A little machinist help

showcrop

Well-known Member
I need to install a 3/8-16 Helicoil in the engine block that I am working on I have the coils but the tap is missing from the kit what should the tap be? it looks like a 7/16-14 matches up but it seems like it should be 7/16-16
 
I would just get the proper heli coil tap ,but yes the threads per inch will be the same.The coil is the same inside and outside. A 10-24 heli coil tap can be used for tapping 1/4-24.
 
Okay after measuring the wrong 5/16 ones . Oops The diameter of the 3/8-16 HC tap is .455 and the diameter of the heli-coil itself is .458".
 
It's a special size tap made just for
helicoils. I really don't care for
helicoils myself, whenever possible I use
solid threaded inserts which are much
stronger and don't need a special size tap
 
Yes, it is a special tap. It's called a "Helical Insert Tap". They are expensive! McMaster Carr has them for
$30. Be careful asking for a Helicoil tap, that's a brand name. An unknowing counter man will sell you a
Helicoil brand, be even more expensive!
 
(quoted from post at 18:47:45 01/03/16) Yes, it is a special tap. It's called a "Helical Insert Tap". They are expensive! McMaster Carr has them for
$30. Be careful asking for a Helicoil tap, that's a brand name. An unknowing counter man will sell you a
Helicoil brand, be even more expensive!

Do they put specially shaped threads in the hole? The outside of the HeliCoil looks like perfectly normal symetrically shaped threads.
 
I don't think they have any special shapenon the thread, but they have to be the same pitch as the thread you're fixing. We use them at work on repairs now and then, and we also use Jergens style thread inserts. Those you use a normal tap, but drill slightly oversize for that tap. Then when screwed in, it gets staked in with usually 2-4 stakes.

I prefer the Jergens, but it depends on how much room you have and what the customer wants...

Donovan from Wisconsin
 
First drill with a 25/64 drill bit and tap the hole with a Helicoil tap #819-6 It is a special thread. They make a tool to insert the Helicoil.
 
The taps are available from most of the well known industrial suppliers (MSC, ENCO, McMaster, NAPA, etc). They
will include "STI" after the size. A 3/8-16 STI is what you need.

You might be better off buying a kit (includes the tap, drill, insertion tool, a few inserts, and a box). It would likely
be cheaper than buying just the tap.
 
It is a "timed" thread, special tap. I prefer to use "Time-serts" or other brand solid inserts as opposed to a coil of wire.
 

Thanks guys I ordered the STI tap I need to get an indexing box or whatever you use for taps. It seems like every time I go to my taps it takes me a good 45 minutes to find the right one, Right now they are divided between 3/8 and under and larger than 3/8
 
The helicoil cannot be measured out of the tapped hole. it is a spring fit into the newly tapped oversized hole. Try the loose heicoil on a bolt and it will be very loose. Thread Sert type are substantially better if space allows. Jim
 

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