Gearbox Recondition and 6-Speed Conversion

timmus

A2OC Donor
My A2 spent the Christmas period with Vince at Stealth for some fairly serious repairs and upgrades.

Some time ago, I posted a query about a metallic knocking noise upon accelerating and engine braking. There seemed to be slack in the powertrain or drivetrain. The knocking was being caused by the slack being taken up in either direction.
The most annoying manifestation of the problem used to happen when driving on cruise control. When driving over the apex of a hill on the motorway, cruise control modulates the accelerator to keep the speed steady as gravity first hinders progress (when going uphill) and then assists progress (when going downhill). There comes a brief point when the engine is neither positively driving the car forwards, but nor is it yet braking the car. At this point, the slack would ‘oscillate’ back and forth, causing a violent, rapid knocking.
With the help of various garages, the slack was eventually traced to the differential. For this to be fixed, the gearbox needs to be removed from the car and opened up. So, I asked Vince to do this for me and, upon rebuilding the ‘box, to add a 6th gear.

The purpose of this thread is simply to show the work that has been done to my car. I realise some members may already know and understand a lot of this, but nevertheless I'll try to give as good an explanation as I can manage with the photos I’ve got. Some of the photos were taken by Vince using his iPhone and others were taken by me using my SLR, hence the varying quality.
 
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timmus

A2OC Donor
Differential Rebuild

This photo shows the ‘core’ of the A2’s differential.
Old Diff 2.jpg

The larger ‘sun’ gears with the splined centres, to the left and right of the photo, go to the front wheels via the drive shafts and CV joints. When driving in a perfectly straight line, the smaller ‘planet’ gears do not rotate about the smooth metal bar through their centre but just ‘orbit’ the sun gears. The smaller planet gears only spin about the smooth bar when one of the road wheels needs to rotate faster than the other, when the car is going around a corner.

The cause of slack in my car was these sun and planet gears having worn slightly. This photo shows the wear to the gears' teeth, creating too much backlash.
Gear Wear.jpg

So, Vince purchased the necessary new parts. In the photo below you can see the diff’ carrier, the final drive ring gear, new diff’ bearings, new final drive mounting bolts, the black plastic ‘sleeve’ and new sun and planet gears.
Diff Parts.jpg

The standard rivets that attach the final drive gear to the differential carrier are weak. These rivets shearing is a common cause of failure of the 02J gearbox. As such, a modification kit is available. As you can see in the photo below, the 8 rivets have been replaced with nuts and bolts, with added reinforcing metal plates between the bolts. Once fully reassembled, the TDI's differential looks like this:
Rebuilt Diff.jpg

Input torque is applied to the ring gear (the final drive), which turns the entire carrier. The carrier is connected to both sun gears only through the planet gears. Torque is transmitted to the sun gears through the planet gears. The planet gears revolve around the axis of the carrier, driving the sun gears. If the resistance at both wheels is equal, the planet gears revolve without spinning about their own axis, and both road wheels turn at the same rate.
If one of the sun gears encounters resistance (from the road wheels), the planet gears spin as well as revolving, allowing that sun gear to slow down, with an equal speeding up of the opposite sun gear. Genius!
 
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timmus

A2OC Donor
Synchronised Gears

This photo shows the A2’s standard 5th gear set. The cog on the left is fixed to the output shaft of the gearbox, whereas the cog on the right spins freely about input shaft. Behind that cog, however, is the synchro hub, which is fixed to the input shaft.
5th Gear.jpg

The two photos below show the same setup from above. The synchro sleeve is able to slide in the left-right direction along the synchro hub, but spins in unison with the synchro hub due to the square ‘dog’ teeth. When the gear is selected, the synchro sleeve slides towards the cog, locking it to the synchro hub, and with it fixing the cog to the input shaft. As such, the input shaft and output shaft are locked together in the ratio of the selected gear.
Synchro Disengaged.jpg Synchro Engaged.jpg

I hasten to add that things are a little more complicated than this, but it explains the basics. What I’ve shown doesn’t allow for rotational speed matching, which is achieved by the synchro ring. The synchro ring normally sit between the synchro hub and the cog but, for simplicity, isn't shown in the photos above.

