Steve and Chris,
The 68 tooth ring I had custom made by Boone Tech. a while back. It's purpose is the drive triple chainring for the triplet I'm building. The campagnolo 170mm triple with rings 68-52-36 (Yes, a 52T for the MIDDLE ring) is combined with a Campagnolo 8 sd. cassette tandem hub, 40 holes, with an 11-28 cogset. Don't bother to figure out the gear with 700c wheels; it's about 167 gear inches. The frame slopes from 52cm to 50cm to 48cm. Basically it's a pint-sized crotch rocket.
I have a Sugino 144 bcd chainring in 62 T; also have same in 64 T. I don't need eBay. I also have 64 T in TA. I put my 62 T TA ring on my Moulton because any bigger ring would saw the frame in half. That bike is set up with a single (62 T) ring and a Campagnolo 7 sp cassette rear hub with 11-26 cassette. I'm running the 17" Moulton wheels so I get a 95" gear out of 62 x 11. I use a Suntour bar end for shifting.
Besides needing large rings upon occassion I get a special kick out of most peoples' reaction to chainrings over 53 T. You'd think the chainring caused a rash or something the way people say stuff like "you have THAT thing on YOUR bike?" OH, MY, GOD! I just reply "I own stock in a company that makes replacement knees." If you're an advid tandem rider (as I am) you should also be familiar with larger chainrings. If you ride on your single with tandems a lot then a bike set up with "overdrive" is practical. Mine is a 58-46 front chainring combo with a 7sp 12 x 28 cassette. I have my normal low gears plus plenty of "top end". About the only thing I DID learn to read in school was a gear chart.
Brian Baylis La Mesa, CA Size DOES matter; love those big chainrings. Wasn't it Teddy Roosevelt that said "Ride a short crank; and carry a big chainring"?