gearbox oil

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Apr 15, 2004
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I'm going to be putting my gearbox back together soon, maybe this weekend if UPS delivers the parts today.

So, I stopped by the local mc supply and bought a quart of synthetic gear oil. I saw an older thread in which BelRay and Redline were recommended. Neither was available locally :roll: so I ended up with "Original Big Twin" synthetic. It's made by Silkolene so I figured it would be ok. But on the back, in tiny text, it says "API GL-5".

I seem to recall some issue with GL-5 vs GL-4. IIRC, one of them contains some additive that destroys the brass bushings. But I can't recall the details nor can I find that thread back. Can anyone re-enlighten me?

I do not pretend to have great knowledge on this, in UK recommended oil was EP90, modern formula is supposed to attack sintered phosphor bronze bushes. I dont know if this applies to Commando. I played safe & obtained AG90 oil from Morris lubricants, supplied direct in 5 litre size, made for just this purpose. There may be something available in US along these lines?

The Porsche gurus recommend SWEPCO gear oil for their delicate transmissions. It's made in USA and is darn expensive. You may want to contact them to see if their product has the deadly phosphor bronze eating chemical :x . I suspect it would be ok for the boxes we have since as I recall the Prosche synchros are bronze.

(wearing my "I survived Hurricane Wilma" t-shirt and wondering when the world will return to normal)

I use Castrol Hypoy C, 85W/140 in my gearbox. It too refers to API Service GL5, which is an extreme pressure specification. Notably, the Norton shop manual lists various acceptable gearbox lubes and Hypoy was one of them.

I can't imagine that oil suitable for extreme pressure service is detrimental to brass.

Low oil level and dirty oil are detrimental to brass. So, keep an eye on your gearbox oil and change it every other engine oil change.

Hi Deb
The one thing i have not had problems with is the gearbox and i just use 90 gear oil and have for years. If it's happy this way so am i .
The EP additives in GL5 gear lube are not compatible with Norton's brass bushings - so sayeth the great Norvil anyway. However, this reminds me of the DOT4 vs DOT5 vs ... brake fluid discussion whereby those using DOT5 silicone fluid noticed none of the "seal eating" problems that were suppossed to accompany silicone brake fluid.

Sure would be nice to have satisfactory explainations and definitive, accurate answers to these queries, wouldn't it. Where I come from, "I use it and I don't see any problems" is anecdotal - not proof positive. With expensive bits at stake (or lengthy teardowns), thats kinda scary.

Redline makes a good GL4 spec synthetic - if you want to be extra cautious.
A local American and British bike shop (used to be British and American but times have changed) which did some engine work on mine put Redline in the gearbox. It definitely passed the right foot test - changes smoother for longer than the Castrol i had been using up until then. I have used red line since - only about 10k miles ago so can't speak for longevity.
Well it must be my bike, but the silicon soaked seals are fine after 20 years and the gearbox has always had EP90 and yes the only ever issue I have had is the layshaft ball bearing disintergrating, the bronze bushes are fine however. Another fairy story started by some salesman, which leads me to a story.

Originally Glacier and Vandervell were 2 separate companies in competition for 70 odd years. The last Vandervell sales director was fond of putting down Glacier bearings to the customers and would get the engineers to over specify the bearings as Vandervell produced high spec but expensive bearings but could never develop a good low cost bearing. Glacier on the other hand had developed a low cost but medium duty range which worked well in most applications but could not develop a high spec bearing to match the Vandervell. Over the years Vandervell lost out more and more as the market for medium duty bearings increased and the high end dropped off, in the end Glacier bought out Vandervell, the Vandervell Sales director became the Glacier Vandervell Sales director and was suddenly faced with visiting customers and having to back track on all the spurious stories he had been putting about for years.
I have always run with the advice given by NOC members of both Melbourne and Sydney clubs.. and endorsed by The Norvil shop UK, that is 80/90 (Castrol LXS90) and 1/2 tube of Nulon G90 Teflon treatment.

I also made up a gearbox level plug with a magnet , the same as the sump plug.

Additionally I regularly check for discolouring of the oil and will drop the oil in a service again to check for moisture, especially if I know I have been doing any great degree of short running..

Works for me ~ 8)
Yep, "Works fer me and my grandaddy". Hear it a lot. I have heard of everything from grease to ATF to Motul in Brit. gearboxes.
The bushes are bronze, not brass, unless you shop for the CHEAPEST parts. Brass has zinc in it, and is chemically rather reactive, bronze has tin, rather inert. Any oil that will pick up water will allow ionization and subsequent reactivity. So use oil that is less hygroscopic. Synthetic oils are much more resistant to water contamination than mineral based oils. They are chemically, physically, and thermally more resistant to the stresses of gearbox life. Synthetic oils are an engineered product, able to be created to accomplish a specific function as effectively as possible. Use them. Redline, Belray, Motul, Amsoil, and even Mobil One offer superior protection for your engine, primary, and gearbox.
I would never use oilite in a high torque application, it is powdered bronze heated and compressed in a mold. Cheap and easy, but not the best idea for a spendy gearbox. Use turned bushes from Aluminum/Beryllium bronze, Phosphor bronze, or Silicon bronze. Even if you have to source the material and have them made.
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