The synchro sleeve is moved back and forth by a fork that is (though a complex mechanical system) connected to the gear stick in the car. In the photo below, the selector that's standing upright actually goes the other way, into the page, but I obviously couldn't assemble it like that for the photo.
5th Selector.jpg

Normally, there is a gear set to either side of the synchro hub, meaning one of two gears can be selected using one synchro hub and sleeve, either by pulling the synchro sleeve downwards over one cog or by pushing it up over another. In the case of the 02J gearbox, the 5th gear is quite literally the odd one out, because it has a synchro hub and sleeve of its own. Consequently, the selector (and gear stick) is designed to only travel in one direction: 5th gear is selected by ‘pushing up’, both with the gear stick and within the gearbox.
 
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timmus

A2OC Donor
Adding the 6th Gear

In order for a 6th gear to be added, the selector mechanism needs to be modified such that the gear lever (and synchro sleeve within the 'box) is not just able to push upwards to engage 5th, but is also able to pull downwards in order to engage 6th. The photo below shows the selector mechanism before modification. The ‘claw’ at the very top of the photo is used to engage 5th. This part is replaced with the part lying on the bench to its right. Compare the claw in this photo to the last photo of the post above and all should make sense.
Selector.jpg

With the selector modified, it’s now able to operate a synchro sleeve that moves in two directions, enabling a 6th gear to be added. Because the normal 5th gear terminates the gearbox’s input and output shafts, it cannot be used in conjunction with a 6th gear. Therefore, when buying a 6th gear, you must also buy a new 5th gear. This photo shows the new 5th and 6th assembly, its synchro sleeve and the fork that moves the sleeve back and forth.
5th & 6th.jpg

This next photo shows the gearbox being rebuilt. On the extreme right-hand-side, you can see the reconditioned differential, turned upside-down. The modified selector mechanism has been put into place. The lower fork selects either 1st or 2nd gear and the upper fork selects either 3rd or 4th gear. The left-hand-side shaft is the input shaft, coming from the clutch. The right-hand-side shaft is the output shaft, which turns the differential. This ratio is the final drive. As you can see from the shiny parts, Vince replaced various synchro sleeves and rings as part of the rebuild. 5th and 6th gear go on the remaining space at the end of each shaft, but not until the first four gears have been enclosed within the main gearbox casing.
Gearbox Insides.jpg

With the main casing in place, 5th and 6th can be installed on the end of the shafts. Note from the previous photo that the synchros for the first four gears are on the output shaft, whereas the synchros for 5th and 6th are on the input shaft.
When fitting a longer gear, the driving cog gets larger and the driven cog gets correspondingly smaller, making the gear 'heavier' and increasing the forces on the gearbox shafts. If you have a heavier gear than intended on the very end of the shafts, furthest away from the clutch, you significantly increase wear and the risk of bearing failure.
Cleverly, the kit therefore no longer uses a pivoting rocker fork to move the synchro ring, as was used to select 5th gear (see the photo at the end of the preceding post). The loss of the pivot means that 5th and 6th have to swap places within the box so as to be in the correct position at the gear stick. Consequently, a standard 5th gear is on the very end of the shafts and the heavier 6th sits beneath it. As such, the heaviest gear set does not end up on the very end of the shafts, meaning it is supported more evenly by the bearings, keeping the forces within VAG's stated limits.
5&6 Fitted.jpg

The original end casing for the 02J gearbox is designed to enclose just 5th gear in its own little compartment. However, with a 6th gear in place, the original end casing doesn't provide enough space. As such, a new end casing is needed, which is shown in the photo below. Whereas the original end casing simply seals the gearbox, the new end casing has an integrated bearing to support the extended input and output shafts.
End Casing.jpg

With the end casing in place, the gearbox is a couple of inches longer and looks like this:
Extension.jpg

It can now be returned to its home under the bonnet. The photo below shows the gearbox fully fitted. Fortunately, there's plenty of space between the end of the standard gearbox and the wheel arch liner, meaning the lengthened gearbox fits without any complications. The VW logo on the bottom of the gearbox is where the base of the shift tower is held in place; this is the very end of the mechanical chain that starts at the gear stick. The extra, '6th' position at the gear stick appears by virtue of having modified the selector mechanism within the gearbox, as shown in the first photo of this post.
Gearbox Fitted.jpg

Once fitted, the 'box was filled with the latest spec' BlueMotion oil.
 
