I ride to work on an aluminum frame bike. It only has one set of holes for a bottle carrier, on the down tube. I'd like to add a second bottle on the seat tube.

1. Can I simply drill an appropriate sized hole, then force a screw in to create the threads? (That works with steel sometimes) Do the properties of aluminum allow this?

2. Or should I just look for a bottle carrier that attaches via some kind of strap or clamp that wraps around the seat tube...no drilling?

Go to BikeParts.com and look up the Problem Solvers Clamp-on Water Bottle Mount. There's even more options in the Waterbottle Hardware/Mounts area.

Also the TwoFish Quick Cage No Bottle. It's held on by velcro.

Don't drill the frame unless you have the capability to also weld in a mount point. Drilling the hole will weaken it, the mount point welded in there will put the strength back in.

There are bolt on mounts for this situation, or some bottle holders simply use hose clamps.

Well put, but wrong.

A round hole will not significantly weaken the frame, but trying to weld stuff to it might.

THe better solution is riv-nuts:

Your source for RivnutŪ engineered fasteners and installation tools - Cardinal Components

These come by other names, such as Pem Nuts
These are generally how water bottle bosses are attached to aluminum frames.

Well, the main reason is that the frame tube is so thin that you'll probably only get one or two threads cut in it, and as soon as you even try to tighten it you'll just rip the threads right out. You need a threaded boss.

I dunno, new Schwinn Varsity might have a pretty thick walled seat tube

[MENTION=397038]12strings[/MENTION]: Measure the seattube diameter and subtract the seatpost diameter, then divide by two. If it's at least 4mm then I'd probably go for it.

Don't tighten the bolts very tight, use some red Loctite.

Agreed, riv-nuts (rivet-nut) is how loose mounts are repaired by bike mechanics.
O.P. why not ask your question in the "Bicycle Mechanic's" sub-forum?
I also feel welding an Aluminum bike is not a good idea. Aren't ALU frames heat treated/tempered after asembly welding at the factory?

Clamp on. Many vintage bikes didn't have any bosses (like my Peugeot)...clamps are cheap. A new frame isn't

Better solution. Cheap too!

Amazon.com : Topeak CageMount : Frame Mount Bike Pumps : Sports & Outdoors

I've been more than dissatisfied with the quality of the cage straps that I'm finding now. And there are always problems with oversized tubes and parts made for the old standards.

I think I'd try RivNuts, although I managed to yank an aluminum rivnut out of my Litespeed, and have another one that is loose. I was considering stainless, but may weld repair them if I ever get comfortable with my welding.

I Own a frame of 7005 Aluminum , the Producing Factory used Riv Nuts extensively . 10 of them..