I had enough pipe and couplers to make 3 3/4" pipe clamps of proper
length. I had two sections of pipe left and an extra set of clamp
ends, but I didn't have another coupler and the two pipes together
weren't long enough anyway. I didn't have time to go get another pipe
so I took a 1 foot length of 2x2 and drilled a 1 to 2 inch deep hole
into each end with a 1" spade bit. I was then able to thread the two
pipe sections I had into this wooden coupler and get my fourth clamp.
The wood creaked a little as a tightened the clamp, but the pipes did
not pull out. Maybe this is a well-known trick, but I hadn't heard of
it and it worked well for me.

Charles

Wow - wear your safety glasses, and let us know what happens when this
thing lets go! I'd guess the only "well-known" thing about this trick
is that screwing into end grain isn't very strong. I'm glad it worked
for you in a pinch, but if it were me, I wouldn't trust it any farther
than I could throw it (and... "with your bad shoulder, Ed, you
shouldn't be throwing anyone!")

I really don't think there's much to worry about when using this method
for a clamp - (I think) the only thing bad that would happen is the
pipes would pull out of the wood and the clamp would fail. I wouldn't
want to hang any weight on a pipe screwed into end grain though.