Internal lugs were the original lugged construction turn of the past century. Some very light frames (not necessarily rigid) were made that way. Typically just a small fishmouth lug. If the lug is inside and you know what you are doing with the braze flow, you don't have to worry about the lug finishing or clean up.
Joe Bender-Zanoni
Whitneyville, CT
Hugh Enox wrote <snip>
>From the outside of the [Zebrakenko track] frame you could just see
tiny fillets (like tig welding) but looking inside the frame there were
short reinforcements. I was VERY intrigued by this as a construction
method (which I think Schwinn had also used a some point).<snip>
For a couple of decades at least, I've used a piece of 1" diameter gas-pipe Columbia frame as my headset race removal tool. The usual drill: several 3 cm or so slots parallel to the tube length, spread 'em out a bit so they catch the head races, engage them, then beat on the other end with a handy hammer. Well, in this case the "beat on" end has an internal tube, as provided when the frame was built. One of the "fingers" has bent, so I'll have to cut it down from its present length, but I haven't been working on bikes with 20 cm head tubes. Ought to give me a couple more decades with a nice purple tool.
harvey "cheap is good" sachs mcLean va (One of my buddies, from the Upper Midwest, used a great ethnic slur one day when I started tapping something on the bike with a hammer. "Harvey," he said, "I didn't know you ever used a Swedish Micrometer." Only Garrison Keilor's bachelor Norwegian farmer remains from those innocent days.)