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GKN Versus Rocky Mountain Overdrive


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Overdrives seem to be a popular topic at the moment.

There is a GKN overdrive up on ebay right now but as we all know these are obsolete now although fairly well supported by Devon 4x4.

The Rocky mountain DX is a new product it seems and so readily available.

My question is does anybody have first hand knowledge of the two side by side and whether there is a winner.

I know most of the stories about GKN and although there are many there are also plenty of successes.

I have not seen any opinions of the Rocky one though.

The Rocky one seems good that it uses the oil from the transfer box but maybe actually this is a waekness rather than a strength.

Also the Rocky is purely mechanical so maybe it is more robust? Just guessing.

I am assuming in the long run they do identical jobs.

So people anybody know anything

Thanks to all

Ed

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Ed, I tried to get hold of a Rocky Mountain 1 a couple of years ago. They were always out of stock and no ETA on when the next batch was due in. Defender 1's seem to have disappeared off the website now, so don't know what the situation is now.

I also looked at this and tried to contact the company with no sucess.

I got a GKN from Devon 4X4 in the end.

Mark.

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Ed, I tried to get hold of a Rocky Mountain 1 a couple of years ago. They were always out of stock and no ETA on when the next batch was due in. Defender 1's seem to have disappeared off the website now, so don't know what the situation is now.

I also looked at this and tried to contact the company with no sucess.

I got a GKN from Devon 4X4 in the end.

Mark.

I had the same problem when looking around for an OD. I thought Rocky Mountain may be the way to go as they were still in production. Contacted RM - eventually got a reply (2 months after emailing - assuring stock being released soon - none has ever appeared on the rocky mountain spares website AFAIK). The prices in the UK seem massivley higher than those in Canada and the States. I contacted Wise Owl in Canada and they replied fairly quicky to say that stock of DX ODs had been promised for ages, but just never turns up and that i'd be better getting something else!

So, on one hand, you have a unit that is not in production anymore that you can get spares for (GKN) or a unit that is supposedly in production that you can't get hold of to buy (so goodness knows if you can get spares even if you can get the actual OD!). I got a GKN from ebay second hand in the end. It has had a few problems, but Devon 4x4 supplied all the parts to sort them out (leaking seals between the tranfer box and the overdrive). I think if they are looked after they seem reliable - Ralph will probably be along soon and tell you his has done 50k without too many problems.......

My advice if you get a GKN unit from ebay is change the adapter plate seals before fitting (its annoying to have to take it off to do this shortly after you've fitted it for the sake of ~£15!). Then treat it carefully, change the oil when you service the rest of the vehicle and keep a good eye on the OD oil level. The Rocky mountain units are supposedly stronger than the GKN - can be used in hi and lo ratio and any gear. I only wanted a 6th gear, so its fine that the GKN cant be used at lower speeds/lower gears (under ~30mph) - the torque is too much for the GKN in 1st and 2nd IIRC.

Of course a discovery transfer box is the option a lot of others seem to go for so could also be considered, but it does affect all of your hi ratio gears permanently....

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I tried to buy their gearlever switch from them a couple of years ago and they were decidedly unhelpful.

Before i bought the GKN I did contact this chap - Tim Payne (i think)

http://www.overdrives.co.uk/landr.htm

and his overdrives are not in production yet. He is still developing them. He said they would be for sale 1st quarter of this year, but i beleive this has been put back again, so you can't get one of these yet.

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Does all this indicate that the Rocky Mountain DX is the better design than the GKN?

Or are they both temperamental

Not going for the Disco transfer box idea as still want the power for towing etc

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Does all this indicate that the Rocky Mountain DX is the better design than the GKN?

Or are they both temperamental

Not going for the Disco transfer box idea as still want the power for towing etc

The power is still there,iIt's just that you can't use it.

Had to fit a Disco box to my 110 coming back from Morocco last year when the !.44 box cried enough and spat some of it's teeth out and ran all the bearings!

Disco box in the 110 is great but not for towing.

Destroyed a GKN on the previous trip, luckily I'd packed the old transfer gears and did a quick change on the roadside near Madrid.

My GKN was nothing but trouble for the 3 months I put up with it and I was glad to see the back of it.

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The GKN overdrive is actually quite an old design being similar to a Laycock-de-normanville type J, been used in a lot of classic 4 sppeders. There's very good advice about them on an American website about TR6s. The only difference to the LR one is the adaptor, speed sensor and the ECU. http://www.buckeyetriumphs.org/technical/jod/JOD1/JOD1.htm

In older installations there was a simple switch that only allowed use in 3/4, the LR installation uses a sensor that equates to about 30 in high range. It's pretty important that it can't engage in reverse as it will explode! (It has a one way roller bearing inside) Operationally it's similar to an auto with epicyclic gears and clutches so can be shifted on the fly with no clutch, get it right under load and the change is really smooth.

From what I've read, the RM unit is similar in principle to the old series OD with normal gears, syncro hubs etc. How strong they are no-one seems to know yet!

