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reb78

GKN Overdrive Strip Down

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My GKN overdrive had a leak the other day from one of the gaskets around the brake ring, so I had to remove it and strip down to fix. I took a load of pictures so thought the process and the pics might be handy to upload as I know a number of you have these. I will make a few comments - I am no expert so feel free to correct things. I have had this apart three or four times along with a couple of others in the past, so I am getting quicker at dismantling them and re-assembling them so they work!

 

So first, some pics of the unit off of the vehicle. The speed sensor is off and at the side of one of the pics - this sits screwed into the unit forward of the breather and stops you engaging at too low a speed. The sump plate is already removed in these pics and you can see the three round discs screwed into the base of the unit - these contain the HP filter (uppermost disc I think), the pump middle disc and the relief valve/dash pot (lower disc when fitted).

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Next, remove the ally housing that encloses the epicyclic gears and the cone clutch - a series of nuts and two allen bolts need to be undone.

 

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You can lift the gears out at this point but you cannot remove the clutch until you delve a bit further.

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Turn this over and you can see where the epicyclic gears run. Note the sprag clutch in the centre of this picture - this stops the OD rotating backwards which is why it cannot be engaged in reverse.

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The gears will lift off of the mainshaft at this point:

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I noted some wear on the gearing here:

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The other side:

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You can then see the front side of the cone clutch as it sits on the mainshaft:

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Note the circlip on the mainshaft. You’d think remove this and the shaft will come out, but it doesn’t, so leave it there. 

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When I first dismantled this, I removed that circlip and struggled for ages to figure how it comes apart further. Even with the circlip removed, you cant pull the clutch at this point and need to dismantle from the other end now.

So, remove the back cover:

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Note the phosphor bronze bush that supports the rear end of the mainshaft:

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Here you can see the two valves that push the clutch backwards when you engage the OD and the pump drive on its concentric bush in the centre:

 

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Remove the two bars that sit across the valves - these are connected to legs on the clutch pack and as the valves push out, they pull the clutch back and engage the OD drive.

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Frustratingly it still wont come apart at this point and you need to remove the circlip in front of the pump drive

 

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Now, even sneakier - it still wont come apart as there is a second circlip behind the concentric pump drive. You need to use something magnetic to pull out the drive and then its really fiddly to remove the second circlip as it is inside the pump drive housing. I made a tiny hooked tool out of an arc welding rod to get underneath it and prise it up and out and off of the shaft:

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Now the unit will come apart:

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The clutch material looks pretty good. Dont lose the springs - these hold the clutch disengaged when at rest.

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Last thing is to remove the brake ring:

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I tidied and cleaned everything up at this point and re-assembled with new gaskets. Re-assembly is pretty much the reverse of the above. Make sure you get the gaskets around the brake ring the right way round - they look the same, but they arent so do a dry fit if you intend to use any sealant otherwise you will find you have wasted an expensive set of gaskets!!

I hope that helps those like Ralph who have a rebuild on the cards.

They are really simple and no special tools are needed. I dont have any torque settings - everything is done up 'reasonably tight' and this OD had done 80k miles in my hands since the last rebuild. Fingers crossed its in for another 80k miles! I am CONVINCED that a key thing with these are frequent oil changes - i do mine every 6000 miles when I service the engine.

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Interesting, thanks.

I’ve been tempted by one in the past to help on the motorways but have always been put off by their seemingly regular reliability issues. Good to know they can be stripped down and reassembled simply enough at least.

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I had that off, stripped, rebuilt and back on the car in less than a day. 

 

I bought this secondhand when my 110 was on 155k miles. It’s on 242k miles now and the OD has needed a few seals and gaskets in that time. 

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It’s brilliant on the motorway. Revs at 70 are about 2600rpm. It quietens the engine a lot! Economy is unchanged but that’s probably because I drive faster than I would without it. 

