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MG MGB Technical - Rebuildiing D-type overdrive and gearbox

Ages ago, I bought a 3-synchro box and overdrive intending to swap it for an exchange unit. I have just managed to buy an input shaft (small type for my 3 main engine) and I was wondering whether it's worth having a go myself. The box has sludgy oil so I suspect the linings in the overdrive are shot.

Is a home-rebuild sensible?
What parts am I likely to need as well as the shaft?

You often hear of people rebuilding gearboxes but overdrives are rarely discussed. It's normally fixing small faults.

Mike
Mike

Gearboxes are bad enough, overdrives are even more specialised, like an auto box. But it would be an interesting and educational exercise!
PaulH Solihull

Hi Mike:

From my own experience regarding just the 3 synchro gearbox; both non o/d & o/d; during the last four decades I have stripped dozens of these crash boxes, finding limited usable parts. You can contact John Esposito at Quantumechanics and he will tell you the same experience. He recently stripped down 3 gearboxes for a fellow MGCC-LICmember and found little usable.

The first thing to do is strip it down, clean and inspect all components for usability. They usually all need attention to:

* layshaft & bearings
* 2nd, 3rd & 4th gear synchros (note late 1965 & on uses a steel synchro, which production ended Nov 2009, so you would need to look for NOS replacement.
* Gearbox ball bearings (3)
* Need to inspect laygear spur, reverse spur and 1st gear spur teeth for wear & chips, damage, etc. These are expensive parts. A chipped tooth means that the case hardness is lost, which results in rapid wear under load.

The most important part of the rebuild is maintaining the laygear to case, and 2nd to 3rd speed clearances, which is essential for correct gear shifting.

Victoria British sells a gearbox overhaul kit (no gears) p/n 2-2650 or 2-2651.

For the "D" type o/d I usually strip it down, clean & inspect. I usually buy Victoria British overhaul kit p/n 2-2698; box everything and send it to John Esposito for reassembly. John will usually install a relined cone clutch ($200). Once assembled John will bench test it for functionality.

There are some UK parts sites that sell components, AutoGear Ltd, Sheffield Yorkshire services (old Laycock folks).


Good luck: Rich Boris
Rich Boris


Mike,

I went through exactly this scenario several years ago. I had to rebuild the gearbox and OD following a laygear bearing failure.

I had never tackled a job of this size or type before and was certainly nervous about the undertaking. I found the most tricky parts were pressing in bearings without a press. I also sent off the first motion shaft to get everything setup correctly (gears, synchros end float etc.) All new bearings, thrust washers,shims, gaskets etc. were required. Parts and work cost me approx $1500AUD

With a lot of advice, time - lots of time! and a volvo overdrive manual from a p1800 (same OD) I managed to get everything back together in one piece. It is my daily driver and has not missed a beat since.

I am more than happy to send some pics and info including the manual etc. if you are interested.

Good luck if you decide to go ahead!! It will be rewarding - as long as you are not in a rush.

C Brennan

I have two old gearboxes with ODs (later type) to rebuild and am going to try that myself. Well, I will at least strip them then get an expert to take a look over them to tell me what's knackered and what's good. The manuals and their 'check for excessive wear' aren't much help when you've never looked inside one before.

Unfortunately the 'send it off to XXX' doesn't work so well when you're on the other side of the world. Shipping on such heavy things isn't a cheap option unfortunately.

Chris, I might be interested in your help and advice when I get around to stripping these things down. Your $AUD costs give me some indication of what it might cost me in $NZ too thanks.

Simon
Simon Jansen

Simon,

More than happy to help. I can strongly recommend taking plenty of digital shots as you go. It was actually quite fun when I look back on the experience. Do you have access to a bearing press at all?

Cheers,
C Brennan

I'm afraid I've chickened out and bought an overdrive off ebay. It has been overhauled but has been sitting on someone's shelf for a few years. That only leaves me now with rebuilding the gearbox.

What differences are there between o/d and non-o/d gearboxes? How much is interchangable?
Mike

The 3rd motion shaft is different as well as the tail shaft extension. The shift lever is also unique to the 3 synchro od trans. RAY
rjm RAY

Hi Mike:

Nice you bought a replacement "D" type o/d unit. But here is where the term overdrive roulette kicks in.

* was the unit professionally overhauled? by whom, when?
* was it functionally bench tested under 400psi?
* was the cone clutch replaced:

The cone clutch is critical on a "D" type, since it is approx 1/2 the size of the LH type. Old clutches may look great, but age and the time immersed in oils over the years can cause them to shatter under heat & load. That is why I always replace them with a relined unit so I don't have play overdrive roulette, and remove & replace engines and gearboxes.

Good luck: Rich Boris 67 B
Rich Boris

Well it arrived yesterday and looks absolutely wonderful. You could eat your dinner off it, both on the casing and on the internals visible from the outside. It looks just as good as C Brennan's overdrive in the picture above. It has definitely not been fitted to a vehicle since it was overhauled, no muck anywhere and no trace of gasket material on any of the facings.

So it's time to have a look at the spare gearbox.

