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We all know the difference between semi and synthetic oils, but gearbox oil baffles me. I have no idea what the various grades mean. I tend to use Silkolene Medium Gear oil myself, and find it perfectly okay. But it came up the other day in relation to ST90, and it occurred to me that I don't actually know what the grades mean in the same way I would an engine oil. I know it's a little thinner, but that's about it. Can somebody explain it all?

All I know is you need to use non-EP gear oil as EP additives don't play well with cork plates, and the number relates to the viscosity, i.e. the higher the number the higher the viscosity

Isn't the issue with EP to do with the phosphor bronze bushes?

rossclark wrote:Isn't the issue with EP to do with the phosphor bronze bushes?

That's right it does,so with the clutch cork issue as well definitely avoid EP oil

ToBoldlyGo wrote:We all know the difference between semi and synthetic oils, but gearbox oil baffles me. I have no idea what the various grades mean. I tend to use Silkolene Medium Gear oil myself, and find it perfectly okay. But it came up the other day in relation to ST90, and it occurred to me that I don't actually know what the grades mean in the same way I would an engine oil. I know it's a little thinner, but that's about it. Can somebody explain it all?

SAE Oil ratings for gearboxes differ from engine oils mainly because the two purposes have different operating temperatures.
SAE90 (ST90) gear oil shares similarilty with SAE30 engine oil. A lot of really old bikes and cars often spec SAE30 engine oil for their gearboxes.

burnside wrote:All I know is you need to use non-EP gear oil as EP additives don't play well with cork plates, and the number relates to the viscosity, i.e. the higher the number the higher the viscosity

That would make sense. So what I'm using is basically just a bit thinner.

85/90W suggests that it would be marginally thinner at its max rated operating temperature, but then all oils have less viscocity as they get hotter. Your brand may just have better labelling?

I tried ATF years ago which cured a slipping clutch fitted with ST90.

No reason for me to fit anything else now, in any engine.

I'm not that clever to claim it to be my idea, as Bantam racers have used it for decades in three speed 'boxes with incredibly high firsts.

Try & get your mind around that they often use steel to steel wet clutches as well, & trying cheap as chips ATF in a Lambretta doesn't appear such a leap for all mankind....

Sometimes, when a gearbox is shimmed up on the tight side (3 thou) it can be a bit of a sod selecting first from a standing start until the gearbox is up to full working temperature.
Using ATF sorts this issue out as well.
I've yet to see a bonefide reason for not using it.

why do you need a 90 oil for a lambretta but only a 30 for a vespa???? i can only guess the 90 maybe better protection for the chain!!!other than the chain there is no real difference in the engines........just my guess i,m sure someone may be kind enough to come along and educate me........

It's like I mentioned earlier in this post. The lambretta uses a 90 weight Gear oil wheras the Vethspa uses a 30 weight engine oil. The viscosity is very similar..

Fast n Furious wrote:It's like I mentioned earlier in this post. The lambretta uses a 90 weight Gear oil wheras the Vethspa uses a 30 weight engine oil. The viscosity is very similar..

so Why do you need a 90????

I use classic 40 in mine now, cheap as chips from Exol

shamrockexpress wrote:

Fast n Furious wrote:It's like I mentioned earlier in this post. The lambretta uses a 90 weight Gear oil wheras the Vethspa uses a 30 weight engine oil. The viscosity is very similar..

so Why do you need a 90????

The weight of gear oil is on a different temperature scale to engine oil.

If you inadvertently put a light gear oil in an engine with a 4 plate cork Surflex clutch, can you take the plates out, soak them in the correct oil and start again? Or, as I suspect, do I need new clutch plates?

For information, I didn’t engage my brain before I put this oil in:

https://www.lambrettaspares.com/mbu0523.html

The clutch, despite having two 1.2mm steels in, drags



I have just noticed this on CamLam’s website, in relation to Surflex plates:

“They tend to come with a thicker amount of friction material, so we always suggest using 1.00 or 1.20mm steels - or a combination of them- to get the desired amount of free play otherwise you will experience clutch drag.”

That’s a new one on me.

Any thoughts as to whether my issue is the Surflex plates or the MB gearbox oil? Or both?

Of no help, directly, to your question(s)

I put this in my engine which claims to be ST90, but its 30 weight (engine?) oil
https://www.torbaylambretta.co.uk/product/gearbox-oil-(st90)-motul-1ltr-lambretta

Per FnF it's the same but it's not, as the weights are measured at different temperature scales depending intended application

Despite the Motul oil seemingly working fine in my initial 100 miles before Alex pointed this out I dumped it, and used trusty Maxima instead

nsaints wrote:Of no help, directly, to your question(s)

I put this in my engine which claims to be ST90, but its 30 weight (engine?) oil
https://www.torbaylambretta.co.uk/product/gearbox-oil-(st90)-motul-1ltr-lambretta

Per FnF it's the same but it's not, as the weights are measured at different temperature scales depending intended application

Despite the Motul oil seemingly working fine in my initial 100 miles before Alex pointed this out I dumped it, and used trusty Maxima instead

Thanks. Agree that the Motul isn’t ST90 by the looks of it.

Until this episode, I have almost exclusively used Rock Oil products for Lambretta clutches, the green pot ST90 for cork plates and their “Lite Gear Oil” for the heavy duty material used in most 5 plate clutches. I only bought this MB stuff as I was placing an order with them and was almost out of Rock Oil supplies.

As regards my clutch, I haven’t got to the bottom of it yet. For the time being, and to use up the MB oil, I have put an old Surflex 5 plate clutch in. That has resolved the drag.

I’ll have a look at the 4 plate clutch in the next few days, taking the steels out and measuring how thick the actual clutch plates are.