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

You’re rebuilding an engine. The service manual says head bolts are “torque-to-yield” and must be replaced. New bolts cost $150. Can you really not reuse the old ones?

No. You really can’t. Here’s why.

What Torque-to-Yield Means

Standard bolts are tightened to a specific torque value. They stretch slightly within their elastic range—like a spring that returns to shape.

Torque-to-yield (TTY) bolts are intentionally stretched past their elastic limit, into the plastic deformation zone. They don’t spring back. They’re permanently stretched.

Why do this? Because a stretched bolt provides more consistent clamping force. It’s actually stronger when holding things together.

The Torque-Angle Spec

TTY bolts use a two-step tightening process:

1. Initial torque: Snug the bolt to a base value (e.g., 25 ft-lbs)
2. Angle turn: Rotate the bolt a specific number of degrees (e.g., 90° + 90°)

The angle turn is what stretches the bolt into yield. You’re not measuring torque at this point—you’re measuring rotation.

Why This Is Better

Consistent clamping force: A torque spec alone is affected by friction, thread condition, and lubricant. Two bolts torqued to 60 ft-lbs might have very different actual tension.

The angle turn removes this variable. The bolt stretches the same amount regardless of friction. Clamping force is more predictable.

Higher clamping force: TTY bolts operate at higher stress levels than conventional bolts. More clamping force means better head gasket seal, more resistant to loosening.

Lighter weight: Because TTY bolts work harder, engineers can use smaller bolts. Modern engines save weight this way.

Why You Can’t Reuse Them

Once a TTY bolt has been stretched past yield, it’s permanently deformed. Reusing it means:

• Starting from an already-stretched position
• The bolt may stretch further than intended
• Clamping force becomes unpredictable
• Risk of bolt failure (breaking during tightening or in service)

A $150 set of head bolts versus a $3,000 engine rebuild from a failed bolt. Easy math.

Common TTY Applications

Cylinder head bolts: Most modern engines use TTY head bolts. The high clamping force is critical for head gasket sealing.

Main bearing cap bolts: Many engines use TTY bolts here too.

Connecting rod bolts: Some (not all) are TTY, especially in performance applications.

Flywheel/flexplate bolts: Some manufacturers specify TTY here.

Intake manifold bolts: Less common but some modern engines use them.

Always check the service manual. If it says replace, replace.

How to Identify TTY Bolts

Service manual: The definitive source. Will specify “replace” or “torque-to-yield.”

Torque spec includes angle: If you see “25 ft-lbs + 90° + 90°”, it’s TTY.

Physical inspection: TTY bolts often have identifying marks. Some have a visible “necked down” section. But don’t guess—check the manual.

Ask the parts store: They can often tell you if a bolt is TTY for your application.

The Tightening Process

Proper TTY installation:

1. Clean threads: Both bolt and hole. Use a thread chaser if needed.
2. Lubricate correctly: Use whatever the manual specifies (oil, moly paste, etc.)
3. Initial torque: Bring all bolts to first-stage torque in sequence
4. Angle turn: Use an angle gauge or torque-angle adapter
5. Follow pattern: Always tighten in the specified sequence (usually inside-out)

Some bolts require multiple angle stages (90° + 90° done in two steps). Follow exactly.

Tools You’ll Need

Torque wrench: For the initial torque stage. Beam type or click type.

Angle gauge: Sits on the socket, shows rotation in degrees. About $20.

Torque-angle adapter: More precise, measures angle electronically. $50-150.

Some modern torque wrenches have built-in angle measurement. Worth the investment if you work on modern engines.

What If You Over-Rotate?

If you turn past the specified angle, the bolt is overstressed. Replace it.

If you’re doing multiple angle stages (90° + 90°) and over-rotate the first stage, you’ve over-stretched early. Replace it.

TTY bolts have no room for error. They’re engineered for a specific total stretch.

Aftermarket Alternatives

For some applications, you can buy aftermarket studs instead of TTY bolts:

ARP studs: Designed to be torqued (not angle-turned) and reused. More expensive but eliminate the replace-every-time cost.

Other brands: Various manufacturers offer similar products.

For race engines or builds where the head comes off regularly, studs can be cost-effective. For a one-time rebuild, factory TTY bolts are fine.

The Bottom Line

TTY bolts exist because they provide better, more consistent clamping than conventional fasteners. The trade-off is they can only be used once.

When the manual says “replace,” it’s not a suggestion. It’s not the dealer trying to sell you parts. It’s physics. A stretched bolt can’t be stretched the same way twice.

Buy the new bolts. Your engine will thank you.