Couple of Gearbox Questions

[John P]

Hi,

I am currently re-assembling the gate change casting onto my 3-speed gearbox.

The casting is held in place by three cheese head screws which sit in counter-bored holes in the casting but when I dismantled it there were no shake-proof washers on the screws.   I thought they had probably been omitted in error but now realise the diameter of the counter-bore doesn't seem large enough to allow them to be fitted.

Should there be some form of anti-vibration washer here or do the screws simply go in as is?

I'm also deciding what oil to use in the gearbox.   I have previously used normal 20/50 engine oil but have seen reference to using a straight (Not EP) SAE 90 oil which apparently can have the effect of slowing the gears down during a gear change making silent changed easier to achieve.

Does this make sense and what are others using?

Many thanks,
John.

[Charles P]

Just do them up tightly. Loctite if you fancy.
Supermarket 20/50 will be fine.

[David Stepney]

When I got my RP saloon (4 speed all crash gearbox) the gearbox had what looked to be SAE 80 or 90 in it. Changing the oil to 20W/50( the high zinc mineral sort that I use in both my Polo Mk2 and the Landrover Series 3 engines) made the gearbox much nicer to use. The factory recommended using engine oil in the gearbox and I tend to trust that the factory knew what it was doing.

[Chris KC]

A7 gearboxes were designed to run on engine oil. I use straight 30 grade.

[Bob Culver]

Many factors influence oil choice; some of those relevant pre war barely apply to moderns with effective seals and quiet all helical gears. Oil was chosen to reach plain bearings if any, quieten, reduce leakage, reduce power loss,  facilitate synchro (thinner). Crash change requires a thick oil to slow for upward changes but a thin one to facilitate downward. And what suits changes at low temp may not suit when hot.

Except possibly the boutique monograde classic oils, all now contain anti wear and or EP additives and SAE 20 or thicker fully adequate for lubrication. Many other cars of the 1950s which ran engine oil SAE30 in gearboxes later recommended 20W50 for the same gearbox. The reference temp for W oils is below 0 degC . According to graphs in the book Which Oil the crossover point for 20W50 and 30 is still below 0 deg C, so bulk SAE20W50 is notably thick at normal use temps. The  multigrades thin out less as temp rises. With 20W50 and 15W40 cheaply available these seem fine in place of monogrades now these have become expensive. 
Gear oil classification is different but SAE90 approximates SAE50 engine oil, and 85W is somewhat thinner. The grades cover a range so comparisons can vary with brands. Mainly because it was used in many other cars, many Sevens ran EP90 gear oil in the 1950s and 60s. (Many now avoid EP oils in diff or gearbox for fear of bronze/brass corrosion but the major oil makers claim OK and doubtful if sufficently high temps reached anyway. Austin publications in 1950s recommended for diffs. Dunno if anyone has ever reported a positively identified case of excessiev wear.)

[Dennis Nicholas]

I suppose I had better not resurrect the last (apparently) controversial issue.......but if you really want to preserve your Austins and remove massive amounts of friction......  been using it for many many thousands of miles in various old and moderns.  AFMT.
I can get the stuff at a club discount; pays for itself in petrol saving alone within a few hundred miles.

   

Dennis

[Zetomagneto]

Same idea as the copper band you can put round your fuel pipe it’s all to do with molecules!

[Robert Leigh]

Excuse my ignorance but what is AFMT, please?
Robert Leigh

[Reckless Rat]

I suspect it's an anagram of snake oil?

[Parazine]

In general, anti friction additives like this can do damage by stopping synchromesh working and causing bearings to skid around rather than turn properly. These things rely on a certain element of friction to work correctly.

Best avoided as the effects may be at best, unknown and at worst deleterious.