I use the term 6 3/4 inch drill collar to describe a heavy-wall tubular component with a nominal outside diameter of 6.75 inches, designed to add weight to the bottom-hole assembly and help maintain drillstring stability. The outside diameter alone does not define the complete product: the inside diameter, overall length, thread connection, end configuration, material condition, and permissible tolerances must also be confirmed. For purchasing, I recommend treating 6.75 inches as the nominal OD and requesting a detailed dimensional drawing before production or shipment.
This guide explains how I evaluate dimensions, thread options, theoretical weight, application fit, and supplier requirements for a 6 3/4 inch drill collar. Because actual specifications vary by drilling program and applicable manufacturing standard, the numerical examples below are reference calculations rather than a substitute for an approved customer drawing or inspection specification.
This reference is intended for drilling contractors, oilfield equipment distributors, engineering teams, purchasing departments, and OEM buyers sourcing drill collars for land or offshore drilling applications. It is especially useful when a buyer knows the required nominal OD but still needs to define the bore, connection, length, weight, and inspection scope. I also recommend it for buyers comparing custom-made collars with standard inventory.
A 6 3/4 inch collar may be used in directional drilling, vertical drilling, workover operations, and other bottom-hole assembly configurations. The correct design depends on hole size, required weight-on-bit, hydraulic requirements, bending behavior, connection compatibility, and transportation constraints. A supplier should therefore evaluate the complete assembly rather than quoting only the outside diameter.
The nominal outside diameter is 6.75 inches, or approximately 171.45 millimeters. This dimension is important for determining clearance inside the wellbore, but the actual finished OD may be controlled by the selected manufacturing tolerance, machining condition, wear pattern, and applicable specification. I advise buyers to state whether 6.75 inches refers to the nominal body diameter, maximum finished diameter, or a different dimensional reference.
The inside diameter has a direct effect on hydraulic flow area, torsional strength, bending stiffness, and total weight per unit length. Wall thickness can be calculated as (OD − ID) ÷ 2; for example, an OD of 6.75 inches and an ID of 3.00 inches produces a nominal wall thickness of 1.875 inches. This is only a geometric calculation, so the final design must also account for machining allowances, stress concentration, connection geometry, and engineering requirements.
In practice, the bore should not be selected only to maximize collar weight. A smaller bore can increase theoretical steel mass, while a larger bore may support required fluid flow or internal tool passage. I recommend confirming the mud system, motor or MWD/LWD requirements, fishing considerations, and pressure-loss calculations before fixing the ID.
Drill collars are commonly supplied in lengths selected for the intended bottom-hole assembly, handling equipment, and transport method. There is no single universal length that applies to every 6 3/4 inch collar, so the purchase order should specify nominal length, allowable length tolerance, end-to-end measurement method, and whether the length includes the connection make-up reference.
The ends may include integral threaded connections, dedicated pin-and-box configurations, stress-relief features, elevator recesses, or other customer-defined details. These features can change the usable length and affect handling, so I recommend reviewing the full drawing rather than relying on a catalog photograph or a short product description.
The thread is one of the most important parts of a drill collar because it transfers torque, tension, compression, and bending loads between adjacent components. A 6 3/4 inch body does not automatically determine the thread connection. Depending on the BHA design and applicable connection standard, buyers may specify a rotary-shouldered connection such as an NC-series or REG-style connection, but the exact designation must be confirmed by the drilling contractor or engineering authority.
I do not recommend substituting a visually similar thread for the specified connection. Thread incompatibility can prevent assembly, damage shoulders, reduce load transfer, or create avoidable downtime. When a buyer does not have a complete connection drawing, I ask for the mating component specification, prior inspection record, or an approved thread designation before preparing a final quotation.
The weight of a 6 3/4 inch drill collar is governed mainly by OD, ID, material density, length, connection geometry, and any machined features. For a preliminary estimate, I use the following theoretical formula for a uniform steel tube: weight per foot ≈ 0.7854 × (OD² − ID²) × density × 12. Using an assumed steel density of 0.283 pounds per cubic inch, this calculation provides a planning value, not a certified shipping weight.
