Hi Chris,

On the X1/9, I saw the manual shows the radiator bleed screw has either a washer or an o-ring with it, is this correct?

It's actually a flat washer made out of a reddish cardboard substance, same as what is on the oil drain plug.

Also, last night I think I over-torqued the bleed screw and might have crossthreaded it (coolant came out). I took the screw out and the threads seem fine, does this mean the threads on the radiator might be damaged?

If you cross threaded it, yes ... but, the washer could have failed, or the thing it screws into in the radiator might have come unsoldered from the radiator tank, a very common problem with the X.

Do you know what would be the cheapest fix for this?

Pull the radiator, observe what is really wrong, then fix it so you can bleed the cooling system without it leaking afterwards. As for the cost? Does it matter compared to what repairing your engine will cost if you run it without coolant?

Also when the engine is running, it seems there is a coolant leak in one of the pipes going underneath the car (just drops at quick intervals). Is this something that I can fix by putting in the coolant the "radiator stop leak" sealer (copper particles type)?

I would never suggest using any kind of stop leak solution. The stuff gets stuck in the radiator (especially true in an X as the cross flow radiator is already prone to this problem!) and causes the car to overheat.

You need to do chassis surgery to see where the pipe is leaking, and then repair it in such a way that you will not need to do the chassis surgery again! Your car could suffer serious engine damage if you have a coolant leak.

Would it damage the internal of the engine cooling passages? If this is not a good practice, do you have a recommendation? Thanks for your help!

Probably not damage them, just coat them with stuff that will prevent the proper amount of heat exchange Fiat assumed should be there! In other words, don't gunk it up, find it and fix it properly the first time.