How come the shims don't need a hole in the middle? I am looking at the FSM diagram and it looks like the fuel goes directly through the middle until it reaches the nozzle insert/spacer (the last thing before the nozzle).

I have a second set of injectors laying around to get shims from. Could I also experiment with swapping the springs as well? Perhaps doing a baseline test of the springs by themselves (from a set of 10) - and then pick the 5 that are closest together?

In fact I have 15, as I have two running cars and a parts car - so I could just rearrange to match in sets of 5, I'm assuming - or is there a problem with mixing injector manufacturers (as long as the internal parts are not swapped)?

Are the OEM shims (from original injectors) different in size, or are they all the standard thickness? (ie does it make a difference to swap with other OEM injector shims)

As per manufacturing my own shims, if the shims don't need holes, wouldn't material from a feeler gauge work as well (if cut to size/shape, but left a solid shape, ie NO drill hole through the middle)?