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Celebrating
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Jamming ClutchWhen I first owned my Box saloon and started driving it in anger, I suffered from a fair bit of clutch slip and squeal every time I changed gear. So, during one of the many recent engine transplants, the clutch underwent a fairly major refurbishment. New springs, new levers, new friction surfaces - all of which was planned to give a fresh lease of life. However, I ran across a snag. The clutch was tested by offering up a gearbox to the back of the engine before it was refitted to the body. Operating the levers by the pedal gave a good clean release, and releasing the pedal locked the drive up solid. Just the job. However, when the engine was back in the car, I couldn't get the clutch to fully release. The pedal was properly adjusted up so as not to drag the thrust bearing, but there wasn't enough movement to stop the clutch dragging when I tried to select a gear. On disassembly, the reason became clear. I'd tested the clutch using an old gash gearbox which hadn't got the thrust bearing return spring in place. This allowed the pedal to travel a lot further than when in the car. What was happening was that with the rebuilt gearbox, the thrust race became "coil bound", that is, it reached the end of the travel, (about 3/4") before the levers were pushed in. Much scratching of head! Finally, the cause was found. The levers pivot such that the fulcrum is the bottom of the slots in the plate itself. On examination, it was found that the plate had worn in sympathy with the old levers. There were trenches at the bottom of the milled guide slots about 2mm deep. The mechanical advantage offered by a good set of levers is about 9:1, meaning that for every 9mm of pedal travel, there's 1mm of clutch plate release. As there was a 2mm wear, this meant that the first 18mm of travel didn't actually do anything. With only 3/4" to play with, that added up to just 1mm of active pedal travel, or a clutch plate clearance of just about 4 thou!! The plate was modified as per the Big Red Book, with some hard steel set-screws down in the slot. Now the levers act at the right level, and I get the full 2mm of clutch plate travel. The major pain of course is that all this necessitated the removal of the engine yet again. When I first owned my Box saloon and started driving it in anger, I suffered from a fair bit of clutch slip and squeal every time I changed gear. So, during one of the many recent engine transplants, the clutch underwent a fairly major refurbishment. New springs, new levers, new friction surfaces - all of which was planned to give a fresh lease of life. However, I ran across a snag. The clutch was tested by offering up a gearbox to the back of the engine before it was refitted to the body. Operating the levers by the pedal gave a good clean release, and releasing the pedal locked the drive up solid. Just the job. However, when the engine was back in the car, I couldn't get the clutch to fully release. The pedal was properly adjusted up so as not to drag the thrust bearing, but there wasn't enough movement to stop the clutch dragging when I tried to select a gear. On disassembly, the reason became clear. I'd tested the clutch using an old gash gearbox which hadn't got the thrust bearing return spring in place. This allowed the pedal to travel a lot further than when in the car. What was happening was that with the rebuilt gearbox, the thrust race became "coil bound", that is, it reached the end of the travel, (about 3/4") before the levers were pushed in. Much scratching of head! Finally, the cause was found. The levers pivot such that the fulcrum is the bottom of the slots in the plate itself. On examination, it was found that the plate had worn in sympathy with the old levers. There were trenches at the bottom of the milled guide slots about 2mm deep. The mechanical advantage offered by a good set of levers is about 9:1, meaning that for every 9mm of pedal travel, there's 1mm of clutch plate release. As there was a 2mm wear, this meant that the first 18mm of travel didn't actually do anything. With only 3/4" to play with, that added up to just 1mm of active pedal travel, or a clutch plate clearance of just about 4 thou! The plate was modified as per the Big Red Book, with some hard steel set-screws down in the slot. Now the levers act at the right level, and I get the full 2mm of clutch plate travel. The major pain of course is that all this necessitated the removal of the engine yet again.
This article, written by Geoff Hardman, originally appeared in CA7C Seven Focus in June 2010 pp18-19.
See also: Clutch Overhaul and Austin Clutch
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