If you have a vintage bike and the forks are bottoming out, adding a few pounds of air to each fork legs helps wonders. In fact, many bikes of the mid-70s ran forks with no springs at all. They just installed air caps, raised the oil level, pumped in 18 - 20 pounds of air (or nitrogen) and went racing.

A lot of air caps were sold at that time, and then bikes started coming with air caps standard and this part of the business fell off.

But there's no reason for you to hunt around for hard-to-find air caps for your vintage bike, when you can simply make them yourself with left-over stuff you've got tucked away somewhere in your garage.

HOW MUCH OIL? HOW MUCH AIR?

For most of the forks we've done, we find that if you pour in enough oil to reach three inches below the top of the fork tube when the slider is at complete bottom, you usually have enough oil to get the proper damping.

If you use this for a starting point, make sure that the damper rod IS NOT exposed when the fork is at full extension. If the damper is sticking out of the oil at this point, pour in more oil until it's completely covered. If the damper is exposed, the forks will top out.

We also found that the amount of oil has direct effect on the amount of air. You'll have to experiment with the amount of air/oil you'll need for tuning your forks. It's best to change the air and the oil after two or three races, since the air tends to break down the oil much quicker. Another tip for riders who are using air forks consistently, is to drain the air when the bike is not in use. This will relieve the pressure from the seals, enabling them to last much longer.

Ideally, nitrogen will be better to use than air, but most folks don't have access to a nitrogen bottle. So if you're using a regular old air compressor, make sure there's not a lot of water in the tank, and if you can, run it through a filter before it goes in your forks.

(1)   Take your fork caps off and remove the springs. If you run springless, you won't have to use them again with the air conversion. Make sure the sealing O-ring is in good condition, since it has to work about twice as hard as before, now that the fork is pressurized.
(2)   Grab an old inner tube from the dark confines of your garage and cut out the base of the stem until it's close to the inside diameter of the stanchion cap. Make it slightly larger and the sealing will be better.
(3)   Some stems have a metal flange that might be larger than the inside of the stanchion cap. If this is the case, grind the flange down until it will fit into the cap. We used a stem from a regular Japanese 400 x 18 tube.
(4)   The fit in this area is critical. Make the stem fit as close as possible.
(5)   Locate the center of the fork cap and drill a pilot hole all the way through.
(6)   Most valve stems are eight millimeter, so a 5/16 drill should do fine. The tighter the fit, the better it will seal. Make sure the area on the inside of the cap is deburred and same for the top section. Any foreign matter will make the seal poor.
(7)   Look through all of the old O-rings you have and find a pair that will fit over the stem. This will increase the sealing qualities of the unit.
(8)   Place the stem into the cap using a blunt instrument; push it in until it bottoms out in the cap. Make sure the hole through the stem is free from obstruction. The valve core shouldn't be in the stem when installing. This will keep it from being damaged.
(9)   Place a small amount of silicone seal around the base of the stem. This will eliminate the chances of it leaking.
(10)   Locate a washer, 8mm, and slip it over the stem. This will ensure that the nuts give even pressure to the surface. Also, this will give the silicone a better surface to seal against.
(11)   Install the jam nut and tighten in place. If the stem starts to rotate in the cap, place the blunt instrument into the bottom of the cap and apply slight pressure.
(12)   We double-nutted the top of the stem to make sure the thing didn't come apart.
(13)   Wipe away all of the excess silicone that oozes out when the nuts are tightened.
(14)   There you have it; an air cap at no cost. Before installing the cap in the tube, we recommend that you install a small amount of silicone to the threaded area to insure a good seal. Spread it over this area in a thin, even layer.
(15)   After doing the proper steps in filling the oil, place the fork cap into the stanchion and tighten. You have a choice on how you cover the core.
(16)   Either use the stock valve cap, or blow everyone's mind by using a short piece of fuel line connecting the two stems. It won't do anything but keep the dirt out, but it will keep everyone guessing.