Replacing a Clutch Carbon Release Bearing with "Roller" Release Bearing onto a Sprite or Midget 1275cc motor
This is an account of my experience in replacing a 1275cc clutch carbon release bearing with a roller bearing onto a standard 1275cc disc diaphram coverplate and onto a Cortina diaphram coverplate.
The term "roller" is not correct. The modern clutch release bearing is an "angular contact ball bearing" designed to take large lateral loads. However the term "roller" will be used in this document.
The original carbon release bearings were made of a more durable compound than today and the roller release bearing offers a longer lifespan than the present carbon ones.
Problems:
Cortina diaphram clutch.
I bought a ready made release bearing made up from a 1098 carbon release bearing, with the carbon removed and a machined spacer holding the new roller bearing suitable for a Cortina diaphram coverplate. The release had an "angular contact" curved face ball bearing.
A. The clutch pedal "pulsed" when the pedal was partly pushed and then would be "steady" when pushed further.
B. The clutch pedal, at times, would go soft to the floor and then after a couple pumps would return back hard.
After +/- 5000 miles the engine and gearbox was removed to inspect the clutch. The coverplate fingers were badly worn and the bearing running rough (spriteclutch4).
Conclusion.
A. The release bearing unit mounted in the 1098 holder was far heavier than the original carbon unit and "tipped" forward resting on the diaphram fingers which when rotating "bounced" the release bearing. The bearing's weight was also wearing the fingers down.
B. The spring fitted inside the slave cylinder (spriteslave3) is designed to keep the carbon release bearing lightly touching the coverplate diaphram disc and take up any slack in the linkages. The weight of the new release bearing,"bouncing" on the rotating coverplate, was causing the clutch fork to be push towards the gearbox to such an extent that all the clutch pedal travel was used to get the bearing back against the coverplate diaphram.
Standard 1275 disc diaphram clutch.
I bought a ready made release bearing made up from a 1275 carbon release bearing, with the carbon removed and a machined spacer holding the new roller bearing suitable for a standard 1275cc disc diaphram coverplate. The release had a light "shaft" ball bearing fitted onto a flat faced spacer.
C. There was an intermmitant "squeak" comming from the clutch while driving and at stop with the engine running. The "squeak" appeared after a while after releasing the clutch and never with the pedal depressed.
D. The take up when releasing the clutch pedal would vary slightly.
After +/- 2000 miles the engine and gearbox was removed to inspect the clutch. The coverplate disc had a circular grove cut into it and the bearing had lost all its grease and was very rough.
Conclusion.
C and D. The spring fitted inside the slave cylinder (spriteslave3) is designed to keep the carbon release bearing lightly touching the coverplate diaphram disc. The new release bearing, not as heavy as the Cortina, was sufficiently heavy to lean forward lightly onto the coverplate disc and with the rotation cut a small grove in it.
Both release bearing assemblies concepts were fine but needed to be redesigned.
Cures:
Cortina diaphram clutch.
The bearing assembly weight must not be allowed to cause the assembly to lean forward onto the clutch cover plate.
This was overcome by two quality plastic cable ties on the clutch fork (spriteclutch2). The ties are initially set to tip the bearing assembly backward away from the coverplate. When the gearbox is mated with the engine the clutch fork must be pushed to force the release bearing against the coverplate and move the cable ties to a position where the bearing will rest against them. The movement of the clutch when driving will always move the release bearing forward away from the cable ties position so the release bearing will always come to rest against the cable ties.
The bearing was replaced with a MG Midget 1500cc release bearing (spriteclutch1).
Standard 1275 disc diaphram clutch.
The shaft bearing was replaced with a flat faced Toyota Corolla release bearing and an assembly made using a 1098 holder (spritclutch3) and plastic cable ties fitted on the clutch fork.
Centering the release bearing assembly onto the coverplate:
The position of the clutch fork bearing holder pivot in relation to the centre of the gearbox input shaft is critical to maintain the "ark movement" of the fork which keeps the release bearing centred on the coverplate. The bearing assembly machined spacer that fits into the original carbon holder and supports the new bearing itself has to be the correct length to achieve this and will vary with the type of roller bearing fitted.
Holding the release bearing assembly in place with clutch pedal in the "up" position.
The bearing must not constantly rotate and must be held slightly clear of the coverplate. An external spring to pull the clutch fork back towards the slave cylinder was installed together with an adjustable threaded rod "stop" plate to adjust the bearing / coverplate clearance (spriteslave1 and spriteclutch3).
To conteract the "sideway" weight of the realease bearing taking the bearing "off centre" to the clutch coverplate a spring onto the "stop" adjustable threaded rod was installed (spriteslave2).
The modified instalations have been in the vehicles for three years without incident.
In (spriteslave3) the spring together with the "metal head" is out of a slave cylinder bought on ebay. These leak air into the hydraulics because they don't hold the rubber cup secure in the cylinder wall. The original plastic cap is what should be on the end of the spring.
Refer to the below photos. (Spriteclutch 1 to 4 and Spriteslave 1 to 3)
