fermenting in 5 gal kegs...how much fits?

For people that ferment in corny kegs, how much are you able to ferment without th e foam bubbling up out of the top? Since I can't think of a way to hook up a blow-off tube, I was planning to just leave the lid off. However if the foam pushes up out of the top, its going to make a big mess. How much foam do lager yeasts produce?

I target my batch sizes at 4.75 gallons, but usually end up closer to 4.5 (I think - I really need to make a dipstick to measure it). I use Fermcap to keep krausen from coming up too high, but sometimes I still get some foam in the blow-off tube, which I just connect using a regular old gas QD. I've never had it clog.

If you're really concerned, you could drill a large enough hole in the lid to connect a proper blow-off tube, but I really don't think you'll find that necessary. An other option is to take off the gas post entirely and stretch a section of tubing over the threads.

I've never brewed a lager, so I can't help you there.

I guess I could take off the pressure-release valve and use it for a blowoff tube. Or get an old lid and drill it.

Occasionally I still will ferment something in a corny, usually 1 gallon of head space is good, as for the blow off I usually remove the liquid side post and dip tube and stretch a blow off over the threads...the tube I have is IIRC 3/8 inside diameter?? But you could also remove the relief valve, and either stretch tubing over the socket for the valve or jam a bubbler lock in the hole.