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timmus

A2OC Donor
Making Choices

There is, for the A2 TDI, a choice of two possible 6th gear ratios, namely 0.65 and 0.588.
  • The former option is almost exactly the same as the 0.659 gear that many people use as their ‘longer 5th’. I had a 0.659 5th gear before this upgrade. Anyone who has driven a factory 6-speed VAG vehicle will have noticed that 6th gear is generally only slightly longer than 5th, meaning that the gap between 5th and 6th is considerably smaller than that between 4th and 5th. It results in a motorway cruiser gear that's closer to peak torque, but doesn't have quite the gains in economy. Choosing the 0.65 ratio as a 6th gear will result in such a setup.
  • Choosing the 0.588 gear results in a gap between 5th and 6th that is equivalent to the gap between all the other gears. This is the 6th gear that MikeMars chose and is also what I have chosen, and when driving, to my mind, it just feels 'right'. For my driving, it also makes sense.
As previously mentioned, a new 5th gear must be bought as part of the upgrade. The 5th gear options are 0.74 and 0.71; the choice being determined by whether you're upgrading a TDI75 or TDI90.
  • The former is the perfect choice for the TDI75, as it keeps the 5th gear (almost!) as standard.
    I think I'm right in saying that MikeMars did not appreciate that this choice existed at the time that he bought his kit. Given that he’s running the longer of the two 6th gears available, the longer of the two 5th gears was chosen for him. Thanks to Mike's pioneering, I knew to choose the standard 5th, which is, I believe, what Mike would have chosen had he known about this at the time.
  • The TDI90 has slightly longer gearing than the TDI75. The TDI90 has a 0.70 5th gear as standard, meaning that the longer of the two available 5th gears is a near-perfect match.
The graphs below show speed (in the horizontal) as a function of engine rpm for all gears.
  • This graph shows the two available 6th gear options for the A2 TDI75. It has a 0.75 5th gear as standard, but, as previously mentioned, the closest match is 0.74. This difference between 0.75 and 0.74 is completely imperceptible, so can be thought of as standard. The drop in engine revs at 70mph is highlighted.
    Audi A2 1.4TDI EWQ Choices.jpg
  • This graph shows the gearing setup for a 6-speed TDI90. Given that the TDI90's standard 5 gears are already longer than those fitted to the TDI75, it would make little sense to fit a 0.65 6th gear. As such, only the 0.588 6th gear is shown.
    Audi A2 1.4TDI GRJ Choices.jpg
The graph below was custom-made for my TDI75 by Radim, the gent who supplies Vince with the 6th gear conversion kit. This graph takes into account the larger circumference of my tyres, whereas the graphs above assume a standard tyre size. At 70mph, my engine revs would be reduced by 94rpm when using the standard 0.75 5th gear, and are reduced by 74rpm in 6th gear, simply by running slightly larger tyres.
My A2.jpg

How does it compare to the longer 5th?

By far the most common of the 'longer 5th' options is the aforementioned 0.659 ratio. Relative to the 0.588 6th gear, it is a '5.56th' gear, sitting almost perfectly between standard 5th and the 0.588 6th gear. The graph below shows this gearing arrangement for a TDI75.
Audi A2 1.4TDI EWQ Longer 5th.jpg

