bike tray mounts on factory roof rack.
 

My wife and I just picked up a great deal on a private sale 1999 Chevy 4 door 4wd blazer. I recently pulled off the Thule Echelon bike trays from my Mini Cooper S to switch over to the Blazer. I didn't want to spend a lot of money on the install so I securely mounted the trays to the factory cross bars using small wedges from McMaster-Carr to level out the trays. My question is, are these factory cross bars strong enough, secure enough to rely on holding my 2 bicycles? I mounted one up and gave it a good shake and I could see the crossbars flexing a bit. I really don't want to lose a bike or 2 driving down the road. Thanks in advance.

Yep
 

Hey, I'm glad I'm not the only one using non specific bike bars on the top of my blazer too. The roof racks on the top of the blazer are Definatley sturdy enough to hold three bikes on the highways and backroads curves. I took the racks of of my moms Volvo xc 70 and they work fine. Good luck!

I bolted some yakima bars (from a first gen explorer) straight to the roof because I didn't have a lot of faith in the stock rack that I could flex easily.

That said, some blazers have a little plate on one of the factory mounts that says something like "200lb max" and there are a number of people with aftermarket baskets and spare tires mounted on top via the stock roof rack. I think a bike should be alright but I would want to test it out with an old bike before gaining enough trust to put a good one up there.