How to fit t/d 5th gear

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How to fit t/d 5th gear

Post by rutter123 »

How much of a job is it to fit a t/d 5th gear into a gti gearbox, im gonna be changing the clutch on the gti soon and thought i would do the 5th gear conversion whilst its all out, i believe this is/was quite a common swap to do, i have a good t/d box here which im willing to sacrifice for this. Will i need any special tools for this job? Having not much experience with the stripping down of a gearbox ant guidance would be appreciated.
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Re: How to fit t/d 5th gear

Post by saintjamesy89 »

I am interested in the outcome of this too, the GT revs quite high at 70 and I will be doing reasonable amounts of motorway miles in her. The clutch in there isn't going to last forever so a good time to change when it's needing doing.
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Re: How to fit t/d 5th gear

Post by JayW »

It's not a home job. IIRC, the 5th gear shaft requires 1mm milled off it.

There will be reference to it on by the user mikeE(uk).
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Re: How to fit t/d 5th gear

Post by frog »

I'm keen to perform this mod too, unfortunately we didn't get diesel BX's in Australia. I'm hoping I could take the fifth from a 405 srdt box and it would be similar.

The most info i've found on this forum is the following from a soundproofing thread:
Mike E (uk) wrote:Hi Oscar, since you asked:

(re-print from

If you rarely use motorways this will not matter to you, but I found the BX16V high revs at speed wearing on long trips. This is a pity as otherwise the car is good for long distance travel.

So I have now modified my 5th gear from
20mph / 1000rpm
to 23mph / 1000rpm.

70mph was 3500 rpm, now 3000
80mph was 4000, now 3500

This reduces the engine noise significantly, and the cruising
comfort is much improved.

The acceleration in top is reduced , but to overtake you would drop
to 4th gear anway.

The modification is to swap the two 5th gear cogs for those from a turbo D.
There is a sleeve that must be ground down 1mm in diameter to fit the larger cog,but apart from that just follow the Haynes manual.
( Ask me if you need help in getting the grinding done.)

Thanks to JBUK for the idea & data and Andy C & Ollie for the donor 'box.

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Re: How to fit t/d 5th gear

Post by Jaba »

Actually for the GTi box which has a different final drive ratio to the 16v you need to use the 5th gearset from a non turbo 17 or 19 to give you a sensible usable 5th gear ratio.The teeth numbers you will need are 37 and 28 to give about 24mph per 1k rev. The TD gears would give about 28mph/1k revs.

The 37/28 gears are used in 16vs, 17 &19 diesel and 16 petrol from memory, i.e. most of the other BXs.

There is no need to take the box out to do it as the 5th gear is accessible through the LH end cover. Its a quick and easy job. Milling of the inner sleeve may or may not be required and cannot be predicted until you try to fit the replacement gears.
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Re: How to fit t/d 5th gear

Post by Vanny »

I never got around to a full write up, but this gallery is a sort of step by step on how to get the gear set out

I had to order the socket for this job, but otherwise it was straight forward enough. This is a 1.9TD set from a Pug 309


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Re: How to fit t/d 5th gear

Post by Defender110 »

Not sure about BX's but the Pugs used to rip the splines out of the 5th gear sprocket, nice simple fix through the end plate.
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