Shallow blind hole with helicoil - bottoming tap required?

[JHZR2]

Another question about my MB manifold project that I think is for the greater good thus I’m posting separately.

I need to tap a blind hole to prep for a helicoil. The blind hole is drilled out with the correct 21/64” bit.

The OE studs were around 10mm. The helicoil is about the same. There isn’t much depth, the blind hole is only slightly deeper.

Should I perhaps get a bottoming tap versus the tapered one that comes with my kit? Start with the tapered tap to get things straight and centered, then move to a bottoming version?

Like this:

Or does that just add more risk of breaking off in the blind hole?

I will be resurfacing the manifold surface, so that will remove a few thou I’m sure.

 

[Chris142]

How deep is the drilled hole and how tall are the helicoils?

 

[Hirev]

Start with the tapered tap to get things straight and centered, then move to a bottoming version?

I would do this with the blind hole.

 

[JHZR2]

How deep is the drilled hole and how tall are the helicoils?

Drilled hole is about 13mm per sticking a drill bit down there. I set the 21/64 a bit shorter, around 11mm.

Helicoil uncompressed is just shy of 11:

My impression is they get shorter (compress together) when threaded in.

 

[JHZR2]

I would do this with the blind hole.

McMaster has a US made bottoming STI tap that is available for will call by 1230 (it’s 1148). I could drive there and they’d have it ready for me.

I’ll probably start with the helicoil tap tonight, and then pick it up from McMaster will call tomorrow when I have time.

 

[Trav]

Tap the hole first with the one that came with the kit and finish with the bottoming tap, it will be easier to start straight.

 

[Gyro Gearloose]

The industrial helicoils I used required using THEIR taps in the hole because the thread pitch on a helicoil OD is slightly different from a bolt that size. It is the same pitch as the helicoil ID, which was slightly different than standard threads for a bolt the size of the OD. Check thread pitch closely before trying to use a standard tap for a helicoil.

 

[kschachn]

The industrial helicoils I used required using THEIR taps in the hole because the thread pitch on a helicoil OD is slightly different from a bolt that size. It is the same pitch as the helicoil ID, which was slightly different than standard threads for a bolt the size of the OD. Check thread pitch closely before trying to use a standard tap for a helicoil.

McMaster has a US made bottoming STI tap that is available for will call by 1230 (it’s 1148). I could drive there and they’d have it ready for me.

 

[D60]

You're never supposed to start with a bottoming tap. STI taps ain't as cheap or readily available but with standard threads (in smaller sizes) I've sanded plenty of taper taps to act as bottoming. Not technically correct but it works (shrug)

Plug taps seem like a good compromise if you can only have one for a given size.

 

[Trav]

The industrial helicoils I used required using THEIR taps in the hole because the thread pitch on a helicoil OD is slightly different from a bolt that size. It is the same pitch as the helicoil ID, which was slightly different than standard threads for a bolt the size of the OD. Check thread pitch closely before trying to use a standard tap for a helicoil.

The op is looking at the correct STI (Screw Thread Insert) tap, helicoils use these taps.

 

[JHZR2]

Tap the hole first with the one that came with the kit and finish with the bottoming tap, it will be easier to start straight.

Perfect, that was my plan. Thanks!!

 

[JHZR2]

You're never supposed to start with a bottoming tap. STI taps ain't as cheap or readily available but with standard threads (in smaller sizes) I've sanded plenty of taper taps to act as bottoming. Not technically correct but it works (shrug)

Plug taps seem like a good compromise if you can only have one for a given size.

Yeah wasn’t going to start a bottoming tap. I have the helicoil tap. The issue is the bottom of the blind hole since there isn’t a lot of room down there.