3rd Gear Chatter, rebuild or swap gearbox internals?

MisterCorn said:

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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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.

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

Dammit said:

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

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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.

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

Dammit said:

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

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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.

I saw this thread and thought - "this is going to be interesting. A DIY gearbox rebuild with different internals from a slave box?"

Mike at Sports & Classic knows these boxes well and is setup with all the tooling to do the jobs on gearboxes and diffs. I doubt anyone could DIY a proper rebuild of the box or diff as there are numerous calculations involved and specialist tools required and a heavy press (as well as specialist know-how) - though there will be an opposing view that says these are just mechanical gearboxes that any competent mechanic can rebuild - and they can with the specialist tools and experience.

It will be interesting how this finishes up - fully DIY'd or at a specialist with all the internals to rebuild using the tools for the job and the proper settings - backlash, heights etc.

If any forum member can figure out how to measure and set up the required shims, it's Mistercorn.

Martin996RSR said:

If any forum member can figure out how to measure and set up the required shims, it's Mistercorn.

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I will be doing my best to get a service manual or equivalent for it, but initial searching has not been fruitful. Dammit, myself, and Infrasilver are all on the case....

Sorting the relevant tools is another issue, but by far the biggest problem at this point is getting the basic information.

MC

cheshire911 said:

I saw this thread and thought - "this is going to be interesting. A DIY gearbox rebuild with different internals from a slave box?"

Mike at Sports & Classic knows these boxes well and is setup with all the tooling to do the jobs on gearboxes and diffs. I doubt anyone could DIY a proper rebuild of the box or diff as there are numerous calculations involved and specialist tools required and a heavy press (as well as specialist know-how) - though there will be an opposing view that says these are just mechanical gearboxes that any competent mechanic can rebuild - and they can with the specialist tools and experience.

It will be interesting how this finishes up - fully DIY'd or at a specialist with all the internals to rebuild using the tools for the job and the proper settings - backlash, heights etc.

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I totally respect Mike and the work he does on our gearboxes, he's the No1 name for me in the UK when talking gearboxes and obviously he has to guarantee all his work plus he wouldn't want any come backs so has to make sure the gearboxes are 100% correct, especially in a race car.

People have rebuilt these gearboxes themselves without a lot of the specialist tools but the main tool being the press which is needed if you are replacing the spinning bits, everything else is fine tuning to a certain extent. I will plod on and see how far I get with this and keep researching to see what more we can find out?

I know people have fitted diffs etc into used gearboxes without a problem but even so I will be measuring up both the gearboxes to see what differences there actually are between the two if any at all. And in the end if it all goes wrong I can get the box rebuilt by someone else, I just enjoy trying to find out how stuff works and then also if I can fix it, the same as I do with everything. Current list of jobs for this week, Rebuild G96 gearbox, Seat Ibiza Timing chain, a washing machine repair and 944 Sunroof slipped worm gear.

Morning,

As you say above one of the tools you need is a very heavy duty press.

We run a 75 tonne press with all the press plates that support the gears, so you can press them on and off safely without damage.

The amount of gearboxes we get through that have come from other places especially gearbox rebuild centres that are damaged is staggering.
They cause more damage than the original fault and in the end it costs the customer more.
We must have at least 3 gearboxes a month brought into us in plastic boxes, partially stripped and the garage / specialist have come to the end of what they are prepared to do, the funny thing is most of these are customers that we have spoken to previously and given a price to do the job and for what ever reason they have not brought the box into us, sometimes it's a financial decision other times it's simply because we can't do the work for 4-6 weeks so they go with the garage that has pulled the gearbox out and sent off to their 'go to" gearbox specialist repairer.

We have actually financially wrote a few gearboxes off due to chipped and damaged gears, that could of been avoided if the correct press tooling had been used.
We have seen things welded to the gears and slots cut into the gear collars so they can pull the gear off the shaft, seen some things that really I do wonder what goes through people's minds to get to that point.

There are differences in casings, machining, bearings and gears. They all have tolerances. There are also a range of shims available from 0.5mm - 1.60mm just for pinion bearing. Input shaft bearing shims from 1.2 to 2.5mm, differential shims from 1.5mm to 2.8mm. That's just the 996/986 gearboxes. I bet we would have over 40 shims per shaft and and at least double that for the differentials.

