Camchain and tensioner seen up close in a cutaway bike engine

Home Repair And Restoration

Changing The Fork Oil

Post Received 17 June 2020

By Mark Noel

Fork oil must be the most neglected lubricant in any motorcycle. For mile after mile we expect it to lubricate, dampen, and protect, while expelling dust, water and road grit without complaint. On most machines there is a convenient drain plug at the bottom of each fork leg, either external to the wheel spindle or accessed after removing this spindle. Unscrew the cap at the top of the fork leg, extract this plug and the oil dribbles out aided by pumping the leg up and down.

To replace the oil the fork cap must be completely removed, taking care that the internal spring does not launch the cap into orbit as compression preload is released. The difficulty then comes in replacing the cap without stripping the threads: the only way being to bear down on the spring while turning the cap until the fine threads engage.

I don't know any biker friends who have bothered to carry out this servicing, simply feeling that 'if it ain't broke then don't fix it'. However, oil is cheaper than a fork rebuild and so adhering to this mantra I decided to change the oil on my Suzuki GN125. But, oh dear, there were no drain plugs and no simple way to extract the old oil other than by removing the top cap and inverting the forks (after removing each from the yokes) or inverting the complete machine.

Fortunately I had built a workshop crane with a lift capacity of 1 Ton for the purpose of moving various machines into position. Problem solved! The GN's tank and seat were removed, plus the fork caps and the bike upended in the garden to allow the oil to drain into a tub. Once upright again and refilled with new lubricant the bike was back together within the hour.

Mark's 125 is hanging upside down from straps in a large wooded rig

I'm glad that I forked out for fork oil!

If you'd still like to advertise on BAT after seeing this... erm... contact

Reader's Comments

Upt'North said :-
Mark you're obviously a genius but wouldn't it have been easier to remove the legs which also means you could have checked the bushings etc.
Fork oil is neglected you're not wrong.
30/07/2020 10:03:08 UTC
Bogger said :-
No motorcycles were harmed during the making of this tread. I think?

30/07/2020 12:55:13 UTC
Bogger said :-
30/07/2020 12:55:35 UTC
Ren - The Ed¹ said :-
We know Mark is a genius from he previous posts - but - this clearly proves the line between genius and madness is quite blurry. Funny though, when I first saw this image I practically spat my brew out.
30/07/2020 13:46:44 UTC
Mark Noel said :-
I am pleased that I am not the only person who recognises my genius!

Yes, I could have instead removed the legs but by inverting the bike I was also able to check the underside of the tyres.
30/07/2020 15:47:29 UTC
Upt'North said :-
Mark! I never thought of that, you are a genius.
30/07/2020 16:28:46 UTC
Ian Soady said :-
Ooh, you are awful! But I like it.

Why on earth manufacturers stopped putting drain plugs in, which makes the job 10 times easier, I'll never know. Crazy to have to remove the forks for a regular job.

As said fork oil is often in a terrible state so changing it - not necessarily using Mark's method - is a good idea.

Or buy a bike with girder forks then the problem removes itself.
Posted Image
30/07/2020 16:40:53 UTC
Upt'North said :-
Just a thought Mark. Did you drain the engine oil before tipping er like a lamb for shearing.
30/07/2020 18:43:13 UTC
Ren - The Ed¹ said :-
MARK! "... check the underside of the tyres".

I now have hot tea coming out my nose, ears and eyes. I believe this could be a cure for Covid-19 infections if the tea has some sanitiser in it. Holy cow that made me laugh.

Ian - how well do girder forks handle? The *look* like they'd be dreadful but never having tried them who am I to say.
30/07/2020 20:46:54 UTC
Mark Noel said :-

Yes I did drain the oil from the battery and the acid from the engine before inverting the bike. I am a genius after all.

Having looked back over what I wrote it is obvious that Ren has misunderstood everything I was trying to say. My article was about four coils, not fork oils, and he has used the wrong picture.

Silly man.
Posted Image
30/07/2020 22:10:18 UTC
Ren - The Ed¹ said :-
Oh lordy, it's turning into the 2 Ronnies sketch, "Fork Handles".

OK Mark you're a genius, you don't need to make me look (any more) stupid. I did wonder what the random stator picture was for though, at least I know now. Prey tell, what grade of engine acid should I be using in the 125? 10pH-40moles?
30/07/2020 22:49:31 UTC
Mark Noel said :-
The four coils are from a computer fan motor which I stripped to form the basis of a light gyroscope in an effort to make a stabilised platform for my kite camera.

