Helicoil tap size

I need to rethread the clutch slave cylinder to bell housing bolt. Found an old helicoil of the correct size (3/8 UNC).
Does anyone have an idea of the thread size that I need to tap the original hole to screw the helicoil in. All my advise is that the thread for the coil is pretty much standard and the coil itself does seem to have an external thread pitch.

Tony

Special tap for helical inserts. Check auto parts store for one.

Reference: http://www.mcmaster.com/#helical-insert-taps/=8r8sod

requires a special tap. borrow one.

The Helicoil kits come with a number of inserts, an insertion tool, and the specific tap needed. They list the drill size to use before tapping - that's a standard size so the drill bit isn't included in the kit. But the tap is specific to the Helicoil inserts.

So if you've got the inserts, it's likely you've got the tap laying around somewhere - they really need to stay together.

If not, you can buy a new kit. They're generally around $40 or so over here.