Spline or bolt-pattern Cobble Boss — which do I need?

Bolt-pattern is fine for intermittent cobble on short bores because it lets the same head handle soft ground and rock without changing tooling. For sustained rock work or long bores, go spline — the connection is stronger in torsional shock and rebuild costs are lower over the bit lifetime.

Does the Cobble Boss work for solid bedrock?

Solid competent bedrock usually warrants a DTH or HDD hammer setup. The Cobble Boss excels in weathered bedrock and cobbly ground where percussion would be overkill. If ROP drops below 10 ft per hour, consider moving to hammer tooling.

What reamer pairs with a Cobble Boss pilot?

In rock-capable ground, match the Cobble Boss pilot with a spline-connection fluted or stacked-plate backreamer sized 1.5-2x the pilot diameter. The Sand Shark and solid-body fluted variants are common choices.

HDD Bits · 29 SKUs

Cobble Boss Rock Bits

Spline and pin connection bits for rock boring

The Cobble Boss family is the rotary rock bit Drillworx builds for the ground that stops pilot bits in their tracks: cobble-heavy glacial outwash, cemented gravels, and weathered sedimentary rock. It is a purpose-built rock bit, not a beefed-up pilot bit — the body geometry, carbide selection, and connection choices are all rock-first.

The series spans spline-connection, pin-connection, and bolt-pattern variants so a contractor can match the bit to whatever head is already on the machine. Spline connections handle the torsional shock of cobble impact without the bolt fatigue that shows up on long rock runs. Pin-connection variants drop into compatible adapter subs for rigs that already run spline tooling upstream.

Drillworx stocks Cobble Boss bits in 4", 4.5", 5", 5.5", 6", 7", and 8". For extended rock bores where ROP is the bottleneck, step to a Cobble Boss III on a spline connection; for lighter cobble work, the base Cobble Boss on a bolt pattern is usually enough bit.

Field guidance

When to use cobble boss rock bits

Cobble-heavy glacial outwash

The bit body shrugs off 4-10" cobbles that would knock carbide off a pilot bit. Expect slow, steady ROP rather than bursts.

Weathered sedimentary rock

For sandstone, siltstone, and soft shale within reach of rotation-only drilling, the Cobble Boss III carries enough carbide to finish the bore without a mid-hole trip.

Long rock bores

Spline connections shed the torsional shock that fatigues bolted heads over long runs. Specify spline when the rock section is more than 100 ft.

Transition formations

A Cobble Boss on a bolt pattern lets a Vermeer or Ditch Witch head run rock when needed without changing adapter subs. Useful for crews that hit occasional rock in otherwise soft ground.

Compatibility

Fitment & sizing

Connections: Spline, pin, bolt-pattern (Vermeer 6/7-bolt)
Common sizes: 4" – 8" OD
Button type: Tungsten-carbide insert — rock-grade
Companion tooling: Float subs, sub savers, rock-frost adapters

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