Combination squares

[BadDog]

I think if it fits within reason for the slip-fit, it should work. The thumb wheel just pulls a sort of hook that in turn uses the rule slot to pull the rule up against the register surface in the head. That should make a rule square within the limits of the head and rule register surfaces (and relationships) regardless of a few thou difference in width, or even thickness (within practical limits). As long as it is tight enough that the can't twist in the head, it should be good to go.

[Mr Ron]

I think if it fits within reason for the slip-fit, it should work. The thumb wheel just pulls a sort of hook that in turn uses the rule slot to pull the rule up against the register surface in the head. That should make a rule square within the limits of the head and rule register surfaces (and relationships) regardless of a few thou difference in width, or even thickness (within practical limits). As long as it is tight enough that the can't twist in the head, it should be good to go.

I'm inclined to believe that is true. For woodworking purposes, I would go that route, but for metalworking, I wouldn't feel secure with using mis-matched parts.

[BadDog]

It's as accurate as it is accurate.

Mismatched parts shouldn't in any way affect how square it may be. Which is not to say that a combination square is square enough for any given task. But a suitably parallel B&S (or Chinese etc; parallel is parallel, assuming same finish) rule in a suitably square Starrett (or any; again, square is square) head should (would) be just as square as a equivalently parallel Starrett rule in a Starrett head shipped as a set. The design is such that the mating/registration surface is the key factor, which is why interchangeable rules are acceptable. I can put my 24" B&S rule, or 12" Starrett, or 6" General (I think?) in any head, and assuming parallel, flat, and good finish on each rule, the results are consistent within the limits of the device regardless of the brand or small size differences. To restate, the brand, or whether it all came as a set, or small deviations in size are not relevant.

Clearly, if you want more confidence in a square, you wouldn't use a combo in the first place, but rather a suitably certified machinists square, cylindrical, etc.