There's no doubting that this is a much cheaper modification, but it's a compromise solution and doesn't necessarily deliver the desired returns in fuel economy.
Whether the longer 5th reduces your net fuel consumption depends greatly on your use of the vehicle. I predominantly do motorway miles, so my previous longer 5th setup would frequently save me money. However, I also do a fair amount of driving in mountainous areas, where I would essentially only have 4 gears due to the longer 5th being inaccessible in such environments. As such, the longer ratio would effectively cost me money.
On the whole, I saved, but many A2 owners with the longer 5th find that their average fuel economy does not go up by much, if at all. Sometimes, all that's gained from the mod is a quieter drive when motorway cruising. The longer 5th is also a down-grade in terms of the vehicle's 'driveability'. On a winding A-road, managing the larger gap between 4th and 5th can get really tiresome, with the driver continually having to choose between rev'ing it harder in 4th or spluttering slightly in 5th.
For all that I've upgraded to 6 gears, I remain a fan of the longer 5th setup. Although it needs to be thought of as a change rather than an upgrade, if I was still limited to 5 gears, it would be my setup of choice. For those who use their A2 almost exclusively as a motorway cruiser, it's a very good option; its pay-back time is relatively short and it makes motorway driving a whole lot more comfortable.

Just for interest, this photo shows the standard 5th (on the left) compared to the 0.659 ‘longer 5th / shorter 6th’. As you can see, there’s visually not a huge difference between them.
Standard & Longer 5th.jpg
 
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timmus

A2OC Donor
6-Speed Gear Knob

I wish to keep my A2 looking OEM. This meant finding a way of making it look like it left the factory with 6 gears. The A2’s gear stick mount is different to other Audis, so simply buying a 6-speed gear stick designed for a factory 6-speed Audi wouldn’t work.
However, I found that the Typ 4F Audi A6 uses exactly the same gear knob emblem design as the A2, except the A6 was made with a 6-speed manual transmission. So, after hunting around a bit, I found this...
6-Speed Emblem.jpg

This has now been fitted into my original A2 gear knob, meaning it looks exactly like it did when it left the factory, except it’s got 6 gears!
This photo shows the gear knob pulled into its new position.
Gear Knob.jpg

Whilst looking into this, I found that the A2’s emblem changed between 2001MY and 2002MY, though only subtly. If you look at the photo below, you’ll see that the font size was increased slightly. My 6-speed emblem has the larger font, which means it’s suitable for my 2002 A2. Personally, I think the larger font is nicer anyway.
Font Change.jpg
 
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timmus

A2OC Donor
After I left Stealth, I drove a few fast, winding roads on my way to the M1. These are the sort of roads where the longer 5th is less than ideal, because there's the need to 'manage the gap'.
A full 6 gears offers even better 'driveability' than the OEM 5-speed setup whilst also giving even better combined fuel economy than my previous 0.659 5th. It is better than the best of both worlds. Furthermore, the gear changes are smooth and tight and the annoying slack in the diff' is gone. I am absolutely delighted with it!

I hope this has made for interesting reading. If you're tempted to add a 6th gear to your A2 TDI, I cannot recommend Vince enough. He was thorough in the extreme, communicated with me regularly about progress, and was, quite obviously, happy to indulge my desire for photos!

Cheers,

Tom
 
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spike

Member
Nice write-up Tom.
Regarding the 6 speed emblem, is there an easy way to remove it without damaging the gearknob

Cheers Spike
 

timmus

A2OC Donor
Nice write-up Tom.
Regarding the 6 speed emblem, is there an easy way to remove it without damaging the gearknob

Cheers Spike

Thanks Spike!

I had a sacrificial A2 gear knob that I used to figure out how it was put together, as I didn't want to damage the A6 gear knob's 6-speed emblem or the gear knob in my own car.
The emblem is held in place with a vertical plastic clip from below. There's no way of releasing the clip, but its hold isn't especially strong. So, you can pop it out by getting a very fine screwdriver and inserting it at the 12 o'clock position of the gear knob, where the leather is stitched together. Get the screwdriver under the very bottom of the emblem, rather than levering it out by the chromed lip around the top.

Cheers,

Tom
 
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MikeMarsUK

Member
A brilliant write up, and great photos :)

You're right, if I had understood that there were two possible 5th gear ratios I would have gone for the 0.75 rather than the 0.71. It's only a very small difference but I find the slightly increased gap between 4th and 5th a little annoying. Obviously the gap from 5th to 6th is quite large with the combination of 0.75 and 0.588, but the 6th gear was always an overdrive gear for me, so would have been fine from my viewpoint.

