Gear Oil & Yellow Metals

The 650 Triumph gearbox contains bushes and thrust washers made of bronze, and when I drained my 1969 Triumph's gearbox last winter I discovered a considerable amount of bronze in the oil.

An inspection of the gearbox revealed that the inboard gearbox bronze thrust washer had turned on its locating pin. But there seemed to be so much metal in the oil that I began considering if there could be another source in addition to the washer.

Bronze is one of the three yellow metals, the other two being copper and brass. It should be understood that not all oil products are friendly towards yellow metals - some can eat away at them and destroy them.

I'd heard that gear oils conforming to the GL1 or GL4 standards were ok, but that GL5 and GL4/GL5 products were not. A look around the Internet disclosed that GL5 products generally have a higher sulphur content (makes things more slippery for modern synchros) and that the sulphur, as well as other additives in GL5, will deteriorate yellow metals through chemical reaction.

In the past I always used GL4 gear oil, but apparently last year I used a GL4/GL5 spec oil. I probably did so because I couldn't find any GL1 or GL4 products!

When I posted about this concern to the Britbike.com forum it was met with skepticism by a significant number of listees, all of whom have a great deal more combustion engine/old motorcycle knowledge than me

While researching this topic it struck me how many discussions on the subject conflate the terms "gearbox" and "transmission". when the terms are used interchangeably it muddies the water, or in this case, clouds the oil.

Anyway, I did find a tractor forum posting from 2014 in which a supposed (this is the Internet) Lucas "product manager" advised that Lucas products are NOT yellow metal friendly and should not be used in machinery whose materials include it.

Even though I concluded that the damaged thrust washer was likely the source of the yellow metal in the Bonnie's gearbox, I decided to use a gearbox oil that explicitly states it's safe for yellow metal.

Easier said than done! A survey of auto parts stores, ATV shops, and big box stores within a 50-mile radius turned up only GL4/GL5 and GL5 products, so I had to turn to the Internet.

There I found:

Is it the Right Stuff?

Sinetific Experiment

Immerse a couple of old bronze bushes in GL5 products: Valvoline and Lucas.

I got a little surprise when I poured the two products over the bushes. The Valvolene was dark honey and totally clear when I poured it. The Lucas product was much lighter in color and began as a clear stream but then came a plume of some fine material. Additives?

Evidently damage to yellow metal from sulphur and additives occurs (more quickly) at higher temperatures: 40C and up. My little windowsill experiment will obviously run under lower temperatures, but we'll see what develops over time.



Lucas oil representative 'Don't use our products with yellow metal' (2014)

Britbike.com "Bronze-friendly gear oil recommendation?"

Hermit.cc "Gearbox Lube"