GT750 Fork Top Seals
by Richard Axford

When you have changed the fork oil or topped it up, the re-assembly process includes pressing the short metal top plug down onto the main fork spring and holding it there, to enable fitting the wire circlip into its groove in the fork bore. The circlip stops the plug coming up again and must be seated evenly and fully into its groove. It is then best to check the air seal as described below, BEFORE reassembling the handlebars and top bolts etc. to the forks.

The problem is that the O-ring seal in the groove in the top plug provides the air pressure seal to retain the specified pressure of 7 - 10 p.s.i. over the oil. This seal and the part on the fork bore on which it rides must be PERFECTLY CLEAN AND UNDAMAGED or air pressure will leak fairly quickly and your front handling will be poor.

Inspect the O-ring for damage; any nicks or cracks mean replacement at about £3-50 per pair. The trick is to wipe the parts (plug, O-ring and top bore) perfectly clean with tissue before inserting the plug, smearing the O-ring slightly with clean fork oil, then pressing the plug+seal slowly and carefully in and fitting the circlip.

The main Tech Tip not mentioned in the Workshop Manuals is to pour some fork oil over the plug until the complete plug is just covered. Now pump up the air pressure to about 15 p.s.i. (I use a small hand pump for a pedal cycle) and see if any bubbles come out past the plug; if they do it's leaking!! It took me a couple of goes on each fork to get a perfect seal. If no bubbles are seen then reduce the pressure to 10 p.s.i., remove the excess fork oil over the plug and repeat for the other fork. Re-assemble handlebars etc. and go for a test ride!