The engine acid is a special elixir comprising 10 parts Hydrogen Hydroxide (concentrate) with 10 parts Dihydrogen Oxide (concentrate) whisked up with hot air of which I have plenty. Obviously you first dilute the two concentrates with water before adding the mix to the engine. Then run until seized.
31/07/2020 09:13:34 UTC
Ian Soady said :-
Girder forks are actually not too bad given the limited travel and agricultural damping via friction discs. In fact well designed ones like those Webbs are actually stiffer than teles.

However if the spindles are worn they can be terrifying, and even in good condition the handling is "different".
31/07/2020 10:04:05 UTC
Ren - The Ed¹ said :-
I suppose Ian a modern one with trick bearings and the damping with the spring (a-la rear shock) might work well. I'm sure there is something like that... Aha I'll add a pic.

Mark - I worked out the coils were from a fan. As for creating a gyroscope... man you need to get out less!

How much do you dilute the acid? I've been running neat and I'm struggling with dissolved engine cases, bearings and con-rods. Mind you the remaining pieces of the motor are incredibly clean although I dare not touch them. I dropped a little acid on the concrete flags while doing an acid change and it's burnt a hole through the flag, the substrate and a large portion of the earth's crust. I do however now have my own super-deep bore hole. I'm due a scientific research visit next week followed by a court case.
31/07/2020 17:34:31 UTC
Ren - The Ed¹ said :-
Aha! Piccy

Just a thought, these might suit Mark as the fork oil won't need changing?
Posted Image
31/07/2020 17:35:53 UTC
Mark Noel said :-
Girders have re-emerged on some modern retro-classics, such as the new Brough Superior and Ariel in an effort to improve handling and to prevent dive when braking.

To my eye they tend to look like a heap of Meccano at the front of a motorcycle, and a bit anomalous, as do leading links, trailing links and missing links. I am often accused of being one.
31/07/2020 18:55:02 UTC
Bob said :-
Ye Gods!
I've seen some strange ideas in my time but this is amazing.
Surely though it's easier to remove the forks and do it properly? You really need to pump the slider to get all the oil out of the damper assembly.
I agree though, most people never bother.
The KLX gets new oil every 6 months (6000 miles) and it makes a huge difference to the ride - but then with 11" of suspension travel I guess it would.
31/07/2020 22:52:22 UTC
Bob said :-
Counter intuitively I have found that as the fork oil degrades the ride tends to become more harsh, or "crashy" - you might have thought that older and thinner oil would make the suspension feel less damped. or more "bouncy"
01/08/2020 06:48:44 UTC
Ren - The Ed¹ said :-
I suspect Mark is enough of a genius to know it would have been easier to remove the forks as most of we simple folks would. But where's the fun it that? "Have 1 ton lift, what can I do with it?" Anyhow he's made me laugh and brought to all our attention the oft overlooked case of the forgotten fork oil. Or was it Four Coils...

The fork oil is single grade and from my very limited knowledge of oils the single grade aspect won't degrade to a lower viscosity. Perhaps contaminants cause it to thicken? I await the wisdom of those with more knowledge.
01/08/2020 09:56:47 UTC
Ian Soady said :-
When I've changed fork oil on new (to me) bikes it tends to be brown and sludgy - it may be that sludge (no idea where it comes from) making the ride harder.
01/08/2020 10:58:13 UTC
Upt'North said :-
Ian, I was informed by someone much more cleverer than I (not difficult) that the fork seals scrape back tiny bits of road crud and moisture, hence sludge. Makes sense, but I am gullible.
01/08/2020 13:32:37 UTC
Mark Noel said :-
Humour is the best medicine, and I'm not joking.
03/08/2020 09:35:27 UTC
nab301 said :-
The last time i was draining fork oil on a friends old F650 , I thought great, they have drain bolts , the forks felt weird , I was in such a hurry I undid the bolt without removing the top nut , the forks weren't in compression but I afterwards realised they were way over filled and before I knew it the drain bolt shot out of the shed and hit the side of the house followed by a lot of the oil....must have been at least 3 or 4 metres away! As above you really need to dismantle the forks to remove any potential sludge otherwise it just ends up contaminating the fork inners.
As for girder forks , I'm sure the reason they have tiny front brakes is because the forks would fail under under any serious braking forces ?
03/08/2020 16:26:55 UTC
Pocketpete said :-
I feel a ren bodge coming on here. Surely it would be easier to drill and tap a hole in the bottom of the forks.

Find a suitable movable tap to let the oil out.

Come on ren, you already have managed this on the 125s engine.

04/08/2020 04:13:34 UTC
Ren - The Ed¹ said :-
There is a better way to drain the forks Pocketpete. Leave them alone for years and years then when the fork seals fail still leave them alone. It might take a while but the oil will drain out over time. This also has the added bonus the if you're lucky some of the oil will dribble down onto the front disk. This greatly increases pad life, but can greatly reduce your own life.
04/08/2020 14:59:22 UTC

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