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MG MGB Technical - Gearbox oil (for 'bos with Laycock O/D)

I see the spec for the 'box is the same as the engine - namely 20W/50.

Has anyone used the Millers Classic MINI oil - which claims to be designed for engine applications which share oil with the 'box and therefore appears a better choice for the MGB O/D gearbox?
R Kelly

If you are in any doubt use gear oil, straight EP90. After all MG specified it for the V8 box which was more or less a 1800 box. I've used it in all my 1800's. Gear oil for gears!!! Makes sense to me!
Allan Reeling

Gear oil for GEARS, yes, not NOT for overdrive cones surely???!!
R Kelly

GT V8 had overdrive, why would you think gear oil would be an issue?
Allan Reeling

Yes I have. Suffered severe overdrive slip. Changed to Penrite gearoil 30, NO further problems in 6000 miles
jim soutar

The OD used in the V8 application used much heavier return springs than those used in the standard B OD. This is why BL recommended using 90W gear oil. When used in a standard OD, the pressure reaches extremely high numbers and can cause the OD cone to lock against the braking ring. Freeing it up requires a good whack with a large hammer. The use of non-detergent 30w oil has been proven to work the best with these OD transmissions. The detergent additives, of regular engine oil, aren't needed in your transmission as there is no combustion taking place. They cause trouble by making the oil foam and slowing down the operation of the OD's internal operating components. RAY
rjm RAY

But isn't EP 90 the same viscosity as 40W engine oil? I've used EP 90 in all my previous 1800 B's,all O/D,and never had a problem.
Allan Reeling

the numbers are not strict as such and cover a range

see chart below

Nigel Atkins

Read the June 2005 topic of the month in this link from quantumechanics.com:

http://www.quantumechanics.com/qm-htm/topic1.htm
Steven Rechter

According to Quantum Mechanics I have been lucky with my previous OD units, but there again I haven't seen the problems associated with using detergent oils either! The chart Nigel has donated seems to suggest EP85 roughly corresponds to 40W engine oil and EP80 to between 25 and 38W. Maybe Quantum should have pursued their experiments with EP80, or some of the multigrades as well. All very interesting stuff.
As an aside I've just acquired a Myford ML7 lathe and, would you believe it, there is the same sort of discussion on their blogs, i.e.., which oil is best for the headstock bearings!!! Ah hem!!
Allan Reeling

I understand the issue can be that the Laycock OD as in the 4-cyl car can be sensitive to oil weight and with EP90 can give rise to higher pressures than desired.

This can jam the cone. Somewhere I read an article where they had tested a series of oils and the 'correct' 20W/50 gave a 400psi pressure downstream of the n/r valve and with EP90 it rose to 700psi and resulted in the cone clutch being jammed as it was driven with excessive force.

Obviously, if you've use EP90 for ever and it's not harmed your 'box then one cannot argue, but if not, perhaps worth sticking to the spec.

EP 90 might 'look' like a 50 weight engine oil at 100 deg C but it will be very viscous at lower temps - see the chart. I would concede that a multigrade EP, e.g. 75W-90 might rate similarly to a 20W-50 engine oil, on viscosity alone.


R Kelly

The only time that I used 90W oil in my overdrive transmission was by mistake. I grabbed the wrong container of oil and filled the box. When I drove the car, I couldn't believe how difficult it had become to shift between gears until the transmission had reached operating temperature. It was still very stiff and I quickly drained the 90W oil and installed the 30W non-detergent oil that had been in the transmission before I had serviced it. RAY
rjm RAY

I've always run gear oil in my V8 OD and engine oil in my roadster - as recommended by the factory - and never had a problem with stiffness of change even when the V8 was kept outside at temperatures below freezing being used every day. The recommendation of gear oil over engine oil for the V8 is more to do with the increased torque through the gear and the OD than anything else, which is also why OD was removed from 3rd gear on those cars.

If anyone has a problem with engine oil (which can be 20W/50, 10W/50 or 10W/40 in UK temperatures) in the 4-cylinder, or gear oil in the V8, then there is something wrong with the gearbox or OD. Then again if using an alternative to the recommended oil - ATF is also mentioned from time to time - 'cures' that problem, then that may well be preferable to stripping the OD and looking for a marginal issue.
Paul Hunt

Hmm, I have just stumbled across the attached document.

Anyone have any background as to how this came about, as personally, I'm still persuaded by the experiments two of us refer to above about SAE90 being risky.

