These billet air fork caps are reproductions of the popular Goki caps that were used in the 1970's. They are designed as an air assist tool to aid in the compression rate of 35mm forks with springs. Add 3-4 pounds of air to each fork leg and it will remove the excess sag in the forks as well as stiffen the compression. So many forks on vintage bikes are sacked out from 30 years of sitting on the same springs or springs that were too light weight to begin with. Or maybe us "full figured" riders weigh just a tad more than the stock springs were made to support.. Either way these are adjustable to the riders preference, weight, ability etc. Screw right on all Husky 35mm black leg forks even the leading axle models 74-79. No modifications needed. When you only have 7 inches of travel, you can't afford to lose 2-3 inches in sag.