Need fork oil recommendation

I just rebuilt my fork on the 75 and put in progressive springs. I filled it with 160 cc of 20 weight fork oil. It seems to be bottoming out/topping out. It doesn't ride bad in general, but it clunks over bumps once in a while (especially the driveway curb). Would you recommend a 30 weight oil? If so, does anyone have a prefered brand or supplier (I can't get it locally).
Other suggestions to improve feel?

Ben,
Did you check/replace the damper top hats ? if worn too much oil can escape between the rod and top hat bore giving similar problems.

Cash

Cash beat me to it. 20W (I used Bel Ray) worked fine in mine until recently. I believe it was damper cap wear that was the problem. Just replaced, but it's been raining like blazes (worst flooding in anyone's lifetime in the Boston area) so I'm still waiting to test ride.

Lansdowne Engineering's response is eagerly awaited....but in the meantime

lansdowne-fork-damper-kit-t4683.html?hilit=lansdowne

Cheers Andy for the link.....John Lansdowne Eng

Did you preload/prime the shocks. Hold the front break and pump them up. 20w should be just fine almost anywhere but if some internals are worn, 30w may be in order. You may want to drain and double check your measurements. Having a slightly longer spring may account for the top out and the 30w will help with that too. Suppliers are stocking up this time a year. I would think you could find it. just google 30w fork oil.

What you are describing sounds "normal" to me. Mine did that until I installed the Fauth fork mod and ATF (as recommended). Solved all the issues with the standard forks and almost makes one go looking for bumps! I know there are "better" (higher tech) mods available for the forks but the Fauth corrects the issues and everything looks totally stock. IMHO this mod (or a better one) is at the top of the Commando "upgrade" list, along with the Brembo brake mastercylinder (if you have a disc-brake model, of course).

Filling you forks will heavy grade oil [30-40 grade ] will slow down the rebound,But higher viscosity will induce greater airation [foaming] Also heavy grades tend to under respond to sharp shock loads...the thicker oil simply cannot escape quick enough...there is no modern bike that use's heavy grade oil norm todate is 10-15. Unless you have the LANSDOWNE KIT!

Thanks for the responses. I'm not sure what a Damper top hat is, but if it is the bronze part that looks like a top hat and goes right below the seals, then yes. I thought about the Lansdown kit when I did the rebuild, but alas, lack of funds. I will keep the 20 wt for now. I'm thinking it might actually be a loose steering head. It has never been off (ever), but if I do as the book says "grasp the front tip of the fender and pull forward and feel for play in the junction of the upper triple clamp and frame" I can feel a very slight little movement. Should there be any at all? This is a MK III.

No, the damper cap is the very topmost, screw-on part of the damper body, which the damper rod runs through into the damper body. New bronze ones from Clubman are about $25 a pair.

the damper cap is the very topmost, screw-on part of the damper body, which the damper rod runs through into the damper body. New bronze ones from Clubman are about $25 a pair.

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Thanks Brian I should have given a better description. RGM make a bronze damper cap, it might be easier to get over here in the UK and should last longer than the alloy cap. I reckon as the alloy cap wears the rider adapts to the change until damping is all but lost.

The Lansdowne kit looks like the thing to fit but is relatively expensive and for me a little too new. I wonder what effect high milage road use will have on the slider spindle clamp due to the rocking couple? probably none, fitting a fork brace would perhaps be a good idea.

Cash

The Fauth mod is a good, less expensive alternative. I just did that along with the Clubman aluminum damper bodies/bronze damper caps and am liking the results.

May I add, Greg's (Fauth) post-sale support is superb. God knows, I must have tried his patience with a couple dozen questions but he was always there for me with a prompt and helpful response. Thanks again Greg!

B+Bogus said:

Lansdowne Engineering's response is eagerly awaited....but in the meantime

lansdowne-fork-damper-kit-t4683.html?hilit=lansdowne

Click to expand...

OK. Its Easter ....and raining! So i might have a re-cap on the Lansdowne Kit , I still have a few guy's looking to improve their forks,and not sure which way to turn.

Risking covering old ground; Stock Norton dampers are little changed from 1953, and due to lack of development and funds these fifties damper's found there way into the Seventie's Commandos.

GOOD ENOUGH FOR A 500 COOKING SINGLE but 850 disc braked 125 mph Commando? Covenant mods give very little improvement...so working with Duncan Fitchett "We" found Rebound and Compression cartridge's a huge inprovement....After winning at Donnington his responce to my question was " Great, i dont have to think about the front end any more,just get on racing"

Duel rebound and compression was now controlling the forks using 10 grade fork fluid , Being fully adjustable ment they could be tuned quickly to suit the track, Belgium proved to be very bumpy but a quick "tweek" improved the action...allowing a great win .

The early Commando design based on Manx internals proved themselves to LAB ,Reggie and Mike [Forum members] Giving a instant improvement, My latest design have a few revisions. Now built in final rebound cushin [15mm] also a retension needle spring ..

The cartridge's have brass long life bearings and brass pistons.. Control via brass valves and a reliable taper needle Designed to last a life time.

Top nuts are aircraft spec alloy,bodie's cold draw hydraulic tube.. Designed to last and track tested ..All the best John Lansdowne Eng