So, I'm putting my Mongoose ATB together and I thought I had everything figured out. It takes a BMX headset, which I found pretty cheap. I also found a crown race at my local co-op that I thought fit, but once I cleaned the crown race and steerer tube up I found that the crown race is marginally larger in diameter than the bottom of the steerer tube where it should sit. Its so close that it stays on when I tip the fork upside down, but I can take it off easily and it spins when at the bottom of the steerer.

My question is this: should I use some loctite or something like that to secure the crown race since its so incredably close, or should I keep looking for a crown race that sits on the steerer tube more securely?

Sounds like you tried to fit a JIS crown race (27.0mm diameter) to an ISO crown (26.4mm). My advice is find the appropriate ISO race.

I don't think so. I measured each and there is less than a .1 mm difference between the two.

An old school way was to hammer a punch at equal intervals around the steerer tube where the crown race sits, in order to push up some metal for the crown race to fit.

What are the measurements of the ID crown race and OD where it is pressed on the steerer? This will tell you which is under/oversize.

I think the race is 26.6 and the steerer is 26.5, but it was a couple days ago and I'm as drunk now as I was when I was working on it, so I might need to remeasure.

You probably just need to clean or calibrate your verniers as standard BMX (and Campagnolo) crown race ID is 26.4 and non-BMX is 26.5 or 27.0.

Why not just use the crown race from the BMX headset? You could use the punch method (it does work quite well) or red locktite. Also, you could try making a shim from the appropriate thickness feeler of a cheaper feeler gauge set. Or blow the holiday drinking budget and buy a Campagnolo crown race.

Unfortunately, the headset I bought didn't come with a crown race. There is a bike shop that specializes in BMX's so I think I'll go there today to see what they have rattling around in the back. If that doesn't work I'll do the punch or loctite methods.

↑ I have had to do this...just punch or chisel into the OD and not the bottom surface...

loctite isn't necessary with the punch method.. you need it tighter add more punches, it works great.