Why is every bolt and nut diffent?
 

Why can't they all just be 10mm head, or something else? Does anyone know why they are all different?

You wouldnt be able to get much torque on something like a head bolt if it had a 10mm head same for something like a cover plate. If that tiny bolt had a 15mm head youd snap them off all the time.

On top of that the XJ is a weird amalgamation of metric and standard fastners.

cause (some brake fittings for instance) are 3/8", which is ever so slightly a misfit for 10mm

there is a mix of metric and imperial fasteners on XJ

Tell me about it. I was removing my heater box and the nuts on the outside of the firewall are 11mm. I have a ratcheted, swivel headed box wrench for 10 and 12 mm but nothing for an 11mm. It was a pain removing those nuts with an regular old boxed wrench. Why couldn't they have just stuck with the 10mm?

That's because they're 7/16.............

hmmm, the 10mm seemed to fit pretty snug. Well, then I'll add to the OP's complaint: why the metric and standard jumble in our Jeeps? (I know, it's been complained about ad nauseum)

I'm confused. You said they shoulda stuck with 10mm but the nuts were 11mm......

And, welcome to the real world. Every US car maker has been intermingling metric fasteners with SAE fasteners since the early 1980s. So, 40 years......

I screwed up. Meant 11mm fit well. Too many numbers in my head. Still working on putting the heater core and Ac evap back in the box and then reattaching the dash. I'm a little frazzled.

IIRC, car makers and others were forced to put some percentage of metric fasteners on all vehicles back in the 1970s. At least I think that is when I first heard about it touted as some sort of government scientific breakthrough to force Americans into metric. Sounds like an awesome rule to repeal.

On an ignition switch replacement, there are at least 3 different sizes of torx at play there. Some are safety torx. The existence of torx used in any manufacturing can be placed squarely on the head of Steve Jobs, who didn't want owner/users breaking into his computer based toys. He upped the game by adding safety torx.

Today, there is no good reason to buy any other set of torx drivers other than safety torx just because of these elitist shenanigans.

What is important is the diameter of the shaft of the fastener - not the head, ie: an 8mm bolt has a 10mm head. Room to reach the bolt is also a consideration as well as required torque. I'm guessing that Chrysler made the transition to metric via the XJ as well as their other vehicles, ie: a replacement USA made front shock for the XJ comes with an SAE nut for the top, but the bottom fasteners are nuts welded to the XJ, so the metric fasteners must be reused.