Definition
Bit Sub — A bit sub is a short crossover sub that connects the drill bit to the bottom of the drill collar or bottomhole assembly, adapting the bit's box connection to the collar's pin and often carrying a float valve or nozzle for the mud circuit.
Also known as: bit sub, bit sub adapter
What a bit sub is
A bit sub is the sub that couples the drill bit to the lowest drill collar in the bottomhole assembly. Most bits have an API Regular (REG) pin connection, while the collar above has its own connection, so the bit sub crosses over between them — presenting a box to accept the bit pin on the bottom and the collar’s connection on top.
Bit subs are commonly bored to house a float valve (a back-pressure check valve) that prevents mud and cuttings from flowing back up the string, and may carry ports or a nozzle depending on the BHA design.
Key points
- Connection matching — adapts the bit REG connection to the collar connection.
- Float valve — a bit sub often carries the drill-string float that stops backflow.
- Fishing profile — machined so the sub and bit can be retrieved if lost in the hole.
Renting bit subs
Bit subs are a routine part of every rented BHA and must match both the bit and the lowest collar. Explore BHA components and drill collars, or contact us with your bit connection and collar size.