On 17 December 2006 at 14:01:26 UK time, ASH Andrew wrote:
"I heard that you can put heavy grade oil in the shocks for racing purposes. .... Any thoughts on this?"

The Armstrong hydraulic shock absorbers werre designed to use for 20 weight hydraulic oil. You can use 30 weight, which will make them quite a bit stiffer, but it is not easy to fine tune them otherwise. Commercially advertised "Shock Absorber Oil" is likely to be expensive. Motorcycle fork oil is another expensive alternative with a lot of different viscosities available (if you insist on using non-standard viscosity to defeat the factory intended function). Otherwise hydraulic jack oil works well, cheap and available anywhere. The proper way to change shock damping action is to change the regulating valve. If you feel the need for competition, you can make them infinitely adjustabe.

To empty the oil from the shock, turn it upside down and operate the arm through full stroke until all fluid is expelled. To fill it turn it right side up, add oil, and operate the arm through full stroke until all air is expelled. Leave about 1/8 inch air space at top to allow for thermal expansion without blowing oil past the shaft seals.

On Posted 20 December 2006 at 17:25:26 UK time, Peter Caldwell of World Wide Auto Parts in Madison, Wisconsin, USA, posted this sage advice:
>"Using a very heavy oil will increase the stiffness, true. It can also cause the splines on the internal piston rocker shaft to strip; it can cause the flow-back poppet valves in each piston to blow apart. Ask the old timers how many shocks all of a sudden just "went dead". We've seen it many times. The weight of the original oil is very close to a 20W motor oil in flow charcteristics. You CAN use 20 or 30W motor oil. It is COMPLETELY compatible with all of the seals. A much better choice is hydraulic fork oil or hydraulic pump oils, as you'd use in hydrostatic transmissions or hydrailc rams on heavy equipment. These have the necessary anti-foaming agents and are suited for the use. Do not exceed 30W.
>
>The better solution to increasing damping rate is to modify or replace the valves. You can find "heavy duty" valves at the usual suspects. They are really standard valves for heavier cars."