Fix-A-Thred... Helicoils?

Question

So I bought the m10x1.5 fix a thred kit from advance auto parts today to fix a stripped hole in the subframe that my upper control arm goes into. These bolts are crucial as they’re the only things holding the control arm to the frame (and thus the knuckle). I bought the tap because I’m using grade 10.9 bolts that are aftermarket because OEM ones weren’t salvageable (also because the nut at the back of the subframe broke off and I don’t want to cut it open). The fix a thred kit comes with a handle, tap, and helicoils. My questions are: -if I drill the holes out with the recommended 13/32 bit and then run the tap through, would that be enough to strongly hold the new bolts? -do I need to install the helicoils? What purpose do they serve(id really like to just put the bolt into the subframe without these)? -if I need to put the helicoils in, are they strong and reliable enough (like the grade 10.9 bolt) to hold up to the stresses of the suspension?

Thank you for any answers, I want to do this job right with a permanent fix and not a temporary patch up!

Answer 1

If the thickness of the metal you are tapping is greater than 1.5 times the bolt diameter then it should be ok, but this also depends on the metal and its grade. If not then all it will do under load is to strip the threads and possibly be the cause of an accident.

You may need to consider welding a nut to a plate and then having that plate with nut welded into position to provide sufficient fixing.

Answer 2

UPDATE: only one of the bolt holes was thick enough to tap, so I tapped and helicoiled it with success! The other hole was like 10mm thick and wasn’t viable for tapping, so I ended up cutting a little flap(3 sided square) in the bottom of the subframe and peeling it back just enough to fit a nut and pliers through so I could get that nut back in. Thank you to all who replied, and good luck anyone with the same issues!!!!

P.S make sure to use a LOT of oil/tapping fluid(wd40 works) when tapping and be patient as to not to break the tap(I didn’t know if the tap was useable for steel or not), I used one from autozone and wd40/a little (2hrs) of patience goes a long way!!!!