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MG MGB Technical - 90W in D-type Overdrive

Hello all,

Regarding my "new" '63 B w/overdrive. I spoke with the former owner who had the gearbox rebuilt (about six years ago prior to the current owner). He said it was rebuilt by Don Ensley in Detroit (I'm not familiar with that name). He said that Mr. Ensley advised him to put 90W oil in the gearbox. That's what's in there now.

From what I've learned so far though, shouldn't the gearbox have the same oil as the engine?

Thanks,
Greg
Greg Van Hook

Hello,

I used 80-90W for years in a non-OD gearbox, which is normal for a standard gearbox. The only drawback could be during winter for stiffer shifting. For OD gearbox it was 20w-50 or ATF recommended.

Cheers,

JGC
Jean Guy Catford

It should have engine oil, but 80/90 is about the same viscosity as 20W50 so won't make much difference. Some say the different chemicalk composition of gear oil will damage the gearbox but the V8 which has the same basic mechanical design is specced to use gear oil, as it resists the increased torque better than engine oil would. I've driven my V8 all year round and in temps well below freezing when it was kept outside and never had a problem shifting. ATF is a 'word of mouth' recommendation. Some have found it useful as a short term measure to flush the OD if having problems, reverting to engine oil afterwards. Some say their OD only works properly if they use ATF all the time, which obviously (to me) shows there is something wrong with the OD, but if ATF keeps it working rather than a major overhaul then maybe. I'd be concerned at what other damage I might be doing to it though.
Paul Hunt

Thanks.

Is the overdrive necessarily a fragile unit? Does it require a light touch or anything?

Greg
Greg Van Hook

My main concern would be that the 90W oil could attack the bronze synchronizer rings in an early transmission. 90W oil has high levels of sulfer and does not co-exist with bronze very well. The later versions of 90W which meet GL5 standards are to be avoided. GL4, if you can find it, has much lower levels of sulfer and can be used. The later transmissions used steel syncronizers that were imune to the sulfer. FWIW I have been using
Castrol 20W50 in my OD trans for 25 years with no problems. As Paul mentions, the biggest problem to look for is hard shifting when the oil is cold. RAY
RAY

I used Red Line MT-90 75W90 GL4 oil in the TF gearbox. Would that be OK in the B as well?

Greg
Greg Van Hook

Quantum recommends 30W oil for best performance and longevity. See here: http://www.quantumechanics.com/categories.php?op=newindex&catid=11
Steve Simmons

Hello,

I disagree with Paul for ATF use in OD gearbox. As I owned two Volvo (142 & 242 GT) with manual gearbox+O.D., equipped with the same O.D. units than our B. And Volvo owners manual specified ATF type F as a regular lubricant for gearbox+OD. I never had an OD problem with such oil, and my volvos retired with more than 300,000 km.

Cheers,

JGC

P.S. One lubricant tha must be avoid supposely: synthetics of all kind
Jean Guy Catford

Hi Folks:

The 3 synchro gearbox has a lot of bronze type components; such as:
* 3rd & 4th speed synchro's
* possible 2nd speed synchro
* 2nd & 3rd speed gear bushings
* 2nd to 3rd speed interlocking ring
* all three (3)gear forks, 1st & 2nd, 3rd and 4th and reverse.

Note; the "D" type o/d unit has bronze bushings

Too many components subjected to a high sulfer lubricant. Use a 30W nondetergent oil (also called compressor oil) to prevent excessive foaming in the o/d hydraulic system, which is running nominally at 420 psi. Foaming can cause cone clutch chatter, thus reducing the cone clutch lining life. All multigrade oils have detergents. You wouldn't put 20W-50 in an air compressor.

Good luck:

Rich Boris 67 B roadster

Rich Boris

It's not me that says you should the same oil as in the engine, but MG. The person who specced the gearbox, OD, prop-shaft and rear axle for the MGB also subsequently worked for Volvo. He said he rcommended the same oil to both but Volvo chose to use something different, their choice.

