The tap drill for a 1/4 NC thread is a no. 7 drill, 5/16 NC is an F (17/64" is close enough). I haven't seen step drills with such sizes.
Same here, I've had good results with knurled rivnuts + retaining compound like the Loctite 648. I always use a little ceramic anti seize on bolts/nuts that might corrode.
The car frame is close to .120 thick? Self drilling TEK screws are more than adequate for a trans cooler or non structural brackets. Use the time/money saved over drilling ,trapping nut inserts etc on more important stuff...
I was going to suggest tacking/brazing a stud or bolt on the frame,, that way no holes to drill...when you are done just grind it off.....
"I was going to suggest tacking/brazing a stud or bolt on the frame,, that way no holes to drill...when you are done just grind it off....." What about having small studs spot welded on? There would be very little heat damage and they could also be later chiselled off and ground flush if needed, as suggested.
OP said the frame is powder coated already, so no welding. @2OLD2FAST has got it right, the way the factory still does it.
Thread forming screws form a closer tolerance thread than drilling and tapping , thread is therefore , better supported/ stronger
Some of the problem with drilling round holes is the included angle on the tip . 2 flute drill bits normally don't drill round holes in thin metal, and the flatter the included angle on the tip, worse they make the hole. Small holes usually are acceptably roundish..... Might use stainless screws to keep rust at bay. If the nutsert spins when you try to loosen the screw, a chisel will knock the top off and then just install another nutsert. Just tapping the frame is not a good idea.
It's a ticky-tacky difference but the factory stamps the holes instead of drilling them and uses thread- forming screws rather than tapping(cutting) threads .......Ahhhh...the language !
In my frame I used the drill and tap method, 10/32 fasteners. For thin body sheet metal I used steel nutserts, 10/32 for most everything since that was a standard Ford size on my Model A. Cliff Ramsdell
The frame certainly is thick enough for #10 threads. When the manufacturer makes a threaded hole in the frame they pierce the hole which forces extra metal around the hole edge, so a little bit more thread is formed.
yep rivnut, nutserts, etc if you think it will be too weighty but I think @Beanscoot is correct punch your holes and use screws... the factory did...when I was doing mine I didn't like the ol' stuck right on the rad, plastic strap through the fins, and foamrubber method dudes used to use back in the day ...so I wracked my brains to find a spot for it on the valence, under the valence, beside the rad, yada yada finally I used two 45 degree pieces of flatbar screwed to the ends of the cooler I found two factory screw holes in the bottom recess of the rad support that match the length and made 45s for the lines which run around the lower hose...in mine it sits right under and slightly in front of the fan (behind the rad because it's a 6 cyl. support) perfectly out of the way and should technically run cooler than the rad because it has airflow all around it as well as through...also didn't have to drill any holes in the body/frame...i'll get you a pic if you'd like
A friend of mine had a '55 Chevy he wanted a newer 235" with side mounts installed in. I built some mounts that would work and told him he'd probably have to holesaw an access hole to get nuts on the inside of the frame. He told me he was just going to used self-threading bolts. I told him I didn't think it would work but he did it anyway and I don't think he ever had any trouble with it. Not that he admitted, anyway. Gary
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