Swapping diffs

Toby_HDi · Post by **Toby_HDi** » 24 Dec 2010, 21:36

Not really a Citroen question but applies to the ML5 box used in some Xantias so is applicable to them too.

Is it possible to swap a diff in an ML5T? Obviously, much work is involved but can it be done, once dismantled is the diff simply bolted off and the new bolted on?

The reason I ask is a bit of research has shown the ML6 box used in certain Peugeots and I'm sure a Citroen or two uses the same gear ratios as the Coupé's ML5T but the 6 speed boxes have a lower final drive. If the ML6 could be fitted with the Coupés diff it would give the Coupé an extra, taller, gear.

Just idle thinking at the moment due to a thread posted on the Coupé forum.

CGATCX25GTITURBO · Post by **CGATCX25GTITURBO** » 25 Dec 2010, 11:27

Don't the answer to your question regarding the gearbox types you list

it's not the diff to swap to get the ratios. It will be the crown and pinion.

I rebuilt my CX with a hybrid gearbox GTI turbo 1 with a CX T2 diesel 'diff' it is actually the crown wheel and pinion 'ouptut shaft 'that are swapped.

All the gears and syncros in my boxes where swappable on the shafts.

Unless the driveshaft splines are different why wouldn't just try fit the gearbox?

GTI 6 link

these guys report the ML6 as being bigger?

Post by **CitroJim** » 25 Dec 2010, 15:45

It might be possible but it's a big job to strip the 'box to do the job.

To do the job properly not only would the crownwheel need to be swapped but the output shaft as well as the pinion is part of the shaft and cannot be removed. You'd need the shaft and transfer all your gears and synchro hubs to it and that's not an easy job.

The ML5 is a grown up MA to all intents and purposes and will be a tricky 'box to strip, needing several types of pullers and a press.

I's go for a complete 'box Toby. Get six speeds then!

Yes, it will be longer to accommodate the sixth gear pinion set on the end in the tin cover. It will fit but you may experience clearance problems on full-lock. 205GTi drivers have had this issue when fitting a 306gti6 (a BE derivative rather than an ML_ gearbox.

spider · Post by **spider** » 25 Dec 2010, 16:08

Ah yes, I do remember the six speed unit is longer.

I can also tell you the 306GTi6 has an (to me at least) appalling lock angle

to accommodate this box the rack on these is different. Makes tight three point turns and (tight) reverse parallel parking annoying to say the least.

Not sure what its like when retro fitted to something else though. I would guess you could catch it if you are not careful. Food for thought.

Post by **Peter.N.** » 25 Dec 2010, 16:49

That takes me back, I used to fit commercial gears into Vauxhall overdrive boxes to make a wide ratio set up for my Perkins 4/108 diesel conversions, had a series of tubes and spacers to hammer it all back together again

Peter

Toby_HDi · Post by **Toby_HDi** » 25 Dec 2010, 23:25

As I say, its all idle thinking at the moment. My drive today confirmed that the Coupé does rev a tad too high for my liking on the motorway.

CGATCX25GTITURBO wrote:Unless the driveshaft splines are different why wouldn't just try fit the gearbox?

The ML6 box, whichever vehicle it is fitted to, has a lower final drive than the Coupé meaning that 6th would be pretty much the same as the current 5th in the ML5. It would also mean I'd have a ridiculously (and uselessly) short 1st.

Jim, thanks for describing what is involved. I'm not contemplating it in the anywhere near future, as said it was just a thread that got me thinking. I knew it would be a tricky job.

As for the steering lock issue, I was aware of the issues with the BE3/6 fitted to the GTi-6 and those retrofitting this box to the 306. I'm not sure if the ML6 would have an effect on the steering lock, all I know is the Coupé's turning circle isn't brilliant as it is.

ACTIVE8 · Post by **ACTIVE8** » 26 Dec 2010, 10:04

Tobi HDi wrote:As for the steering lock issue, I was aware of the issues with the BE3/6 fitted to the GTi-6 and those retrofitting this box to the 306. I'm not sure if the ML6 would have an effect on the steering lock, all I know is the Coupé's turning circle isn't brilliant as it is.

Which gives on lookers more of a chance to view the lovely body (the cars) styling & design while a three point turn is being executed!

CGATCX25GTITURBO · Post by **CGATCX25GTITURBO** » 26 Dec 2010, 11:31

Toby_HDi wrote:As I say, its all idle thinking at the moment. My drive today confirmed that the Coupé does rev a tad too high for my liking on the motorway.

