TTY Bolts Explained — Torque-to-Yield Fasteners
A torque-to-yield bolt is designed to be tightened past its elastic limit so it stretches permanently and holds clamp load through that stretch rather than through friction. That design is why TTY bolts clamp so evenly — and why they must never be reused.
What a torque-to-yield bolt actually is
A conventional bolt is tightened within its elastic range: it stretches slightly, behaves like a stiff spring, and returns to its original length when released. A TTY bolt is deliberately taken past its yield point so the shank deforms permanently. The payoff is very consistent clamp load across every bolt in a pattern regardless of small differences in thread friction, which is exactly what a cylinder head gasket needs.
How to identify one
TTY bolts are usually longer and thinner than you would expect for the load, often with a reduced-diameter waist along the shank. The decisive clue is the torque procedure: any spec written as an angle rather than a number — "30 ft-lbs, then 90 degrees, then a further 90 degrees" — is a torque-to-yield fastener. If the service manual gives you a torque-plus-angle sequence, treat the bolt as single-use.
Why you cannot reuse them
A TTY bolt has already used up its plastic deformation on the first installation. Re-torquing a stretched bolt to the same angle takes it further along a curve it has already climbed, so it reaches its breaking point at a lower load than a new one. It may survive assembly and then fail weeks later under thermal cycling — typically as a blown head gasket or a bolt snapped off in the block. New bolts every time, without exception.
Where they are used
Cylinder head bolts are the most common application, along with connecting rod bolts, main bearing cap bolts, and many flywheel and crankshaft pulley bolts. Ford has used TTY head bolts as standard since the early 2000s, and most modern aluminium-head engines from any manufacturer use them. Always check the service literature rather than assuming an old-style reusable bolt.
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