Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
A soda bottle cap is obviously designed to withstand both CO2 and pressure.
I suspect what's really going on here is that when drilling the cap, if the drill bit binds or catches at all instead of cutting smoothly, it creates tiny cracks which radiate out from the hole. Under pressure, these cracks tend to grow. Especially the amount of pressure it takes to drive a glass diffuser, which many people don't recommend using with DIY CO2 at all.
Use a sharp drill bit. Use lubrication, a bit of water is sufficient. Don't force the bit, push gently and let it do the work. Use a drill press if possible, with some scrap wood under the cap. After drilling, you can use a heat gun, flame, or soldering iron to melt the edges of the hole; sealing any cracks enough to keep them from growing.
I have never had a cap fail, or show any signs of failing or becoming brittle. One has been in use for years.
I also don't use glass diffusers with DIY because of the high back pressure. Any failure that causes sudden depressurization will cause the mixture to instantly foam up, like shaking a bottle of soda then opening it; creating a nasty mess. If the sintered glass diffuser element breaks, that mess ends up in your tank.