I would like to re-use my gearlever, the correct one is missing from the spare box. The overdrive levers are hard to get, is there an alternative?

Mike
Mike

You can take 2 non od levers, cut them up and weld them together to make an od lever, but they are still available new from Moss. However, they're not cheap. RAY
rjm RAY

I've had a dig around in the archives and discovered that the gearlever from a 1622 Magnette IV, Morris Oxford, Austin Cambridge etc. also fits, just needing cutting to length. Not many here in Berlin, I'll have to look when I get back to England next.
Mike

I have opened the inspection plate on the side and had the remote control tower off my gearbox today.

I have discovered a couple of teeth on the second gear pinion with slight rusty pitting, one 4mm x 1mm, the other 2mm x 1mm, apart from that only the reverse gear looks to be worn. Am I right in thinking that this pinion is scrap?

The gearbox has obviously been filled with EP90 gear oil. The whole thing stinks! There is an unpleasant sludge at the bottom, quite thick

Unfortunately something nasty has happened to the gear lever socket. It isn't perfectly round, it appears to have been squashed and one of the retaining bolts has broken off.

It looks like I'm repeating Rich Boris' experience.
Mike

Nothing wrong with gear oil in an MGB gearbox, it was standard for the V8, it's extra 'cushioning' isn't required for the 4-cylinder. Yes it smells different, but that means nothing. Whatever oil is used it's all down to how it's maintained.
PaulH Solihull

Hi Folks:

Gear oil has too much sulfer to be used in a 3 synchro gearbox & especially in an overdrive configuration. Gear oil is usually used in high pressure steel gear applications, such as a rear end crown wheel and pinion, spider gears, etc.

The 3 synchro gearbox has bronze/brass synchro's, gear bushings, and shifting forks. Sulfer attacks bronze/brass components especially under heat & load. Same for the D type which contains bronze/brass components.

Use the standard "engine oil". most folks prefer 30W non-detergent oil, which limits foaming in a 400+ psi pressure environment.

BTW Mike, if your second speed gear wheel has chips, you must trash it and replace with an acceptable used or new part. Must be mindful of the 2nd speed gear change in 1965 (brass synchro to steel) and the later uses 22H0230 & 22H249 components, which are available @ Anglo Parts (Beligum) and a German company that sells MGB parts.
Rich Boris

I say again, gear oil is standard for the V8 which is the same basic design to the 4-cylinder. You seem to be thinking of diff oil, which is different again. That's not an argument for using gear oil in the 4-cylinder, just an argument against it being harmful.
PaulH Solihull

I'm not a connoisseur of gear oil, I'm afraid. I sloppily referred to the oil in the gearbox as "EP90 gear oil". It has that pungent smell of the extreme pressure additive which is not what I expected. Does proper gearbox oil also have that strong pong?

The second gear doesn't have chips, more like small rusty pitting on the surface. This will clearly have weakened the hardening and I expect the gear is scrap.

So if the oil has attacked the brass of the synchro rings (it's an all brass ring box) I'll be needing to replace those too, right?

Mike
Mike

If you mean 'proper gearbox oil' for the 4-cylinder that should be engine oil the same as the engine and so smell just the same. There is a difference between gear oil and diff oil, but it is not as great as either of them to engine oil. If what you are smelling isn't engine oil then it needs changing anyway. Gear oil won't have done any harm.
PaulH Solihull


Mike

Do you still have the OD, I am looking for a cheap "D" type OD to rebuild and install in my 63
J Rosbon

One of the items that a club member that does all our transmission rebuilding told me was that one of the weak areas on the D type is the layshaft bearing. On my OD unit he modified the shaft to accept the longer bearing from the later unit. We feel this should assist in reducing the wear in this area.

I can get details from him on the modification if anyone is interested.
Bruce Cunha

Is the modification shown half way down this page the same as your club member advises Bruce?
http://mgaguru.com/mgtech/gearbox/gt204.htm
T Aczel

As I remember, the bearing he replaced the needle bearings with was longer than the needle bearings. I know he had to slightly modify the lay shaft.

I will see if I can get the exact information on it.
Bruce Cunha

Here is what our club transmission rebuilder wrote to me. Hope it makes sense.

Moss has done essentially what we did on your transmission. We modified the laygear to accept the longer first motion shaft spigot needle bearing
rollers into the back (small gear end) of the laygear where originally it had only one row of short needle bearings. The front of the original laygear had two rows of needle bearings despite the fact that the load here was considerably less. Don't know what the original engineers were thinking!
Bruce Cunha

The factory modified the layshaft bearings in later boxes with a larger diameter shaft and four bearings - two at each end.

Wear on the layshaft is 99 times out of 100 at the front end which is under load at all times except top gear - the rear end is under max load in 1st, less so in 2nd and less still in 3rd.
Chris at Octarine Services

A great thread: so good that it has put the spare D Type overdrive I've owned for about 20 years firmly on my things to do list, which should mean a rebuild next winter!
John Prewer

This thread was discussed between 22/01/2011 and 25/01/2014

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