We also custom grind shims for special applications, like gearboxes that run aftermarket differentials with billet aluminium side covers. Sometimes the diff work alone can take us anywhere from 4 - 6 hours. We have invested heavily in tooling, we also use clam shell style pullers that can pull a bearing off a differential with out damage to the bearing cage/rollers as on some applications there is no clearance behind the bearing to pull on. Although I admit this is more aftermarket applications we do still use the puller for 996 turbo gearboxes as clearance is very tight on the larger differential carrier bearing.

I can remember my very first pinion bearing replacement that I did £200 for a press tool...! I had no idea I would be using it again and again. Borrowed all the setup tooling and a friends 50 tonne press. Worked through it and sorted the gearbox out. After that I had another one and another one so we took the plunge and invested around £10k into the gearbox side of the business for tooling. A 50tonne press arrived and that served me for around 10 years, I finally bought an air assisted 75tonne press to make the job easier as pressing those gears off and back on can be hard work.

To do the job right the pinion height needs to be checked and the diff needs setting up, all clamping pre tensions need doing to input shaft bearing and pinion bearing. Differential rolling torque setting and finally backlash between crown wheel and pinion head gear. That's just basic gearbox fundamentals.

Mike.

Thanks for your in depth explanation Mike regarding these gearboxes, they do seem to be very fiddly in respect to setting them up. I was looking at the amount of shims that are available on PET and there are loads of them.

I haven't got my current gearbox out quite yet as I've been away for a few days but I got very close today before I stopped what I was doing.

I was just about to undo the last bolt from the bellhousing and then slide the jack under the car to take the weight when I spotted a shiny part on the gear selector bars weight, I then realised it had been rubbing on the driveshaft/CV joint for what looks like for a while.

I deduced that this was where my 3rd gear noise was coming from and I sent Mistercorn some pictures, but we both couldn't work out why other gears weren't making the same noise. I connected the gear shift cables back up to the gearbox to check how it actually shifted, it was then I realised when in third gear (and 1st and 5th) this weight is actually furthest away from the driveshaft so this couldn't be the problem noise but then I ran out of time today in the end with finding this so will have to carry on finishing getting the box out on Tuesday now.

Some pictures of what I thought was my problem, I guess the gearbox does still need stripping down after-all. Its strange that I hadn't noticed the noise from this rubbing previously though, will have to work out why that weight is so close to the driveshaft when it obviously wasn't before once I get the casing off.

You can see the clean part of the CV joint and where it has been rubbing on the shifter weight.



And it's worn into the same shape as the CV joint.

Got my gearbox off today and stripped it down, it was indeed 3rd gear making the noise and I found 3 teeth missing on 3rd, two together and another on the opposite side and they were all stuck to the magnet in the base of the gearbox. I just need to weight up what path to take now.

I'm guessing a bad shift may have caused this but I don't remember a bad shift? There is no other damage or wear on any other internal parts.

Has that gearbox been apart before ?
The reason I ask this is that normally when the teeth break off like that is because they have been chipped previously when the gear has been pulled.

Take a picture of the shift arrestor detent please. (The pin that is held in with a spring and a M18 threaded plug with 8mm Hex drive. If you can measure the roller in the end of that...

Mike

sportsandclassic said:

Has that gearbox been apart before ?
The reason I ask this is that normally when the teeth break off like that is because they have been chipped previously when the gear has been pulled.

Take a picture of the shift arrestor detent please. (The pin that is held in with a spring and a M18 threaded plug with 8mm Hex drive. If you can measure the roller in the end of that...

Mike

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Yes Mike, both have been rebuilt, my spare box had Input shaft, new 2nd gear and the major bearing, the other box that was in the car was reconditioned but I don't have any info of what was replaced, although looking at that input shaft it also looks quite new.

It also makes perfect sense that a gears teeth could be weakened by the amount of force applied to it, I did check the teeth today that had broken off but there were no chips noticeable on them. It has done quite a few thousand miles with this gearbox attached.

I have measured the roller as you suggested, I see what you are saying with this, if it is worn it may lay the shifter bar over more. I will wait for your reply in relation to whether it is worn or not from the diameter (picture below) but I also have a feeling it may have been bent somehow the last time it was remove/installed?

While I was at it I also inspected the flywheel, I found the spec for this DMF and it should only rotate 2 teeth of the starter ring or 6 degrees, it was easier to mark the middle position on the teeth and move the flywheel in either direction.

The DMF manufacturer part number is 415 0102 10



It's moving around 3 teeth so is out of range and will require a new one.





I also checked the RMS and IMS seals but these are fine so I won't be disturbing them.