Whether you are performing a complete engine rebuild or just replacing a head gasket or bearings, it is important to know when bolts may be reused and when they should be replaced. Reinstalling bolts that are compromised can lead to blown gaskets or catastrophic engine failure.
One of the first things to remember is that a bolt with any damage, no matter how slight, should be replaced. Bolts that are damaged are likely to fail under pressure, resulting in engine damage.
By design, bolts stretch when they are tightened and spring back to their original length when the tension is removed. However, if a bolt is tightened beyond its spring back point, it will remain permanently stretched.
Torque-To-Yield (TTY) bolts are used in many applications including cylinder head bolts, connecting rod bolts, and main bearing cap bolts. These bolts are designed to be torqued beyond their spring back point to maintain a consistent clamping force. Never reuse TTY bolts. Since they are permanently stretched the first time they are used, they could break or provide insufficient clamping force if reused. The result is blown gaskets or connecting rod damage leading to total engine failure. Always check your engine service manual if you are unsure if you have TTY bolts.
Even if you have non-TTY bolts, it is advisable to replace them if they are old. Bolts can weaken over a lifetime of heat/cold cycles. It is a good idea to check thread pitch with a pitch gauge to see if the threads are stretched.
Although it may be tempting to reuse bolts if they appear to be in good condition, the damage caused by a broken or overstretched bolt will cost much more time and money than exchanging the bolts in the first place.
Jordan Good
HooverTec
