Torque-to-yield (TTY) bolts: angle tightening and why not to reuse them
A torque-to-yield bolt is tightened to a modest torque, then turned a set angle that stretches it permanently. GM's rule is blunt: a torque-plus-angle-to-yield fastener must not be reused and must always be replaced if loosened. Some makers allow reuse after a stretch check; only the service information for that bolt can tell you.
Contents8 sections
Key takeaways
- GM names three spec types: Torque (T), Torque Plus Angle (TA) and Torque Plus Angle to Yield (TAY, also called TTY) 1.
- A TAY spec leaves the bolt permanently deformed. GM: it must not be reused and must always be replaced if loosened 1.
- The angle cannot be converted to ft-lb. Use an angle gauge; FCA says not to use a torque wrench for the angle step 2.
- Reuse rules differ by maker and bolt: Honda measures head bolt diameter, FCA checks for necked threads, GM replaces 3 2 1.
- Angle tightening past yield controls preload far better than torque alone: ±3 to 10% versus ±15 to 30% 4.
Quick facts
| Spec format | A torque, then a stated angle in degrees |
|---|---|
| Preload accuracy, torque past yield | ±3 to 10% 4 |
| Preload accuracy, torque only | ±15 to 30% 4 |
| Reuse | Only if the service information allows it |
What TTY means
Every bolt behaves like a spring up to its yield point. Tighten it within that elastic range and it springs back to its original length when you loosen it. Tighten it past yield and it stays longer: it has been plastically deformed.
GM’s service bulletin on fastener reuse describes three ways a torque can be specified 1:
| Spec | How it’s tightened | What happens to the bolt | Reuse |
|---|---|---|---|
| Torque (T) | Torque wrench, one pass | Elastic stretch | Generally reusable unless the service information says otherwise |
| Torque Plus Angle (TA) | Torque, then a set angle | Elastic stretch | Generally reusable unless the service information says otherwise |
| Torque Plus Angle to Yield (TAY, or TTY) | Torque, then a set angle | Permanent deformation | Must not be reused; always replace if loosened |
Scroll the table sideways to see every column.
The TA and TAY specs look the same on paper, for example 30 N·m (22 lb-ft) + 120 degrees 1. You cannot tell from the numbers alone whether a bolt is single-use. GM’s service information flags them with a warning to install a new torque-to-yield or single-use fastener 1.
Why engines use them
Clamp load. GM ranks the three: a TA spec gives more clamping force than a T spec, and a TAY spec gives more than TA. GM’s example is an M12 bolt with a TAY spec replacing an M16 bolt with a plain torque spec, saving weight 1.
Consistency is the other reason. Most of a torque goes into friction, so torque alone controls preload loosely. The tightening-accuracy table reproduced in NASA RP-1228 rates torque control at ±15 to 30% of preload and torque-past-yield at ±3 to 10%, using “soft” bolts tightened well past the yield point with a consistent snug torque 4. Bolt Science notes yield-controlled tightening has been used in critical applications such as cylinder head and connecting rod bolts to get consistently high preloads 5. The angle bypasses most of the friction: once a joint is snug, further nut rotation stretches the bolt in direct proportion and is “not affected by variations in friction” 6.
Why you can’t reuse them
A bolt that has yielded is longer, and it necks down where it stretched, which is what FCA’s straightedge check looks for 2. Bolt Science notes that a bolt tightened beyond its elastic range “can only sustain a limited number of re-applications before it fails” 5. Reinstalling it and turning it the same angle again stretches it further from an already-yielded start.
GM’s explanation in the bulletin: with T or TA, the bolt “returns to its original length” when loosened; with TAY it “is overstretched (plastically deformed) and does not return to its original length. For this reason, the Torque Plus Angle to Yield (TAY) tightening specification requires the externally threaded fastener to ALWAYS be replaced” 1. The nut or threaded hole is not affected, and a nut can be reused 1.
It is not only engines. Subaru changed a rear stabilizer bracket bolt to a new style with a special thread lubricant and made it one-time use, while the original bolt stayed reusable at the same 35 N·m 7. Single-use status belongs to a specific part number.
Makers’ reuse rules differ
Some makers allow a torque-plus-angle head bolt back in after a check. Follow the rule for the bolt in front of you:
- GM: TAY fasteners are always replaced 1.
- FCA (Pentastar heads): the bolts are tightened by torque plus angle and “must be examined BEFORE reuse. If the threads are necked down the bolts must be replaced.” The check is a straightedge held along the threads: if all threads don’t touch it, replace the bolt 2.
- Honda (2013 Odyssey and Pilot cylinder heads): measure each head bolt’s diameter at 50 mm and 45 mm from the threaded end and replace it if either is under 11.3 mm. New bolts get an extra 90° compared with reused ones 3.
How to tighten one
This is the pattern the procedures share. The values belong to the examples cited; use the numbers from your own service information.
Get new bolts if they are single-use
And any one-time-use gasket or washer the procedure lists.
Prepare the threads as specified
Clean blind holes; FCA warns that oil, coolant or sealant left in a hole can crack an aluminum casting when the bolt is tightened 2. Lubricate exactly as written: Honda applies new engine oil to the threads and under the heads 3, FCA says not to add oil to the threads 2.
Torque stage in sequence
Every bolt to the first torque in the printed order, then the next torque stage. Honda specifies a beam-type torque wrench for its 29 N·m (22 lb-ft) stage 3.
Angle stage in sequence
Turn each bolt the specified angle, relative to the mating part 1. Use a torque angle gauge: Subaru requires both a quality torque wrench and an angle gauge, each calibration-checked if not calibrated in the past 6 months 8. If the angle can’t be made in one pull, FCA allows splitting it, for example 125° as 35° then 90° 2.
Don’t back off an overshoot
Honda: if a bolt goes past the specified angle, go back to the start of the procedure; “Do not loosen it back to the specified angle” 3.
Listen
Honda and Subaru both say that a bolt that creaks or doesn’t tighten smoothly is loosened and the procedure restarted 3 8.
What not to do
- Don’t convert the angle to torque. There is no ft-lb equivalent of “+ 90°”. The point of the angle step is that it doesn’t depend on friction.
- Don’t use a table torque on an angle-spec bolt. Our calculators produce elastic-range torques at 75% of proof load. They don’t apply to TTY bolts.
- Don’t re-torque a TTY bolt “to check it”. Turning it further takes it further past yield. Re-tighten only if the procedure says to.
For the order of the passes, see torque sequence.
Sources
- 1
- 2
- 3
- 4
- 5Bolt Science Ltd, Bolt Science, Methods of Tightening Threaded FastenersChecked 21 Oct 2026
- 6
- 7
- 8
Spotted a value that doesn’t match its source? Report an error
Related
- Torque sequenceWhy multi-bolt joints are tightened in a star or crisscross pattern and in several passes, with the sequences GM, FCA, Subaru, Ram and NASA actually specify.
- Torque vs tensionTorque only estimates bolt tension: about 90% goes into friction, so preload scatters ±25% or more. The figures from NASA-STD-5020B, NASA RP-1228, Fastenal and RCSC.
- How to use a torque wrenchHow to set and pull a click, beam or digital torque wrench, what ±4% accuracy means under ISO 6789, when to recalibrate (12 months or 5,000 cycles) and what not to do.
- Extension calculatorFind the torque wrench setting with a crowfoot or adapter: T_set = T × L ÷ (L + E). 100 ft-lb through a 2 in adapter on an 18 in wrench means setting 90.