As posted before, the problem I had with my GKN was it working loose on the back of the Tbox which damaged the odd size bearing inside. Ignore the instructions and thoroughly clean the Tbox holes, loctite the allen head screws and do them up tight. I did it by feel but reckon it's 50% tighter than recommended. The guys at D4x4 said the hadn't heard of the problem though. It's behaved itself since and changes are getting smoother with use.

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As posted before, the problem I had with my GKN was it working loose on the back of the Tbox which damaged the odd size bearing inside. Ignore the instructions and thoroughly clean the Tbox holes, loctite the allen head screws and do them up tight. I did it by feel but reckon it's 50% tighter than recommended. The guys at D4x4 said the hadn't heard of the problem though. It's behaved itself since and changes are getting smoother with use.

Now you've got my mind worrying - i can;t remember if i did use loctite on the allen head screws! I did em up tight though - i couldn't go much tighter than the recommended 40nm as i think i'd have broken the allen attachment on my torque wrench! I'm sure i used loctite or did i?!!

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I'm probably scaremongering, the manual doesn't mention loctite but the the only thing holding the bolts is the friction from the torque setting. It was also on my old Tbox which had a fair amount of vibration and backlash (and I was pounding down the M4 at 85 :rolleyes: ). Since fitted Ashcroft main and Tboxes and it's been fine.

I thought a lot about it when buying the new Tbox but I live in the back of beyond and a lot of my driving is on back roads where the 1.41 ratio is spot on plus the occaisional towing, the OD just helps on long runs (and the odd stretch of local road that is straight and not full of potholes, v rare!) If I still lived in UK I would have probably gone for the 1.22 disco box.

As for how tight I did the bolts, it was with an allen key adaptor in a 3/8 drive wrench. After years of practice it was somewhere more than tight but just short of 'kin tight, around that point before you know the cheap allen key is going to round off!! ;)

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i had a GKN unit that i fitted at 22k miles (no loctite!) and it was till going well when i sold it at 96k miles, i changed the oil when i changed the engine oil every 6k and apart from a speed sensor issue it was fine.

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Are you sure you will win a willy waving competition? ;)

I wasn't instigating one :huh: merely suggesting that, apart from slightly larger tyres, a fully laden Disco 200TDi is not very far removed from a fully-laden 110 200TDi so if a slight increase in gearing is a straw that breaks the camel's back, it could be worth feeding the camel some extra vitamins in the first place.

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A big intercooler and some pump/turbo tweaks should see you with at around 150hp for very little cash, which will make a huge difference to the motors performance. Much more than this and you reach a point of running the injectors for too long and causing the EGT's to skyrocket, as you cant increase the pump pressure without an expensive rebuild, and i've never been able to find larger injectors for the TDi to get more diesel in without increasing the pressure.

A huge ebay intercooler can be had for not much money (£100ish) as long as your willing to make up some brackets and pipework to make it all fit. Turning the knobs on the pump and actuator costs nothing, although you'd be wise to invest in a boost guage and EGT guage.

I just picked up a disco2 transfer box from ebay for £90, the D1 items go for less, but i wanted a newer box with the Q gears.

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A big intercooler and some pump/turbo tweaks should see you with at around 150hp for very little cash, which will make a huge difference to the motors performance. Much more than this and you reach a point of running the injectors for too long and causing the EGT's to skyrocket, as you cant increase the pump pressure without an expensive rebuild, and i've never been able to find larger injectors for the TDi to get more diesel in without increasing the pressure.

A huge ebay intercooler can be had for not much money (£100ish) as long as your willing to make up some brackets and pipework to make it all fit. Turning the knobs on the pump and actuator costs nothing, although you'd be wise to invest in a boost guage and EGT guage.

I just picked up a disco2 transfer box from ebay for £90, the D1 items go for less, but i wanted a newer box with the Q gears.

I was tring to increase my MPG as well as reducing revs. A sixth gear for motorway driving.

The RM overdrive seems like a preferable choice but is untested.

The GKN on ebay just sold for £680 with an alleged 5500 miles on it. Second hand means that that could be a make up mileage or a problematic unit removed out of exasperation.

101Sean meantions that the GKN has one way roller bearings

Does the RM overdrive have one way bearings in too

This could be a bit of a problem if the overdrive is accidently engaged by the wife and then puts it into reverse

I would like to think that the overdrive I end up with is not a liability but a strength.

I assume the speed sensor on the GKN stops this happening but the RM one is totally manual.

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AFAIK, the RM unis uses normal gears and syncro hubs and is similar to the old series OD, no one way bearings and totally manual. The speed sensor in the GKN unit automatically drops it out below 30 so it can't be engaged in reverse.

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Ah I see, rather than spend £1k or so on an overdrive, spend it on the engine and fit disco gears. That makes sense. :)

Exactly - It strikes me that a damn good service & overhaul, with perhaps a few quid spent on trick bits on a ~10+ year old engine is going to give better value than bolting £600+ worth of extra potential failure to the back of the gearbox. An engine making better power should be giving better MPG for the same performance or better performance for the same MPG, depending on how you modulate your right foot :P

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