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MG Midget and Sprite Technical - 1275 Gearbox Oil for Racing

I have a 1275 gearbox with straight cut gears. When it is cold it changes gear beautifully, even during a reasonable drive on the road, gives not problems. When the gearbox gets hot on the circuit, second gear becomes very difficult to engage.

I am wondering if the oil that I am using is thinning out too much and I should be using a heavier oil. I have asked several UK based gearbox specialists but they have been no real help at all! I'm currently using a 20-50 oil. Any suggestions?

Many thanks, David (currently based in Oxfordshire!)
David JM

I use 20/50 and had a similar problem. Try connecting a breather - i brazed one onto the filler plug and then ran a pipe to the catch tank. I then filled the box from the remote, after removing the gearstick. The other thing is make sure the clutch hydraulics are all OK - after I bled mine again I had no problem.
John Collinson

I use Duckhams 20/50 in a s/c box, & have done races over 1 hour long with no problems. Could be worn synchro or selector, who is your gearbox builder?
David Smith

What sort of 20/50?

Have you tried used a synthetic engine oil like a 15-50 (Mobil 1)
Daniel Thirteen-Twelve

(had to go looksee) Duckhams Q Classic mineral oil.
I've used Mobil 1 synthetic 15/50 in the Honda PG1 boxes in Montys for years, but heard 'old wives tales' about fully synth attacking the bronzy bits in midget boxes so haven't risked it.
David Smith

Maybe an enlightened person can advise what it is in gear oil that attacks the bronzy bits and we can then check to see if:

Synthetic engine oil has it in it (which I doubt)

Synthetic gear oil has it in it (which we aren't going to use, I also doubt but want to know out of interest)

I ran Mobil 1 in Spridget boxes for many years with no problems (car regularly used 7,500 rpm)
Daniel Thirteen-Twelve

IIRC having done a quick bit of digging about this recently with regards making some PB sun gear thrust washers for the diff, the issue is with certain EP additives containing sulphur. At higher oil temperatures and with moisture present compounds can be formed which attack the copper and can lead to degradation of the bearings or copper containing parts. That was only after a short search and I haven't spent too much time looking, IIRC there are EP additive which don't pose a problem and the oils do have a rating as to whether they attack copper and its alloys to any degree.

Just this morning I ran across this page and it notes the grading for a copper corrosion test http://www.mobil.com/USA-English/Lubes/PDS/GLXXENINDMOMobil_Vactra_Numbered.asp .

Regarding the diff washers, the planet washers seem OK in what looks to be bronze, so the sun gear ones should be OK. Having stripped a few diffs I have 2 steel sun gear washers, 4 in bronze, and haven't seen much of the phenolic ones to be worth saving. Most likely a cost cutting exercise changing the material.
David Billington

Dave Billington,

There was a thread some time ago that I contributed to re. pb diff thrust washers (using a Phantom Grip).
There was discussion about getting some made - are these the ones you refer to in your post?

Am still interested in some pb washers if they can be made up.

John Turner
John Turner (Midget & MGB)

John,

Yes, I have acquired some PB sheet with the view to making some. I wanted to make punches and dies for them and have started a bit but with a higher workload currently and trying to juggle various other projects it's not the highest priority. If you want to give some a try with a phantom grip then maybe I'll make a small number up by boring the centres and mounting them on an arbor to turn them round. Not quickest per washer but quick to produce a small number.
David Billington

David,

No rush, happy to wait until you are 'tooled up'!

When you have some please contact me at

g dot john dot turner at blueyonder dot co dot uk

Thanks

John
John Turner (Midget & MGB)

This thread was discussed between 16/04/2009 and 17/04/2009

MG Midget and Sprite Technical index

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