Metallics 7814CTB Carbon Steel Tap Bottoming Type 7/8-14 (1pc)

The 7814CTB cuts a 7/8-14 thread, meaning a 7/8 inch nominal diameter at 14 threads per inch. That is a UNF (fine) pitch rather than the 7/8-9 coarse series, so use it only where the mating bolt or rod is fine thread. On a 7/8-14 hole the usual tap drill is 13/16 inch.

A bottoming tap is for threading close to the bottom of a blind hole. It has almost no tapered lead (roughly one to one-and-a-half chamfered threads), so it cuts full-depth threads nearly to the end of the hole, but that same short lead makes it hard to start square in bare metal. Standard practice is to open the hole with a taper or plug tap first, then finish with the 7814CTB. For a through hole you usually don't need a bottoming tap at all.

It's carbon steel, not HSS. Carbon steel taps are suited to hand tapping in softer stock, mild steel, aluminum, brass, copper and plastics, and they're commonly used for cleaning up or chasing existing threads. For production work, powered tapping, or hard and abrasive alloys like stainless, a high speed steel tap holds its edge far longer.

Yes, that's a common use for a carbon steel bottoming tap. As long as the existing thread is 7/8 inch at 14 TPI, it will clean up burred or paint-fouled threads, and the short chamfer lets it work right down into a blind tapped hole. It will not correct a 7/8-9 coarse thread, and it won't re-cut metric.

You'll need a tap wrench or holder sized for a 7/8 inch tap shank (a T-handle won't do it; this size calls for a bar-type wrench), cutting fluid, and a 13/16 inch drill if you're cutting a fresh hole. A taper or plug tap in the same 7/8-14 size is worth having alongside it, since a bottoming tap is meant to finish a hole rather than start one.

One tap per order, supplied in a pouch. If you're standardizing a shop or a truck stock on 7/8-14 and want spares, order the quantity you need rather than expecting a multi-pack.