That was an interesting solution to the 6-speed gearknob, I wondered if it was possible to change the emblem but because I was unsure, I went for the ICT gearknob instead. Have been really slack and still haven't fitted it!
 
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a2sumo

A2OC Donor
Great wright up- this is something that I would like to do in the future as i always feel the tdi is reving so much harder than the fsi. Hmmm maybe next year. Suspension and rnse next.
Thanks for sharing and another great promition for the guys at stealth racing. Cheers mike
 

timmus

A2OC Donor
A good write-up that, a good read.

A brilliant write up, and great photos :)
That was an interesting solution to the 6-speed gearknob...

Great write up-
Thanks for sharing...

Thanks guys! I had fun polishing the photos and arranging the info in what I deemed to be a logical order.

The emblem took a bit of 'research' (I looked through the windows of parked Audis for a month!) but it was worth it because I think the solution is perfect. I simply couldn't get rid of the original stick, not least because it matches the steering wheel and handbrake lever.

Yeah, I agree that the TDI is crying out for an extra gear when driving on the motorway, especially at 70mph. At 60mph it's fine, but the added 6th gear makes the engine noise virtually inaudible. That's when I fire up Bose! :)

Tom
 
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andybaggies

Member
Cracking write up and don't you just love gears & gearboxes! I'm no great expert myself and I'll need a few more re-reads before I even get close to full understanding.

One question though - at 70 what rpm are you turning in 5th & 6th? IIRC 'ol Hector is spinning at ~2,500 rpm.

Cheers,

Andy B
 

MikeMarsUK

Member
Cracking write up and don't you just love gears & gearboxes! I'm no great expert myself and I'll need a few more re-reads before I even get close to full understanding.

One question though - at 70 what rpm are you turning in 5th & 6th? IIRC 'ol Hector is spinning at ~2,500 rpm.

Cheers,

Andy B

in the 0.588 6th, 70mph = 2000rpm, it's really perfect for cruising
 
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DJ 190

Member
It will take a certain amount of force to propel a vehicle at a constant speed and that can be calculated. I understand that your extra 6th gear reduces engine revolutions and as a consequence, friction is reduced. What I find a little difficult to understand is that in itself should result in such significant improvements in fuel economy?

David
 

timmus

A2OC Donor
Mike (Mars) posted some brake-specific fuel consumption graphs somewhere that tell a more complete story, but basically...
Every rotation of the engine results in a certain amount of fuel being burnt. If you revolve the engine fewer times, you use less fuel. If you can therefore travel the same distance for a reduced number of engine rotations, your fuel economy will go up. A 'threshold' amount of torque/power is required to maintain a given cruising speed, but as long as you're still generating torque/power above that threshold with the reduced revs, there's no problem. This is obviously the basic principle of the longer 5th too, in all its variants.

Tom
 
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spike

Member
Just to add a bit more 'meat' to Tom's comments on fuel consumption -

Lower revs means lower engine friction and pumping losses so less fuel is wasted just to keep the engine spinning over. To give you some idea, a typical 14 litre truck engine takes about 100 Hp to spin it over at peak revs
In addition an engine runs at varying efficiency levels depending on load and rpm. In the old days, optimum engine efficiency was usually around peak torque rpm but with turbos and electronics this is not always the case.
Depending on revs, the engine uses different ammounts of fuel to produce the same horsepower. During development this is plotted and the dots joined up to produce fuel consumption curves - often known as 'onion' curves because the mapping looks a bit like the rings of a mis-shaped oinion when sliced. These curves can be used to evaluate the effects on fuel consumption of cruising at lower engine rpm.
In the extremes, lower rpm is not always best as somewhere below peak torque rpm the fuel curves bottom out and consumption starts to creep up. In addition if the car starts to 'chug' in a higher gear than conditions require, increased torsional activity in the drive train can result in increased wear on gears, joints and bearings.
The trend towards 7 and 8 speed transmissions means there is a correct gear for every occasion, both from a fuel consumption and driveability standpoint.

Hope this makes sense.

Cheers Spike
 
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