I have a 3/8 straight hole. How can I step it down to take a 5/16-18 shoulder bolt? It's already beyond the size of the helicoil drill, which is 21/64. If I use the helicoil tap in that size hole, will it hold?
I need a 3/8 shoulder bolt to fit through a bearing, but the securing hole needs to be reduced and threaded. I don't have a welder, or anything like one!
You prolly need a couple of cans of Add-A-Thou to make the hole smaller.
You could drill the hole larger and thread for say 3/4, then run a 3/4 plug in and drill it and thread for 5/16. I've done that to repair stripped out holes on machinery where I didn't want a helicoil.
If you have a lathe (?) make the plug with a shoulder on it so that it will jamb once seated rather than possibly travel all the way through when installing. if the shoulder is made wide, a small hole can be drilled through the face and a set screw or roll pin used to prevent it unscrewing if you have to remove the 5/16" bolt. I used this method for fixing a stripped out helicoil on a spark plug hole in a motorcycle engine head.
Thanks for the ideas, guys!
If I'm doing a plug to fix something like that, I've had good results with high-strength thread locker, even without threading the plug in.
I've used Greg's technique many times with good success. Some high strength thread lock or a roll pin will make it permanent and as strong as the original part.
Tom
Vinny has the best solution.