They are thin little sleeves on the bottom inside.
They can be tapped one size larger but that makes the walls of them pretty thin.
There ARE some new replacment 'crush' sleeves available to replace them.
Basically you drill the old ones out, stick one up in the hole and when you tighten up on them a bulge area below the female threads at the bottom of the new sleeve crushes, expands and rivits its self inside the new hole.
This is an ideal fix! I found them at my well stocked local industrial 'fastiner' store.
Another practical solution is to drill all the way up through the top of the frame rail then drill a little larger bottom hole and insert a crush tube much like how the side frame motor mount holes are designed.
Then run a long bolt all the way to the top of the channel and use a nut and washer up there. The crush tube prevent the frame from caving in and loosen the bolts. Unfortunately, this method weakens the frame somewhat as the top and bottom channel sides are the most critical.
The 'cut the lower frame side and weld a nut in there' method is your other option ... but that IS a messy operation.
The 'weld a stud' is a pretty good idea ...
You could cut a slightly larger fresh thread, screw in a stud and then penetrate a weld all around it.
Take yer pick ! [img]/wwwthreads_images/icons/smile.gif[/img]