Head Bolts: The Torque-to-Yield Fasteners You Can't Reuse

Modern engine head bolts are torque-to-yield (TTY) — they’re tightened in stages, with the final stage being a specific angle of rotation beyond yield. The bolt stretches plastically (permanently) during this final tightening, which gives a more consistent and higher-tension clamping force than a torque-only specification. The downside: TTY bolts can’t be reused. Each new head gasket installation needs new head bolts.

Why TTY is used

A TTY tightening procedure achieves higher and more consistent bolt tension than a torque-only procedure can. The angle-of-rotation step beyond initial torque exploits the bolt’s yield strength precisely, giving each bolt the same final stretch and the same clamping force. This is especially important with multi-layer steel head gaskets — the gasket needs precise clamping to seal correctly across the full operating temperature range.

Why they can’t be reused

The first tightening permanently stretches the bolt. The bolt is now longer than its original specification. Tightening it again to the same angle wouldn’t achieve the same stretch — and the actual clamping force becomes unpredictable. Used TTY bolts can also fatigue and break under repeated tightening.

What’s in a head bolt set

The set covers all the bolts needed for the head — typically 10-12 main bolts on a 4-cylinder, more on 6 or 8. Different bolts may be different lengths (the corner positions sometimes use longer or shorter bolts). The bolt set comes with the correct specification for your engine.

Installation procedure

Always follow the manufacturer’s torque sequence and the specified tightening stages. A typical TTY procedure: 1) hand-tight all bolts; 2) tighten in sequence to a specified torque (e.g., 40 Nm); 3) tighten in sequence to a second torque (e.g., 60 Nm); 4) tighten in sequence by a specified angle (e.g., 90 degrees); 5) final angle stage (e.g., another 90 degrees). The exact procedure varies per engine — consult the workshop manual.

Brand

Elring, Victor Reinz, BGA cover OE-equivalent head bolt sets for most European engines. Mahle and Ajusa also strong. The bolt material specification (grade, coating) varies per engine — match the OE part number, not just the bolt count and size.

Find head bolt sets for your engine on the Head Bolts collection. Always new bolts with a new gasket — non-negotiable on modern engines.