A bolt acts like a spring. When it is tightened it stretches a little, and this stretch holds the parts together. A bolt can be checked by turning force (torque) or by its length (stretch). Some connecting rod bolts are set by stretch: the manual says if so, and gives the length.
Practice on a junk bolt
- Measure the length of an old bolt with calipers or a micrometer: Choose and use calipers and micrometers. Write it down.
- Put washers and a nut on it and clamp the head in a vice.
- Tighten the nut to the torque in your manual for that size and grade: Use a torque wrench correctly.
- Measure the bolt again. It will be a little longer.
- Loosen it and measure it a third time. A good bolt returns to its first length. A bolt that stays longer has failed.
- Tighten a second bolt until it gives way, so you know how it feels when a bolt yields: the wrench turns on without the force rising.
In the engine
- If a bolt gives way (yields) as you tighten it, replace it. Do not reuse it.
- Rod bolts, cylinder head bolts and studs are under high stress. Replace any that are stretched or damaged: Bolts, nuts, studs and washers.
- Check rod bolt stretch only where the manual calls for it.
Check your work
- You know how long a good bolt is, and what a bolt that has taken a set looks like.