Front Fork Oil

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FreeRadical

FreeRadical

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Hello everyone,

Can someone please tell me the proper oil to use in the front forks. Springs are stock. The "classic motorcycle guy" at Autozone sold me some Roayl Purple 10W-30 fully synthetic. The Norton manual is not very specific, but from what I can gather a 10W-30 or 10W-40 would be fine.

Thanks!

Kevin
 
I use good old ATF. Do they have a 'classic motorcycle guy' at every autozone? :mrgreen:
 
If on smooth roads ATF may be nice but I found it too thin in ordinary un modified standard 750 Roadholders so was better with the 20 grade fluid oil or 20-50 to 15-40 grade engine oil, which are as thin as the lower number when colder than inside running engine. Bottoming resistance can be improved some by filling with 175 mll but risky to go much over as air pocket pumps down and forks lock then become angry bucking broncho out of control.
 
FreeRadical said:
Hello everyone,
The Norton manual is not very specific, but from what I can gather a 10W-30 or 10W-40 would be fine.

Thanks!

Kevin
Click to expand...

I started with 20W synthetic fork oil, but found that 15W suits my flimsy wrists better. Most of my riding is one-up, lightly loaded. So far, no ill effects. And yes, as Steve points out, increasing the volume of oil to 175cc (as compared to the factory-prescribed 150cc) helps with the jounce spring rate rise. This change may be what allows me to get away with what some feel is too thin of an oil.

Nathan
 
I have 20w PJ1 fork oil in mine - just right. Have used 30w, but a little too stiff for me. 15w is too thin, not enough damping.
 
+1 on ATF. I use that in ALL my bikes, Duc, Honda, HD, Triumph, etc. etc. etc. Buy it by the gallon and change it often.
 
Just rebuilt a Seeley fork, it was filled with heavy gear oil! The dampers took some effort to work. stiff as hell!!! been raced like that?
 
Norton specified "150 cc of SAE 20" oil for the Commando forks. The PR specs called for 180 cc of "Castrol CRI30 (SAE 30)" oil for the same forks. Another PR reference called for SAE 20 oil in the forks, with SAE 30 for particularly bumpy circuits. I tried both on my PR, and generally used the 20 weight. Other service manuals call for SAE 10-30 or SAE 20W. Note that these are all engine oil grade specs, which do not necessarily apply well to fork oil. You may find that if 20W engine oil works well, you could need anything from 15 to 30 grade fork oil to get the same results, depending on the brand of fork oil used. That's not a bad thing. It just means that if you are experimenting, you should probably stick to one brand of fork oil to play with.

ATF is generally considered equivalent to 10 weight fork oil, and too thin for stock Commando forks. That doesn't mean you can't use it, but it won't work as well for really spirited riding, or for racing. A fairly common mod for racers back in the day was to braze the stock metering orifices closed and drill new, smaller holes, sometimes also relocated, and use thinner oils, like ATF. Sometimes they also modified the fork internals to take an o-ring seal in the damping tube cap and use another drilled orifice to get a more consistent rebound damping. Probably not worth the effort of the typical Commando street bike. The Commando forks actually work pretty well stock, at least compared to other period forks.

Another reminder. Fork oil does wear out, and should be changed regularly. I think Norton recommended it every 5,000 miles. Judging from forks I've taken apart, not many owners followed that advice.

Ken
 
When you say they work better than other period forks , which type are you comparing to? AJS. Matchless,Velocette . these are far better made. Early Norton long roadholders had no dampers,just a taper peg in the bottom. This give the last 2 inch of hydraulic stop.
 
john robert bould said:
When you say they work better than other period forks , which type are you comparing to? AJS. Matchless,Velocette . these are far better made. Early Norton long roadholders had no dampers,just a taper peg in the bottom. This give the last 2 inch of hydraulic stop.
Click to expand...

John, Ken said they work better, not that they were better made. An important distinction!
 
Castrol 15 W. Fork oil Synthetic. Fit rubber accordion long fork tube covers but some don't much like the looks so they should change fluid more frequently (road dirt). :wink:
 
Dear dudes its well known by serious intense vintage racers that over the decades have tested viscosity and charted many fork fluid brand to find them all over the place so the weight/grades listed on fork oils nil relation to anything but that particular factory batch or internal record keeping or sale pitches. Motor oil is likely more predicable viscosity and over kill on anti foam and rust and etc etc for mere fork dampening. So if one fella says 20 wt works in his as could actually be 10 to 30 will apply to your brand of fulid and state of forks and condition of operation. The racers charting this used a flow meter not by guess and by golly like me.
 
FreeRadical said:
Hello everyone,

Can someone please tell me the proper oil to use in the front forks. Springs are stock. The "classic motorcycle guy" at Autozone sold me some Roayl Purple 10W-30 fully synthetic. The Norton manual is not very specific, but from what I can gather a 10W-30 or 10W-40 would be fine.

Thanks!

Kevin
Click to expand...
A good quality fork oil will do eg Bel ray...Use it...Try up or down W weight too suit your driving style or issues...change it often.
Yearly.
T
CNN
 
john robert bould said:
Just rebuilt a Seeley fork, it was filled with heavy gear oil! The dampers took some effort to work. stiff as hell!!! been raced like that?
Click to expand...


John - for your damper kit you recommend to "Fill with 150 ml of S.A.E 10 grade fork fluid in each leg." As others have pointed out, the viscosity can vary significntly for 10w oils from different manufactures.

Do you have a recommended target cSt @ 40C?
 
Keep in mind when the factory manuals were written and what was available at the time.

Try a good 10-wt. fork or suspension oil and if you need an increase in damping, change to 15 wt.
 
I mention again that vintage racers most particullary Kenny Cummings, over a decade ago objectively measured dozens of brands for each grade offered to find it Had No Meaning What So Ever with dozens of grade points variation on either side of what label said so best is just eyeball the pour ease and trial error from there. Btw its perfectly acceptable to put one grade in one leg and diferent grade in oppostie leg and to mix match thick and thin oils together for custom blend. The risky thing is creeping up on volume just short of possible air pocket pump down to hydro lock. I lost the pages long fluid test list d/t 'puter failures from cosmic rays and power surges so maybe some digital seracher could find and post to put some caution in firm statements of what grade & Fork Fluid you were are pleased with as may not work the same with another brand of 'same' grade' or slang term "weigth" which is mis use of what the W on labeles means. Auto parts and bike shops will not notice this slang term mis used but a pow wow with fork fluid tech staffer will.
 
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