Cylinder Head Screw Inch BSW

What is a BSW Cylinder Head Bolt?

The BSW cylinder head bolt (British Standard Whitworth) is a special bolt with a British imperial thread according to the Whitworth standard. The defining characteristic of BSW threads is a 55° flank angle, whereas metric threads and the US UNC/UNF system use 60° .

BSW is the British coarse thread and is part of the historical Whitworth thread standards, along with BSF (British Standard Fine) . These bolts are primarily used in classic British vehicles and machinery.

Applications

Cylinder head bolts with BSW threads are typically found in:

• Classic British automobiles (e.g., Jaguar, MG, Triumph)

• Vintage motorcycles (e.g., BSA, Norton)

• Restoration projects of classic and vintage vehicles

Technical Specifications

An important point about BSW cylinder head bolts is the spanner/socket size: Whitworth threads use larger hexagon sizes compared to comparable metric or UNC threads. For example, a 3/8" BSW bolt requires a 0.6" wrench (marked 3/8" BS), while a 3/8" UNC bolt uses a 9/16" AF wrench .

Important Note on Confusion

Caution: In modern automotive contexts, "BSW" is often used as an engine code – for instance, BSW designates the 1.9 TDI engine from VW and Skoda with 105 HP . This refers to the engine type, not the thread of the cylinder head bolts! These engines use modern metric cylinder head bolts (e.g., M12 x 1.75) . Therefore, always verify before purchasing whether you actually need a Whitworth thread bolt or a part for a BSW engine.