Use 6.5gal Carboy for Secondary?

All of my secondary fermenters are currently occupied and I want to get a brew out of the primary it is currently in, for filtration purposes. I have an empty 6.5 gallon primary I could use. Anybody ever use a primary for a 5 gal batch secondary fermenter? What if any ill affects would/could occur? Excess exposure to oxygen? Thoughts appreciated.

You'd be fine.

There have been many threads around here about this- but I don't think anyone will ever be able to say definitively that it will harm your beer. When you rack, you minimize oxygen exposure and then some co2 is released and still blankets the top of your beer. So, probably no harm. I've never done it, but I'm sure many others have.

Still, it does no harm to keep it in the primary a little longer, either. If it were me, I'd probably keep it in primary another week, rather than using a larger carboy. But, I'm a winemaker, too, and in winemaking headspace is evil! Not so much in beer.

In short- should be fine, either way!

should be just fine. the amount of surface area between beer and air is what you want to watch, and a carboy will have the same contact area if you have 1 gallon or six. As long as there is enough beer (and 5 gal will be plenty) in there, it will kick out some CO2 and purge the headspace of air. brew it!

You could always use some sanitized marbles to bring the liquid level up.

It'll be fine. The original brew kit I bought at my LHBS came with a 6.5 Gallon glass carboy for the secondary. That's what I've been using. No need to worry about things like sanitized marbles or anything else. Just rack it over, relax and have a brew (home or commercial).

This thread got me thinking.... I brewed my first 10 gallon batch last weekend and ended up with more wort than I planned for. I guess I wasn't boiling vigorously enough to get the volume down, so I ended up with about 12 gallons of wort split between my two 6.5 gal Better Bottles.

The liquid level was about an inch above the part of the bottle where it starts to curve inward towards the neck.
I knew I had too much liquid, but I decided to go for it anyways. I experienced the most amazing ferment for the first 48 hours that I had to change my gallon blowoff jug three times!

I was sad to see all that potential beer go to waste, but I'm also concerned that I may have lost other valuable components of the beer like dissolved hops, some of the apricot puree I added, etc.

Can anyone attest to beer quality going down by blowing off the top gallon of the primary?

From what I've read it can actually improve the flavor of the beer.

I currently brew in smaller 3 gallon batches and primary ferment in a 6.5 gl. glass carboy and use a 3 gallon glass carboy for secondary... I usually end up with about 2.5 gallons in secondary (which is actually quite a bit of headspace) and everything works out just fine. As others have said, some CO2 will come out of solution to purge the O2 and will actually create a blanket of CO2 to protect the beer.

Besides, oxidation takes a while to get a good foot hold, right?

I've done it. No problem.

Used cornies make great secondaries.