fork vacuum

Montesa’s, Ossa’s and Bultaco’s over the years of production used Betor suspension components on many models. Many years have passed since the production of these “vintage” motorcycles and many of them have been neglected in their maintenance. When they have been resurrected from their dormancy it is common to replace the fork seals and fork oil. Usually there is evidence of fork seal failure by an oil/dirt accumulation on the fork slider. The fork seals may not have been the entire cause of this mung accumulation. Many of these forks share the same construction.

As an example the Montesa Cota 247 throughout its production used Betor forks with the Montesa logo cast into the fork slider. On this fork you should look to the fork cap for some of the fork maintenance. When you look at the fork cap you should see an oval head common machine screw in the center and a small hole drilled off to the side. There is a check valve and spring located beneath the screw. The fork, thus configured, is designed to operate under a partial vacuum with this arrangement. As the fork compresses air is pushed out of the vent hole and on extension the check ball should seat and prevent air from returning to the fork chamber. There is considerable volume to the fork chamber so there is only a partial vacuum formed. There will be little visual evidence that the valve is operating properly. The benefits to this system is that there isn’t air pressure to add to spring compression allowing the springs to operated as they were designed and pressure is not going to add to leakage or help in blowing the fork seals.

Inspect the fork cap upon removal by removing the screw. Beneath the screw you will find a small spring and a 5/32” steel ball. Rust or debris may have stuck the ball against its seat. The ball and spring can be removed by tapping the cap against a wooden bench or by inserting a small wire into the hole on the bottom of the cap. Once removed the cap should be thoroughly cleaned. If the spring is rusted or broken it should be replaced. The ball should be cleaned and, if necessary, replaced with a new one.

Upon reassembly you should be able to blow against the ball on the bottom and have air pass. When air pressure is applied to the vent hole it should resist air passage. An air hose may be used for this test.

Upon reassembly you may hear air hiss through the vent hole with the forks are compressed but not on extension.

John R. Haberbosch