Roadholder fork modifications:
The first thing to do is to dismantle the forks completely, I will not go ianto a detailed step by step "how to", this has been posted before and most manuals have a good description of how to do it.
The first thing to do is to plug up the holes at the very bottom of the damper tubes, on the older shorter damper tubes, it is easy to do with an aluminum sleeve of the correct size. On my damper tubes, I turned down the end a bit to make it really round and made little sleeves that I pressed on. On the Commando damper tubes, the hole is midway up the taper, plugging those is best done by tapping the holes for a 1/8" pipe plug and gluing it in place before filing down the excess outside the tube (be carefull about not making the hole too big and getting material inside the tube, I haven't done this myself, but I guess it can be done fairly easily)
Next the holes must be relocated to a point just above the taper. By doing so, a proper hydraulic slowdown is acheived.
Next the stock damper rods will need to be replaced with rods that are 2" longer than the original ones. Greg (a.k.a. NORBSA) makes a kit of parts that includes longer alloy rods, he can be reached at firstname.lastname@example.org
Now on to the springs, the regular Norton springs are 18.5" in lenght and get coil bound (will not compress anymore) at 13.25" for a total possible travel of 5.25". The commando forks and even the shorter Atlas forks all have a possible travel of 6.25". This is considering going all the way to the bottom and extending the sliding and fixed bushing contact each other. An unmodified fork has about 4.5" of travel before things click and clank.
To gain more travel, a "helper" spring is added to the now longer rod, this allows more than 6" of travel before the main and helper springs get coil bound, Greg has parts similar to these in his kit. The combination of the stock spring and a helper spring works well on Commandos according to Greg, however on the shorter Atlas forks, these springs get coil bound before full possible travel is acheived, but there is a way out by fitting progressive springs (Greg has some), these progressive springs have a free lenght of 19" and get coil bound at 11" which provides more than enough range for Atlas forks and they would probably enhance the handling of Commandos as well.
The stock Norton damper tube cap is made out of aluminum, it is reccomended to use dissimilar metals when they rub on each other, this is why I made bronze damper tube caps although one of my friends told me not to worry since the whole assembly is running in oil and there is not that much movement and no side loads. At any rate, these bronze caps are available from Clubman Racing (I made mine)
Here are the fork innards all appart:
Now to keep the oil inside the forks, a stock Norton oil seal can be used, but I have heard very good reports of a brand called "Leak Proof Fork Seals" http://www.motohaus.com/html/seals/leak ... 0intro.htm
To use these, the top hat bronze bushing in the forks must be modified by filing grooves to allow oil to get under the seal
The fork leg must also be very clean and free of gunk and debri since the seal can move up and down. There are lips on both the inside and outside of the seal so good surfaces must be present on both the fork leg and the fork tube. Obviously the fork tube moves much more so it must be in very good condition, free of bumbs and pits.
By doing these modifications, the forks should have 6" of travel and a soft compliant ride. The forks will extend more than the stock ones but will settle down 1 to 1.5" when sitting on the bike, this gives a possible 4.5" of compression damping and 1.5" of rebound damping, and there is proper hydraulic control of both compression and rebound.