R Kelly

If gear oil of 80 or 90 has caused a problem that engine oil of 20W/50 didn't, then the implication is there is something wrong with the OD or gearbox concerned.
Paul Hunt

Just a query - Is the OD relief valve spring different in the V8 gearbox? Are there any other differences besides the springs mentioned by Ray between the 4 cylinder and V8 gearboxes (I am aware the casing is different to allow for the larger clutch and that the ratios are a little closer). Why does running gear oil in 4 syncro LH gearbox give a higher pressure if relief valve is the same - less leakage? Is the higher spring rates in the V8 box to unlock the cone clutch (outer lining) from the brake ring or to insure there is no slippage between the cone clutch (inner lining) and the annulus when switching to direct drive?
Mike
Mike Ellsmore

Yes it is, to develop the higher pressure required by those boxes - 510-530 psi as opposed to 400-420 of the 4-cylinder.

There are differences in the gearbox casings for the larger manual clutch of the V8, also different gearbox ratios. The casings are the same, as is everything bar the cone clutch return springs, the valve assembly, and the method of attaching to the main gearbox in the shape of one different stud and a different gasket. The OD ratio was exactly the same as all the other 4-synch gearbboxes at 0.82:1.

The manual describes giving the casing a sharp smack with a mallet to free a clutch, which may stick on a new unit because of insufficient bedding-in. The different release springs are as much likely to be because of the higher hydraulic pressure as anything else.

As the comparative viscosities 80 wt and 90 wt gear oil are the same as 30wt and 50wt engine oil I can't see why leakage would be any different either.

In direct drive forward motion is transmitted through the Sprague or one-way clutch which is locked, not the clutches, so spring pressure is not relevant. The clutches are only used for reverse (non OD) and OD i.e. 4th gear. The springs only come into play for reverse, so I suppose they could cater for anyone that might reverse at full throttle. But in a description of the functioning of the OD the manual states that "During over-run and reverse, additional load is imparted to the clutch sliding member by the helix thrust of the sun wheel, thus helping to retain direct drive." That says to me that the more torque that is applied in reverse, the harder the clutch faces are pressed together.
Paul Hunt

Hello,

If advice from a yankee mechanics could be of some help:

http://www.mgtoronto.com/pdf/Tech/Overdrive_oil.pdf

Cheers,

J.G.C.
J. G. Catford

That's the same text that Stephen gave a link to several days back.

I'm not sure what he means by giving pressures before and after 'shifting'. According to the Leyland and Laycock manuals there should be NO pressure with OD disengaged, only when engaged. If they saw those 'before shifting' pressures with OD disengaged then I'd say the low-pressure part of the relief valve wasn't functioning correctly. And there's no getting away from the fact that 30 wt engine oil falls between the viscosity of 80 and 85 wt gear oil.

Like laying all the scientists in the world end to end, a single conclusion on this topic will never be reached. Except perhaps to use what works best for you.
Paul Hunt

So what should I put in a "re-built to V8 spec" unit for a supercharged (higher torque- no more than 140 at the flywheel) 4 pot? I use 20w 50 engine oil as I did before it was rebuilt!
Michael Beswick

Well Paul,

I am not an OD specialist, but every hydraulic system may have a residual pressure before being to zero, an inertia. In my mind it is not an on/off situation like flipping a light switch.

One basic point why I add that info is on our side of the world, that company has a very good reputation in OD overhauling.


JG
J. G. Catford

"So what should I put in a "re-built to V8 spec""

If they used V8 springs and relief valve, than I'd say gear oil as recommended my MG for the V8. If they didn't, then engine oil as recommended by MG for the 4-cylinder.

200 to 400 psi before engaging OD is hardly 'residual' pressure!
Paul Hunt

Well Paul,

Nobody could know exactly the answer if they haven't run test in real condition with a gauge. But for me one oil I would not use is S.A.E 90, for high pressure as yankie cie mentioned.

Another example is Jag. XKE, which may serve as a reference. I searched Jag lovers for some infos, and found one answer S.A.E.30, but it was from private owners.

Another advice is from Penrite and they mentioned S.A.E. 30.

http://www.groups.tr-register.co.uk/nene-valley/docs/Library/Penrite_brochure.pdf


Wish it could help...



Jean G.
J. G. Catford

I've been using EP90 in my V8 as specified by the manufacturer for 20 years and 100k and not had any problems. Ditto engine oil in my 4-cylinder for 25 years and 50k. Like I said, this discussion will never reach a conclusion. Use what's best for you, but use other than the manufacturers recommendation at your own risk.
Paul Hunt

This thread was discussed between 24/09/2014 and 04/10/2014

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