Slave Cylinder Internal Spring

[Argess]

I’ve been thinking about removing the slave cylinder internal spring for some time, so last year I did just that, and it‘s turned out to be a success….. So far.

I’ve never been happy with the clutch adjustment on my car. As you can see in the sketch below, with the clutch engaged (pedal up), the fairly strong internal spring in the slave cylinder is at maximum compression. In order to maintain an air-gap at the throw-out bearing (so it doesn‘t constantly spin), an external return spring is used.

The external spring must be strong enough to overcome the internal spring, but keep in mind the external spring will be stretched as the clutch is disengaged. This makes for a very strong spring. Strong enough it adds to the pedal force a bit.



The master and slave cylinders are BMW parts and the internal spring comes as part of the slave cylinder assembly. There is no standard external return spring for the combination of BMW hydraulics, Ford and aftermarket clutch parts used on my car.

One purchases a universal spring such as the one pictured below and attempts to bend it in precisely the right spot to obtain the correct preload. It’s a lot of fiddling, and usually a few springs are ruined in the attempt.



Before I go much further, I should detail the clutch adjustment procedure. Refer to the picture below:



It’s a touchy thing and I can never tell if my throw-out bearing has an air gap or not due to the spring pressures.

I decided to remove the internal spring and see what happens. So I did that today. Because the slave cylinder is an expensive part, and seal kits are rare, I bought a rear wheel brake cylinder for an 1987 Honda Accord which has the same cylinder ID of 13/16” The thickness and ID of the Honda seals are not the same, but the seal OD is at 13/16”. Best of all, I got it brand new at the closeout price of less than $5 USD! (I like to go cheap when experimenting).

Here’s a pic comparing the brake cylinder parts to the slave cylinder parts.



Taking the cylinder apart, I get two pistons with seals. I‘ve modified one as shown below and installed it without spring in my slave cylinder.



Adjustment was easy and a test drive was successful. Oddly, if I slowly let out the clutch at idle, I can hear when the throw-out bearing touches the pressure-plate fingers. It’s not really noticeable and I had to strain to hear it, but I could and that was satisfying. Makes me suspicious the old throw-out bearing was always spinning before. Probably why it failed.

Overall, the pedal feel is better, although I can’t really explain why. Crisper maybe.

As a final note, some throw-out bearings are designed for constant rotation, although apparently this is increasingly common the more modern a vehicle is. I’m quite certain my old 1960 Ford stuff (or aftermarket equivalent) should have clearance and not spin when cruising along.




Info: 1986-1988 Honda Accord Rear Wheel Cylinder, $4.77 USD Closeout Price @ Rock Auto, Bore 13/16” Thought to be 7/8”

[DanEC]

Thanks for posting. I’ve pondered this also and I’m actually running two fork return springs - concentrically - and I’m still not sure my TO bearing is disengaged. So why did you use the Honda piston and Seal in the BMW master and not just pull the spring out of the BMW master and reinstall it with the BMW piston/seal?

[Argess]

Oh... my SC was leaking slightly and I wanted a new seal. The Honda thing was my cheapest alternative, but only the OD dimension was correct, so I modified one of the Honda Piston and used it.

There may have been a problem with the BMW piston spring end... using it without a spring, but I can't remember now. Probably not an issue.

I did take a video of the clutch and you can clearly see the throw-out bearing backing away from the PP fingers as it's released. I was never confident before that it did that.

[DanEC]

Got it - thanks. I just changed out my Master so not wanting to dive into it again immediately but I think I will put this on my to-do list for next winter.

[patrickt]

Well that is a nice video. But you know, whether or not your throw-out bearing can be in constant contact depends on whether or not it is of the “self-aligning” type. If it is, then a very light preload is absolutely acceptable. The Federal Mogul #614038 that is in my car is clearly labeled as sealed and self-aligning, so a little bit of preload is just fine. But I vaguely recall from a previous thread that DanEC's TOB is not self-aligning and I think it even has a funky zerk fitting on it. But that was a very nice analysis by you and I love a proof video.