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| Nominal OD | Example ID | Approximate Wall | Theoretical Weight |
|---|---|---|---|
| 6.75 in | 3.00 in | 1.875 in | About 97.5 lb/ft |
| 6.75 in | 2.75 in | 2.000 in | About 101.4 lb/ft |
| 6.75 in | 3.25 in | 1.750 in | About 93.4 lb/ft |
These examples show why the bore must be stated when a buyer asks for weight. A collar with a 3.00-inch bore has a calculated steel mass of approximately 97.5 pounds per foot before accounting for threads, recesses, chamfers, tolerances, and actual material density. For final logistics, I recommend using the supplier’s measured net weight and packing list rather than a theoretical value.
Drill collars are typically produced from high-strength alloy steel selected for demanding load conditions, but the exact grade and heat-treatment condition should be defined by the required specification and service environment. I avoid presenting one material grade as universally correct because fatigue, sulfide exposure, temperature, connection loading, and customer standards can change the selection. The purchase specification should identify chemical composition, mechanical properties, heat treatment, hardness limits, and traceability requirements where applicable.
Inspection scope should be agreed before manufacturing begins, especially when the collars will be exported or incorporated into a critical BHA. I can support buyers by organizing the required dimensions, documentation, inspection checkpoints, and packing instructions into one quotation package. This reduces the risk of receiving a product that is dimensionally correct but unsuitable for the mating assembly.
Start with hole size, expected dogleg severity, drilling direction, required weight-on-bit, mud flow rate, and the tools positioned above and below the collar. These factors help determine the acceptable OD, bore, length, connection, and quantity. I also recommend checking rig handling limits because each collar can be heavy enough to affect lifting, storage, and makeup procedures.
Confirm the exact thread designation from the mating tool or approved drilling program. Do not assume that a nominal 6 3/4 inch body uses one universal connection. If the connection is custom or obsolete, provide a sample, drawing, gauge information, or detailed dimensional data for technical review.
Request both unit weight and total shipment weight, with the calculation basis clearly identified. Lead time depends on raw material availability, machining capacity, heat treatment, thread work, inspection, and packing, so I recommend asking for a production schedule rather than accepting an unqualified delivery promise. Minimum order quantity may also vary according to material procurement and whether the order is standard or custom.
The most common mistake is specifying only “6 3/4 inch drill collar” without stating ID, length, connection, material, or inspection requirements. Another frequent issue is comparing supplier quotations using different weight bases, such as theoretical body weight versus measured finished weight. Buyers may also overlook thread protectors, packing dimensions, marking, spare connections, or export documentation until the order is ready to ship.
I recommend using a written technical checklist and obtaining drawing approval before production. If the project is time-sensitive, identify which dimensions are fixed and which may be optimized by the manufacturer. This allows the supplier to propose a practical solution without changing critical interface requirements.
At Longway, I approach each inquiry as a complete steel-pipe and drilling-tool specification review rather than a simple OD quotation. I can help organize the required dimensions, thread information, material requirements, inspection documents, packing details, and delivery expectations for a 6 3/4 inch drill collar order. For custom requirements, the most useful starting information is the nominal OD, ID, length, thread designation, quantity, service application, destination, and requested documentation.
Before I issue a final offer, I recommend confirming the technical drawing and acceptance criteria. This gives the buyer a clear basis for comparing price, lead time, quality controls, and logistics. It also helps prevent costly revisions after machining or thread cutting has started.
A 6 3/4 inch drill collar is defined by more than its 6.75-inch nominal outside diameter. The inside diameter controls wall thickness and theoretical weight, the thread determines connection compatibility, and the length and material condition affect handling and application suitability. Reference calculations can support early planning, but final weight and acceptance must come from the approved design and inspection records.
My practical recommendation is to prepare a complete inquiry containing OD, ID, length, thread, material, quantity, inspection scope, packing, and delivery destination. Send that information to Longway for a technical review and quotation based on your actual BHA requirements. With the correct specification confirmed in advance, you can compare suppliers more accurately and reduce sourcing risk for your 6 3/4 inch drill collar project.
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