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Belt drives and oil contamination of the clutch.

Submitted by tony_ripley on Tuesday, 30 September, 2025 - 11:17
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Nortons - General

There are a number of modifications to the clutch pushrod mechanism to prevent oil coming from the gearbox into the clutch basket. As my commando is standard, the clutch runs in oil anyway.
so my questions are.
1. Does gearbox oil affect clutch plates in some magic way that primary drive oil does not?

2. For those that have a belt drive and thus run the chaincase dry (obviously not 850 MkIII's) do you get a problem with oil contamination coming along the pushrod?

your thoughts...

Submitted by stan_thomas on Tuesday, 30 September, 2025 - 12:04

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I'd be interested in answers to those questions too.

The standard bronze clutch on my Mk3 slipped badly at about 5,000 rpm - dangerous if passing a line of cars in 3rd. Cleaning helped for a few miles but it kept coming back. I've fitted the RGM pushrod seal and changed to Surflex fibre plates and changed from a semi-synth car oil to a 20/50 4T specified for wet clutches. Seems to have worked. But having changed three things I don't know which has had most effect.

I would note:

  • Clutch slip was never a problem on any of my Commandos (including Mk3) when they were in daily use back-in-the-day(TM). Then I was using Castol EP90 in the gearbox and and a mineral 20/50 in the primary; now I use a 75/90 synthetic in the gearbox.
  • Running Surflex fibre plates dry in belt-drive race bikes, I never saw any oil contamination of the plates. But those bikes were parked on track stands so they never spent much time leaning over to the left.

So is it that modern oils are much "slippier" or infrequent use, with the bike parked on the side-stand for days/weeks, allows the oil (primary or gearbox) to permeate the clutch where previously it was flung off? Or something else?

Submitted by tony_ripley on Tuesday, 30 September, 2025 - 12:37

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Hi Stan,
I had the same problem with clutch slip. Solved by modifying the plates in the same way as the RGM hi torque ones, reduces the contact area by about half, plus using ATF in the primary.

hopefully we will get a good number of responses from those running belt drives.

Tony 

Submitted by stan_thomas on Wednesday, 1 October, 2025 - 16:21

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The subject has been covered extensively over many years:
https://www.nortonownersclub.org/support/technical-support-commando/clutch

and, of course, Mr Leadbeter's exposition on the shortcomings of the clutch itself:
https://www.nortonownersclub.org/comment/16032#comment-16032

But, having read these articles, I'm still trying to understand why I never, ever suffered from clutch slip when using my Commando regularly in the 70s. And the several Commando owners I knew never complained about their clutches either. Yet now, using the exact same parts, it can be truly awful. Yes, the parts are half a century old but they've just been in a box on the shelf for 40 years.

Things that are different are: oils and time spent parked. Is that it?

Submitted by tony_ripley on Wednesday, 1 October, 2025 - 17:17

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Hi,

Thanks for details about slipping clutches. But PLEASE could responders read what the question actually asks.
I am interested if those who run a dry primary, have issues with oil coming down the pushrod tube and contaminating the clutch plates.

If not, then why are there methods suggested to prevent this happening?
If so, then I will know that it does happen, and then try to decide if it is actually a problem.

Tony

As noted, I never saw any oil on the plates running a dry clutch and belt drive on a race bike so I'm as mystified as you to whether clutch pushrods seals are a panacea or snake oil.

The link to the NOC compendium on the clutch does offer suggestions on how oil might get onto and down the pushrod but also how this can't happen. There was a trace of oil on my bronze plates before I swapped them out this year, but the usual smell test didn't identify gear oil. I suppose one could add a dye to the gearbox oil and look for it in the clutch.

Submitted by ashley_cutler1 on Thursday, 2 October, 2025 - 08:12

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To answer the question on the Gearbox oil, it usually contains extreme pressure additives that could affect the clutch plates more than other oils.
I don't use the side stand and don't get gearbox oil in the in the clutch. 
My clutch used to slip but on the first aggressive overtake of the day, after that it would be fine all day.
If you fit Barnett plates then they should work fine in oil as they are designed to run in oil but will also run dry. 
Most race bikes with belt drives most probably don't experience this much as they are usually on paddock stands and thus upright. 

 



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