pre-loads.
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