What size tap do you use for a 5/16 Heli-Coil?


Tap Size: 5/16-18 STI. Drill Size: 21/64"
Do helicoils need special taps?

Heli-Coil STI (Screw Thread Insert) taps are used for general and machine operations. The standard STI tap can be used as a basic tool to create threads ideal for threaded inserts. It is recommended that these taps are used with a CNC machine or other high-precision equipment in order to ensure a proper class of fit.

 

It's about the size of a 3/8 tap but will have 18 threads per inch.

 

Well I just found this tap that says, "Helicoil tap, M8x1.25 use drill bit 21/64.

Now of course, that sounds like its used as a metric tap, BUT, why is the drill bit given in fractional size, which is the correct size for the drill bit used for a 5/16 coil?

 

Well I just found this tap that says, "Helicoil tap, M8x1.25 use drill bit 21/64.

Now of course, that sounds like its used as a metric tap, BUT, why is the drill bit given in fractional size, which is the correct size for the drill bit used for a 5/16 coil?

easy go out and try to buy a metric drill bit at any normal store
my metric tap sets at home and at work both state standard drill bit sizes

 

21/64

 

5/16 is .312
8mm is .315

 

I just found a chart on the tap. The min size is .32, max is 33. I'm going to try Ebay, and see if I can buy just the tap.

 

.

 

Helicoils do have their own specific tap for each particular thread size. I was able to buy them at my local Ace Hardware store.

 

STI Tap (Screw Thread Insert)

But I think he has the taps , he wanted the drill size for a 5/16 STI Tap

 

The helicoil taps are specific for the helicoil being used. Look at your taps and find one that says helicoil on it. They usually print or stamp the size on it and the drill bit to be used. 21/64 is the size drill bit for a 5/16" helicoil.

 

STI charts say : Size Q

 

Just buy the kit.

BILITOOLS 5/16-18 inch Thread Repair Kit, HSS Drill Helicoil Repair Kit SAE https://a.co/d/6gDhVnU

 

I grabbed the tap I have for my 5/16 Helicoil kit. It had no markings other than "tap for 5/16 Helicoil". I measure it and as someone already stated, it has 18 threads per inch and was .390 diameter. You can probably go thru the collection of taps and find one that has that thread count and dimensions if you want to try that.

 

Like others have said. Will say 5/16-18 sti.
Or heli coil 5/16-18 if heli coil brand tap.

 

Make sure to use a bottoming tap or else you'll be doing it again and again. Going to have to charge extra labor on that build due to the previous builders fuckups!.

 

Grab a die and find one the helicoil will fit. Use a tap that size.

 

1 inch,
then a mig welder.

 

Use a helicoil specific tap or you will have issues screwing in the insert.

 

This.

A 3/8 is slightly too small.

A 10mm is close but the pitch is all wrong.

Luckily the hole size for a 5/16 Helicoil is easy. Just completely strip the threads, nothing else is needed.