Wheel cylinder questions...

A few general questions about drum brake wheel cylinders:

1.) Do they all share a common "bolt pattern" to attach to the backing plate?

2.) Is the distance between the ears that mate with the shoe common?

3.) What happens if the bore diameter is larger (or smaller) than what was originally there for a given drum size? That is to say, what happens if you put a 1-1/8" bore wheel cylinder in place of a 13/16" wheel cylinder on a 10x2 drum brake? Is that even possible (reference questions #1 & #2 above...)?

4.) How do you measure the actual bore diameter? Is it the smallest diameter inside the cylinder, or is it the diameter of the lip that is at the very end that engages the rubber cap?

Thanks for the help/clarification!

:dunce_cap_on:

 

Just so you know somebody's out there listening, I'll take a shot at #4 first. Diameter is the actual cylindeer bore. #3 gets a little dicey. You only transfer so much fluid to push against a given surface area (the pistons in the cylinder); as the surface increases, the pressure against that area is spread out. Too large of an increase, and you have much less force against the shoes. The master cylinder will only move the same amount regardless of the size of the pistons in the cylinders, right?

#'s 1 and 2, I don't know. Why are you wanting to change?

I'm well known for being completely full of it, so don't take it too seriously.

 

If I read that right you have it backwards. Force = pressure X area, knowing that as area increases fro the same pressure then force must increase. So if you have a bigger piston in the wheel cyl, then it will have more braking force with less pedal force however it will require more volume so the pedal travel may be increased.

BUBBA

 

Well, the biggest reason I ask about swapping them around is local availability and pricing. For whatever reason, the stock 3/4" bore wheel cylinders on the D44 rear axle with 10x2 drums are running upwards of $35 each, and are 6-days out for local delivery at NAPA. The fronts however, for the D30 with 10x2 drums, are 15/16" bore and run only $14.60 each, and can be had overnight. :? So, I was thinking I would just put 15/16" all the way around, assuming it would not wreak havoc with the master cylinder and actual brake performance... Anymore input on that?

As for questions #1 and #2, I was able to answer them myself when I fianlly got things torn apart late last night. It appears they do all have a common two-bolt mounting pattern, and the collapsed and expanded dimensions between the ears are also common. Thus, at least dimensionally, they are swapable.