Most moving parts are assembled within certain measurements. These limits or tolerances are usually given as + - after the optimum clearance required. They are common in engine rebuilding. Pistons, piston rings and bearings all have published measurements that include the allowable variations (+-).
Limits that may be harder to find are those in the brake system for the Model A. Each clevis and pin must fit within given limits for the brake system to work the way it was designed to. Also important are the other moving parts in the brake system.
Excess clearances allow play in the system that will require more pedal travel to properly actuate the brake shoes.
There are also wear limits that allow the system to operate for a period of time or miles. If there is too much clearance between parts at assembly (that would be the + side of the limit) then the system may require attention before its normal interval. Wear at the backing plate can cause uneven application of the brake shoes against the drum. Wear at the actuator cam can prevent the shoes from being moved into proper contact with the drum, resulting in poor braking. Each fit and clearance along with wear has a cumulative effect. They may only be a few thousands of an inch each. Combined they can cause poor braking without any sign of excessive wear.
Experienced mechanics can often “feel” the right fit on some parts. When exact fits are required measurements must be taken with a micrometer or vernier. Using and reading a micrometer or vernier requires some practice. If you are considering using either for working with close tolerances, then it would be a good idea to get a little practice and become familiar with the instrument before you take important measurements. This could avoid an expensive “REDO” if an error is made.
If the fits and clearances are too tight the system could seize or jam. Again everything accumulates.
A little drag (friction) at each moving part could result in very stiff pedal movement with no sign of any one problem.
