Shimano Clipless Pedals Cross-Compatable?
 

Do the cleats that come with Shimano clipless pedals fit ALL Shimano clipless pedal models? Or just the model they came with? For example I have a different model of clipless pedals than my friend, but can I clip into his so long we both have Shimano's?

Rick, your cleats (presuming they are the usual SM-SH 51) will fit just about all MTB shimano SPD pedals:

PD-M 959, 747, 737, 646, 636, 545, 536, 535, 525, 515, 434, 324, 323, PD-A 525, 515.

For some reason PD-M 858 pedals have their own cleats (SM-SH 52) but I am reliably informed that they can be used with the SM-SH 51 cleats). :) But don't quote me.

they are actually the SM-SH55's

Should still be no problem. The SM-SH 55 give various options of clipping out from the pedals (left, right, up, whatever). Have you got the PD-M 324 pedals by any chance?

I have an old set of 323s (the predecessor to the 324s) and they came with the SM-SH 55 cleats I believe. I used them on all sorts of SPD pedals so you should not have any problems. :)

You left one out and that is the 424.