Hi,
I am trying to install shutoff valves below an old sink (house built 1926 in Buffalo, NY). The old water supply had a 3/8" FIP connection on the bottom and what looked to be a 1/2" compression connection on the top. A local plumbing store that specializes in old plumbing also thought that a flexible hose with 1/2" compression connection should be a suitable replacement. Yet when I try to attach the hose to the male brass threads, I get no purchase on the threads. The threads are not visibly corroded or worn away, and a 3/8" compression connection is far too small (I tried it). Any suggestions? For instance, is there an easy way to build up the male threads so they will grab the apparently too large female end, or do I need to hunt down an off-size compression connection..

Take a pic and post via "Go advanced/manage attachments" . Put something in the pic so we can gage sizes. A ruler, a penney, a dime.

Yes please post a pic. Are you sure that they didn't sell you flexible closet(toilet) supplys. Closet supplys won't fit on sink fixtures, they are to big. Make sure you have lav(sink)supplys. There is no off size connection on sink faucets. IF you get the correct flex supply lines, it will work for you. Good luck. Lee.