CGATCX25GTITURBO wrote:Unless the driveshaft splines are different why wouldn't just try fit the gearbox?
The ML6 box, whichever vehicle it is fitted to, has a lower final drive than the Coupé meaning that 6th would be pretty much the same as the current 5th in the ML5. It would also mean I'd have a ridiculously (and uselessly) short 1st.

Jim, thanks for describing what is involved. I'm not contemplating it in the anywhere near future, as said it was just a thread that got me thinking. I knew it would be a tricky job.

As for the steering lock issue, I was aware of the issues with the BE3/6 fitted to the GTi-6 and those retrofitting this box to the 306. I'm not sure if the ML6 would have an effect on the steering lock, all I know is the Coupé's turning circle isn't brilliant as it is.

are you sure the engine your coupling it up to will have the power to pull the desired ratio change? otherwise it could just become too over geared.

The reason I put the diesel set up in is beacause it was reving out in top too easy

Toby_HDi · Post by **Toby_HDi** » 26 Dec 2010, 11:47

Yes,

The ML6 fitted to the 407 and 607 petrol V6 (same engine as my Coupé) or the petrol 2.2 have exactly the same ratios as the Coupé's ML5 but a different final drive - hence my question about swapping it, which would mean I keep my current 5 ratios and add a taller 6th.

CGATCX25GTITURBO · Post by **CGATCX25GTITURBO** » 26 Dec 2010, 20:22

there are those saying don't fit the ML6

the ML5 is stronger

I never listened either

fynnbar · Post by **fynnbar** » 02 Jan 2011, 09:40

Toby, I think you're getting 'final drive' and 'top gear' confused.
The final drive ratio is the crownwheel and pinion ratio (which is often called the diff ratio, as the crownwheel and diff are built as a unit).
Top gear is the last gear you select i.e fifth on an ML5
The new trend for 6-speed boxes is to introduce a high top gear in order to get better fuel economy at high cruising speeds.
So, 6th gear is always higher than top gear on a 5-speed box, like for like.
Using the diesel ML5 as an example, the gear ratios are generally the same, first through fifth - fifth is 0.61 (31:51) - it's the final drive (diff) ratio that's changed, depending on the model. For example, the Expert van uses a 5.07 (71:14) final drive, the C5 2.0 HDi 110 uses 4.06 (65:16) and the 2.2 HDi uses 3.94 (67:17)
The diesel ML6C has either a 3.94 or 4.18 final drive, but uses a higher 0.53 6th gear.
As CitroJim says, with the pinion being machined into the output shaft, it's a complete gearbox strip and output shaft swap to change the final drive ratio. Better to swap gearboxes really. However, again, the difference is in the bellhousing, not the gearbox case. As far as I can tell, putting an ML5 onto a different model is a matter of swapping the bellhousings and using whichever gearbox has the correctly toothed pinion gear to suit your crownwheel gear.
Incidentally, the petrol gearboxes use lower ratios throughout the gearbox (V6 Coupe fifth gear is 0.733 - 33:45) as petrol engines rev higher than diesels

Toby_HDi · Post by **Toby_HDi** » 02 Jan 2011, 10:23

Thanks fynnbar.

I hadn't got them confused.

As you rightly point out, the final drive is different. The actual 1st to 5th ratios themselves on certain ML6 boxes and my ML5 box are exactly the same. The whole point of the ML6 box is for that longer 6th

Fitting a complete box would be nice and would be the simpler option but it would be pointless, as with the different final drive of the ML6, it would mean that the 6th gear would be no longer than the 5th of my current ML5.

fynnbar · Post by **fynnbar** » 02 Jan 2011, 11:23

Yes Toby, you'd probably have to mix and match parts from the petrol and diesel 'boxes - a pain, but depends on how badly you needed the longer ratios. Can't see the point really, unless you had a tuned motor that could pull the higher ratios without compromising performance.

CGATCX25GTITURBO · Post by **CGATCX25GTITURBO** » 02 Jan 2011, 12:18

can you just swap the 5th gear for the 6th on the end of the shafts? if it is just an over drive you seek? I'm not familiar with the box so I don't

Toby_HDi · Post by **Toby_HDi** » 02 Jan 2011, 13:39

Indeed, I'm quite fond of the current 5th though as at 60mph ish it is right where the VVT kicks in.

It is just when you settle down for a cruise at 70 that it could do with revving slightly lower. It would basically just mean adding the 6th gear on to what I already have.

As I mentioned anyway, it was all idle talk at this stage and I was merely curious as to what would be involved. Its not something I'm considering, at the moment.