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3.9 diffs


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Hi everyone.

As part of the coiler axle swap on my Tdi powered 109 (standard transmission with Roverdrive), I'm considering regearing the diffs. I won't use the 4.74s from the existing axles because the vehicle is significantly under geared, and I am concerned about retaining the "new" axles' 3.54s as i think they will be over geared (based on experience of uphill slopes on motorways with a full roof rack with the overdrive engaged - 3.54s are taller in crease than overdrive).

I think 3.9:1 would be the perfect gearing for my application, and I know that the Rover P4 used this ratio in its Rover diff, as did the MGB GT in its Salisbury diff. The question is, will their crown wheels and pinions fit the LR carriers and diff casing?

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The Rover P4 has been done before, and (IIRC) is a direct replacement for the Rover diff assembly. Main issue is finding a serviceable pair. Alternatively, the 3.9 crown wheel and pinion might build into a 10 spline RRC diff with a thin spacer between the crown wheel and carrier to compensate for the slightly smaller diameter of the pinion.

Don't know much about the MGB diff and never heard it mentioned as a source of components before.

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I'd be most surprised if MGB crown wheels and pinions would fit a Rover axle. Rover was an independent company till the mid/late 60's when it was first amalgamated with Leyland then BMH to become British Leyland. Rover unusually made their own axles.

MG were a division of Morris which amalgamated with Austin at the start of the 50's to become BMC (BMH after amalgamating with Jaguar in about 1967)

I mention the history to show that each vehicle was developed by separate companies - so there were no common bits except electrics and small components supplied by others.

Rover made only large cars - BMC made a whole range - and I'd expect an MGB rear axle to be smaller and lighter than a Rover one - as well as being designed and probably made by entirely different people. (you're never quite sure who made what in the British car industry)

All Rover axles are simple spiral bevel with the input and output shafts in the same plane. MGB's may have hypoid axles with the input shaft lower than the axle shafts. If this is the case the tooth shape will be entirely different.

If they do fit I'd be very interested to know.

Pair of 4.11's for sale here - http://forums.lr4x4.com/index.php?showtopic=54536

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Thanks, Dave. I think the crown wheel is thicker to compensate for the pinion diameter. I know that Salisbury axles have a different carrier between the 3.54 and the 4.71 for that very reason, the cut off being 4.1 (I can't remember for certain which side the 4.1 falls on, but I think it's the 3.54 carrier). KAM make spacer rings to allow the higher geared rings to be used on the 4.71 carrier. I assume it's similar on the front Rover diffs, too, but I'm not certain.

Thanks too, Jeremy. MGBs, TR7s and Rover SD1s used Salisbury rear axles with 3.9 gearing, but I'm not sure if it's the same size. The V8 MG and SD1 used 3.1 gearing, but I think it's the same carrier, so it'd need to be pretty sturdy. I'm just not sure they fit the LR axles. I'm trying to work out which type of diff this fits: http://www.rimmerbros.co.uk/Item--i-TKC3282

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Ah yes - the Salisbury axle. This is rather different to the 'Rover' axle - and was fitted to the rear of late 109 Land Rovers. The immediate problem is finding a front one.

I think Salisbury's were used by Triumph (on proper cars before the TR7) and Jaguar - even with independent rear suspension. They are recognisable by the detachable back cover and the claim that a spreader is needed to get the things apart.

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If you've got a TDi and coiler axles surely the easiest solution by far is to fit a coiler box & LT230, that way you have a complete standard LR drivetrain using standard bits which are easy to find. The LT77/R380 and LT230 combo will give you a 5th gear (overdrive), a choice of high range ratios (1.6, 1.4, 1.2, 1.0), are stronger and quieter than the Series transmission and are matched to the ratio of your diffs.

By the time you've sold the Roverdrive you should easily break even on the deal.

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If you've got a TDi and coiler axles surely the easiest solution by far is to fit a coiler box & LT230, that way you have a complete standard LR drivetrain using standard bits which are easy to find. The LT77/R380 and LT230 combo will give you a 5th gear (overdrive), a choice of high range ratios (1.6, 1.4, 1.2, 1.0), are stronger and quieter than the Series transmission and are matched to the ratio of your diffs.

By the time you've sold the Roverdrive you should easily break even on the deal.

I did manage to get a Defender LT77 and a 1.4 LT230, both MoD recon, for £100 with that installation in mind, but I didn't want to modify the chassis to move the mountings or the front cross member or chop about the floor and tunnel, which would prevent me from using my Wright Off Road matting. While the LT77 is certainly stronger than a Series gear box, the LT230 isn't as strong as the Series transfer box. The quiet and leak-free nature of the LT77 certainly had its appeal, as did losing the extra lever for the OD, but I quite like the old fashioned nature of the SIII transmission, so I'd rather keep it - otherwise, with the mods I've already made, I might as well just have sold the 109 and bought a 110.

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I run R380 + LT230 in my 109 and the chassis mods are minor, in fact it makes it neater as you remove the welded-on bits and put a standard bolt-in x-member under the gearbox and then drill & tube 6 holes in the chassis and bolt the standard mounts on. A friend did my transmission tunnel in fibreglass so not only does it fit beautifully, it also keeps noise & heat (and water, and mud) out amazingly well.

I'm not sure why you think the Series transfer box is stronger than an LT230, they're one of the few bits of LR that are very hard to break. We've run them behind a 500hp LS1 reliably, and mine lives happily behind a 4.6 V8. You can convert the LT230 to RWD so it behaves the same as a Series transfer box.

I don't think a transmission swap stops it being a Series, I think that's when you coil spring it and put a Defender front end on <_<

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Ah yes - the Salisbury axle. This is rather different to the 'Rover' axle - and was fitted to the rear of late 109 Land Rovers. The immediate problem is finding a front one.