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Most of the trouble with mine (*) was caused by buildup of sludge in the valve block, blocking the filter and preventing the oil pressure building and releasing properly. I had to spend a lot of time cleaning those out properly.All of the o-rings are standard imperial sizes and easy to buy if you want to replace any - theres a webpage that lists them all, think it's probably the type-J rebuild articles on buckeyetriumphs.org.

(*) - once you exclude the fact it wasn't really man enough for a small saloon car, and didn't change significantly in Defender form.

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Interesting info Richard, you can do mine next :P 

I haven't touched it since I removed last year.

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41 minutes ago, reb78 said:

It’s brilliant on the motorway. Revs at 70 are about 2600rpm. It quietens the engine a lot! Economy is unchanged but that’s probably because I drive faster than I would without it. 

Noise would be my main gain, the car is happy enough to sit at 70 mph all day long but it’s too noisy for my liking. I’d have thought with an overdrive though the revs would be less than that? What percentage drop does the GKN unit give?

Standard LT77/LT230 is about 2,850 rpm at 70 mph - on 235/85/R16 tyres.

The other option is the Roamerdrive of course, but you’re looking at £1,600 new.

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Oooo. Ill check the revs. What i said might be at 80 actually! Its a 28% ratio change i think. 

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43 minutes ago, western said:

Interesting info Richard, you can do mine next :P 

I haven't touched it since I removed last year.

Haha. Happy to advise!

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56 minutes ago, TSD said:

Most of the trouble with mine (*) was caused by buildup of sludge in the valve block, blocking the filter and preventing the oil pressure building and releasing properly. I had to spend a lot of time cleaning those out properly.All of the o-rings are standard imperial sizes and easy to buy if you want to replace any - theres a webpage that lists them all, think it's probably the type-J rebuild articles on buckeyetriumphs.org.

(*) - once you exclude the fact it wasn't really man enough for a small saloon car, and didn't change significantly in Defender form.

I havent found time to strip your old one yet, but thats interesting. What frequency were you changing the oil at? I dont seem to get any sludge in mine. 

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really miss my OD on A roads & motorway, at 60mph you should running at 2100rpm with od engaged IIRC

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3 minutes ago, reb78 said:

Haha. Happy to advise!

You might regret that answer,  maybe a day here fixing mine.

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Being sure whats wrong is the hardest part. Get it apart and check the clutch but it could be as simple as weak springs in the clutch unit legs. 

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Interesting to see inside!

Looks like part of it has got very hot from the colours on it.

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Yes, the blueing always makes me think that. They run very hot but I think that’s partly because they carry a tiny amount of oil (700ml). Vulcan Bomber is looking at whether he can make an extended sump to increase capacity and cooling. (There are US versions)

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18 minutes ago, reb78 said:

I havent found time to strip your old one yet, but thats interesting. What frequency were you changing the oil at? I dont seem to get any sludge in mine. 

Initially probably every 12k, later less often. I was working flat out at the time, driving 100+ miles/day and I seemed to be servicing the car almost every month! I don't think my O/D was right from day one though, it was never smooth shifting like I remember a mates MGB GT V8 overdrive was. Couldn't ever shift it under power without a huge shunt, could lift off, but usually dipped the clutch as well. The sludge I took out was very metallic, I suspect heavy wear in the oil pump.

I think the real benefit of a larger sump on it won't be so much the extra cooling (though it can't hurt), so much as the extended oil life (ie the life of the additive package). Double the volume of oil means every drop only works half as hard per mile! 

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Absolutely. 

I know what you mean. I’m doing 500+ miles a week in mine at the moment so it’s serviced at least every three months!

Edited by reb78

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20 minutes ago, landroversforever said:

The bit that's got hot, is it still flat/concentric etc?

Yep. It looks fine. It’s a thick hunk of metal though so it’s take something to distort it. 

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I'll link this to the tech archive, probably in the gearbox section. 

 

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I've split this thread into two parts.

The one above that was a tear-down with great pictures and the remainder now in a rejuvenated thread from 2017 when @western had the original Overdrive issues....

 

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