Also as I say gear oil will NOT attack the gearbox as it is the recommended product for the V8 which is materially the same.

The OD is pretty strong, if it is good enough for full throttle changes on the V8 it certainly is on the 4-cylinder. The only difference between the two is the V8 was changed to have OD on 4th gear only as the torque reversals in 3rd were proving too much for it. The change on late North American cars to have OD on 4th gear only is for completely different reasons associated with their emissions stuff. Some people are paranoid about the OD and insist on declutching or feathering the throttle when switching in and out, but that rather defeats the object. The manual specifically states NOT to declutch. The ODs were designed for 'throttle on' switching out which is why the earlier D-type had the vacuum switch that prevents it switching out on the overrun. On the later LH type you do get a slight jolt when switching out on the overrun, but it is only a comfort issue.

Just switch it in and out regardless and enjoy the benefits.
Paul Hunt

Interesting to me as I have been running Castrol GTX 20w-50 in my OD tranny since 1992 when I installed it with 23K miles on it. It now has about 75K miles on it since I installed it and about 98K miles since new. Still works just fine and as it always has. Either blind luck or??????
RK Muenchausen

Unless I remember incorrectly, the V8 also used a larger OD clutch.

RK, I've also used 20W-50 Castrol GTX in mine. It's probably the best thing to use, other than 30W ND perhaps. I'm passing 50K miles on my LH gearbox and it still works as good as new.
Steve Simmons

Paul
I always heed your wise counsel and I note your comments with respect to de-clutching or not when changing in and out of overdrive. I always de-clutch when changing out (on my 4 cylinder car) and mostly do when engaging - at all times matching engine revs with road speed. I accept your assurance that no de-clutching is necessary and will adopt that practice in future as it will stave off left leg fatigue. I have a competion clutch and find it very heavy - a pain in the (a***) leg.
Thanks for your ongoing contribution to this great forum.
Peter M
Peter M (Member)

Please remember that Paul Hunt's opinions are just opinions, not statements of fact. Stick to what the experts say, it will work out cheaper in the long run.
peter t

To quote the experts (ORS / Laycock, who built the OD units) "the OD is designed to operate with oil viscosities from ATF up to EP90"

EP oils up to GL4 are suitable - GL5 is NOT recommended as it WILL attack the yellow metals within the gearbox & OD unit.

The recommended oil grade was determined by the gearbox manufacturer.

One of the engine oil recommendations is the normal gearbox oil in an MGB. I use ATF or a mixture of ATF and engine oil for the high detergent qualities to clean out a sticky OD unit. This often restores patchy operation of the unit to normal.

V8 & MGC OD units have the same size cone clutch but use a different lining, stronger return springs and higher oil pressure.

Type D ODs used the vacuum switch to protect the OD from sudden torque reversals on switching out on the over run - the LH type do the same thing by making the operating oil mix with the pump output on its way from the piston chambers to the sump, thus damping the cone clutch movement and providing a smoother change.

One recommendation I make is to use a different colour oil in engine & box so that oil leaks can be traced back to the engine or box when they are spread over the bottom of the car!!

Chris at Octarine Services

peter t is entitled to his opinions and make his comments, but what I have said is from the manual, not my opinion.

According to the Parts Catalogue the only differences between the 4-cylinder LH-type and V8 ODs is different pressure relief valve and springs to give more pressure on the friction surfaces in both the OD and non-OD positions, the cone clutch (which carries the friction material) is shown as common to both.
Paul Hunt

I agree with Paul, based on literature from the manufacturers as well as personal experience. Using the clutch when shifting is a waste of a good clutch lining. I have a competition overdrive in my MGC and I do occasionally give it half clutch as the OD engages, because it's really a strong jolt and my engine puts out in excess of 200HP. On a standard gearbox in an MGB, there is no need.
Steve Simmons

This thread was discussed between 20/03/2009 and 26/03/2009

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