I think Salisbury's were used by Triumph (on proper cars before the TR7) and Jaguar - even with independent rear suspension. They are recognisable by the detachable back cover and the claim that a spreader is needed to get the things apart.

It's definitely a Salisbury gear set, I just don't know if they made different size diffs like Dana do. It could be that the SD1/MG carrier is smaller than the LR, or that the axle case's diff nose is a different length, affecting the pinion.

I asked the same question on the Defender forum and got pointed towards KAM 3.8 gear seats made for LR Rover and Salisbury diffs. It's an expensive alternative to this, but at least the parts are easy to find and a guaranteed fit. As I said on there, their price mean I'll have to fit the axles with the existing 3.54s first and try them out. If using 3rd+overdrive on 3.54s gives a comparable overall gearing to plain 4th on 4.71s, then I should be able to use that selection on those motorway hills where I currently can't use 4th+OD (where, with the 3.54s, I would unable to keep 4th gear engaged). It all depends on the drivability on hills, as I really just want to use the OD as a cruising gear.

Thanks for your help, everyone. I think the 3.9 idea was a nice theory, but impractical in practice as there are just too many unknowns to be worth the effort and expense. If the 3.54s turn out to be bad for driving, I think I'll have to save up for the 3.8 KAM parts.

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I run R380 + LT230 in my 109 and the chassis mods are minor, in fact it makes it neater as you remove the welded-on bits and put a standard bolt-in x-member under the gearbox and then drill & tube 6 holes in the chassis and bolt the standard mounts on. A friend did my transmission tunnel in fibreglass so not only does it fit beautifully, it also keeps noise & heat (and water, and mud) out amazingly well.

I'm not sure why you think the Series transfer box is stronger than an LT230, they're one of the few bits of LR that are very hard to break. We've run them behind a 500hp LS1 reliably, and mine lives happily behind a 4.6 V8. You can convert the LT230 to RWD so it behaves the same as a Series transfer box.

I don't think a transmission swap stops it being a Series, I think that's when you coil spring it and put a Defender front end on <_<

I was worried about the combined engine and transmission length requiring the front cross member to be moved forwards or cross member behind the transfer box to move aft. I don't want them to be so squeezed between cross members that you have to lift both and slide both against their respective nearest cross members to separate them for removal of one or for clutch replacement.

Regarding the strength of the transfer boxes, I was going on what I had been told by a LR engineer and also some LR Experience instructors. I see a lot more posts about broken LT230s than broken Series transfer boxes, and have never met anyone who has heard of a broken Series transfer box. I just wish the gear box had the same reputation, though mine seems to be running fine (I cured all but the tiniest weep by applying sealant to the top cover and selector detent rubbers, and by sealing the rear bearing carrier in the casing to prevent migration, and made the refilling and level checking a few seconds' job with the MoD top fill and a Rocky Mountain dipstick).

With coiler axles, I really can't see the point in changing an LT230 to part-time 4wd. In fact, I'd rather convert a Series transfer box to full time 4wd, if it was a straight forward job, using the viscous unit from a late RRC/P38.

I mainly want to keep the SIII transmission because I don't want to customise everything. I like SIIIs, and mine has little enough of that character left already, even though it's only bodywork and the engine that have been modified so far.

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I suspect you hear about more broken LT230's because there are so many more out there doing hard work. The Series transfer box lived behind the 2.25, the LT230 lives behind everything up to 4.6 V8 and bigger, in heavier vehicles.

In a 109 there is plenty of room for the gearbox & transfer box, it's only on 88's that it gets a bit marginal with the rear prop length.

The point of retaining RWD is personal preference and of course

:P
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I suspect you hear about more broken LT230's because there are so many more out there doing hard work. The Series transfer box lived behind the 2.25, the LT230 lives behind everything up to 4.6 V8 and bigger, in heavier vehicles.

In a 109 there is plenty of room for the gearbox & transfer box, it's only on 88's that it gets a bit marginal with the rear prop length.

The point of retaining RWD is personal preference and of course

:P

Hmmmm...donuts.... :)

Fair point on all of that, but the shafts are hugely thick and the gears are pretty substantial. I think you'd have to be doing something fairly stupid to break a Series transfer box, especially as there are so many much weaker points elsewhere. the LT230 is obviously the better unit because of its centre diff, but i don't really need that as my 109 is for daily commuting, family and domestic chores and holidays, rather than heavy off roading.

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I would go with the 3.54 gearing myself and see how it goes. It might be a bit tall, but you can downshift for a hill if you have to. The advantage of running your gearing nice and tall is that your consumption will potentially improve, as the TDI runs at a more favourable RPM, near max torque.

If youre worried about low range, you can always fit an S1/S2 transfer box which drops low box by 20 procent. I ran that for years this way behind a 2.5 litre petrol with plenty of power.

Daan

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I would go with the 3.54 gearing myself and see how it goes. It might be a bit tall, but you can downshift for a hill if you have to. The advantage of running your gearing nice and tall is that your consumption will potentially improve, as the TDI runs at a more favourable RPM, near max torque.

If youre worried about low range, you can always fit an S1/S2 transfer box which drops low box by 20 procent. I ran that for years this way behind a 2.5 litre petrol with plenty of power.

Daan

Thanks Daan - I already have that in hand. It's the SII Suffix B gears you need, which are 2.81:1 instead of the 2.35:1

used on Suffix C boxes and on, and all SIII boxes (except V8 and 1-ton). I have the new output shaft gear from Dunsfold and intermediate gear cluster from PA Blanchard, both genuine, for an outlay of about £100, to go into my existing refurbished transfer box. I'm pretty sure that once the speedo housing is off, I can replace both gears in situ.

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