Clutch slave cylinder bore sizes? 1" vs 1-1/6"?

[Chuck78]

Has anyone else noticed that parts stores (or at least RockAuto.com) list both 1" & 1-1/16" hydraulic clutch slave cylinders for our old GMC's?
Luckily I was wise in the past decade and went on a few buying sprees of anticipated spare parts, & bought both size slaves, which I now need - to replace an 11-12 year old unit that is leaking like a seive currently. But the question is... Which one is the proper size, and if they both do fit our trucks, which trucks get which bore size??? Is one size to give more leverage for a much stiffer/bigger clutch&pressure plate?

[Chuck78]

this shouldn't have any relevance on these matters, as my clutch master in place now is the same bore as the original was, but...I modified my system to use a 67 Chevy 1-ton dual circuit brake master cylinder (bore size chosen in anticipation of front disc brake swap for 1991 Suburban front end, just need to remove residual valve for front circuit when swapping to disc), and then use a single circuit brake master cylinder off of a mechanical linkage clutch Chevy of our era, dismantled and residual valve at the end of the spring in the front removed. This was to get rid of the single master cylinder unit for both clutch and brakes, and add a dual circuit brake system for safety, as well as adding a power booster on a custom bracket & linkage that I made to space it in front of the clutch master.

I do definitely recall that a dozen years ago when I did these modifications, that I did purchase a brake master cylinder for clutch master cylinder conversion use, that was the same bore size as the original combo master cylinder's clutch bore...


EDIT - I can't find my darn factory service manual right now, otherwise I might have a chance of answering this question tonight!!!!!

[Chuck78]

Well I found my manual! It states that there are three different varieties of clutch master cylinder, the double barrel brake combo unit, a horizontal unit, and a vertical unit. It states that they are ALL 1-1/8" bore.
The factory manual also states that the slave cylinders are all 1" bore.

So I'm not really sure where this identical appearing (same casting) 1-1/16" bore slave cylinder comes from. Any ideas?

[James]

Originally Posted by Chuck78

Has anyone else noticed that parts stores (or at least RockAuto.com) list both 1" & 1-1/16" hydraulic clutch slave cylinders for our old GMC's?
Luckily I was wise in the past decade and went on a few buying sprees of anticipated spare parts, & bought both size slaves, which I now need - to replace an 11-12 year old unit that is leaking like a seive currently. But the question is... Which one is the proper size, and if they both do fit our trucks, which trucks get which bore size??? Is one size to give more leverage for a much stiffer/bigger clutch&pressure plate?

This is only for information only and it is my point of view.

The 1-1/16" bore will provide more force to the clutch but require more pedal travel to move the clutch the same distance as the 1" bore did. And it will take less foot pressure on the pedal. If the old unit is a 1" and you didn't go all the way to the floor to release the clutch then the 1-1/16" should work, providing all external and cup dimensions are the same.

Using the same size that came off of the truck it would drive and feel the same.

After working the math base on the master cylinder size of 1-1/8".
If it took 100 lbs of foot pressure to disengage the clutch with 1" slave cylinder.
Then it will take 88.6 lbs of foot pressure to disengage the clutch with 1-1/16" slave cylinder, this require pushing the pedal a little farther.

If my math is correct and the slave cylinder dimensions are the same I would use the 1-1/16" slave cylinder.