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Oil for Mono-spiral Differential

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number 5 Peter H
Richmond, nsw, Australia   AUS
Hi all,

Its been 18 years since I I have needed to acquire differential oil, which is for both a 3.667 and 4.125 momo-spiral diff, not an hypoid unit. I honestly cannot remember what oil I last acquired when I put my '48 convertible back into service, but one needs to get an oil that does not have any additives that could affect the small amount of soft alloy used in the diff. I have spoken with a number of tech's here, and with the exception of one, none seem to be 100% sure what is best. Hence, it would be good to get what others may have been using, without any hassles.

The diff removed on this car that I am now restoring had 0.018" backlash, and it was filled with "tar"!!!!! It had done a mere 300,000 miles, and when last driven, despite the backlash measured, was functioning without any apparent problems. Not sure what oil it was, but it was in the car when I acquired it.

Putting it aside, I stripped down another that had 0.0085" backlash, and after washing it clean, and replacing the pinion bearings, and resetting the bearing pre-load torque, after re-fitting the crown, the backlash measured 0.007, so that was pleasing to see.

So, if anyone can advise what oil they use in their mono-spiral , even if it is not available here in Aussie, I should hopefully be able to get a cross reference.

Thanks-Peter.

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NaDaDawgRacer John Jones
Waxhaw, NC, USA   USA
Peter
I don’t know what the oil designations are for you down there but up here in the States Lake Speed Jr told me that you should use a GL4 gear oil where “yellow” metals are involved. GL5 oils have a larger amount of sulpher in it which is good for heavy shock applications but is bad for brass and bronze.
You had 300,000 miles on the old oil? You would be amazed how much better things operate with regular oil changes. At a minimum I change tranny every 2 years and diff every 5 years.
John

number 5 Peter H
Richmond, nsw, Australia   AUS
Thanks John---I have been assured by Penrite that their their GL5 rated oil will in no way have any adverse affects on yellow metals, as the modern sulphur additives are "benine". There are one or two GL4 rated products, though these are 75W90, not any higher value on the latter. Caltex here said that a 140 would only be needed for for extreme loads, and a 90 would be more than adequate.
I have contacted a coup[e of other suppliers and will see what they have to say. There are several industrial grades which fit the need, but alas, they are sold in minimum 20 litre containers.
Again-thanks for your response.

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NaDaDawgRacer John Jones
Waxhaw, NC, USA   USA
Peter
You definitely don’t want to run a 140 grade oil it is much to heavy for your application and would be like driving with the brake on.
John

number 5 Peter H
Richmond, nsw, Australia   AUS
Hi John-Thanks for your comment. The car I am restoring is an A90 Atlantic coupe, in which I travelled 300,000 miles before "shedding" it in 1981. In 1980. I acquired my first road registered A90 Atlantic convertible, undertaking in it over a 20 year span, 475,000 miles. With the coupe, I never changed the diff oil, though with the convertible, I think twice during the miles covered. In it, I did use 140 oil, and it was a rather "spirited" set of wheels, and did not hesitate to cruise around the 80~85 mph figure. Unlike my 100-4, I must admit an A90 is not as quick off the mark, but once rolling, it was, in my humble opinion, a drivers car. Our club undertakes only one run a year, and many of these are for very long distance. The 100 has long been gone, but in its place I now have the only A90 Atlantic prototype that is known to exist. Unlike the above first two A90's I have used, in the past 10 years in the prototype I have only undertaken just over 8000 miles.

I honestly cannot remember what oil I used in the diff, which I filled back in 2004,m but I know it was a GL4 rated product, though I did think it was a 90/140, though these days I cannot see any GL4 products with this rating. It will be interesting to see what I am recommenced in the next few days. I will post the details when known.


All the best-Peter

Michael Oritt Avatar
LUSBY, MD, USA   USA
In your all steel diff you should be using a GL5 oil. GL4's are designed for transmissions that have yellow metals such as the synchro rings.



Best--Michael Oritt
1954 Austin-Healey 100 (street)
1958 Elva Courier (track) FOR SALE
1959 Elva MK IV sports racer (track)
1961 Ginetta G4 (track)

number 5 Peter H
Richmond, nsw, Australia   AUS
Hi Michael--Thanks for your comment.

Having dismantled both the 3.66 and 4.125 diffs, these do have a small content of "yellow metal", being the curved shims between what Austin refer to in the Atlantic manual, (as item 8), differential pinion gears. I have also head of others referring to these as planetary gears, or sun gears.

There are also shims between the axle spline gears and the inner case. These though, I think, were originally steel. However, quite some years ago when I had my 3.66 checked when I didn't have the time, the chap used some 0.003 bronze shim to set the gap correctly. He said that it was common to use bronze shims there when it was not possible to get it right with all steel ones. As at that time, I had a GL4 oil, it was OK.

Yesterday, I had a call from Penrite, who advised that their GL5 oil was OK now to use with yellow metals, as they have altered the nature of the Sulphur to be "benine", so as not to cause any problems.

I also rang the Melbourne Healey Factory, and another Sydney organisation, one saying that they used a GL5 product, and the other saying they used a GL5/6 oil.

Hopefully, all will be OK.

Cheers-Peter.

BobAH100 Bob Markovich
Patterson, NY, USA   USA
An excellent question my friend and I pondered for my spiral-bevel-equipped early BN1. I ultimately settled on Royal Purple 75/90 synthetic, which has the Extreme Pressure (EP) rating essential for this diff. Interestingly, this was the only diff oil specifically stated to be EP in the store's ordering catalog, though I believe Mobil 1 also is EP-certified.

Valvoline apparently makes an EP diff oil, if you can find it. Meanwhile, Royal Purple (which really is purple and costs a pretty penny!) is definitely EP. Will it harm the soft metal inside this diff? If so, all of us are likely to be long dead by that time ...

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