Rod and main bolts-replace?

FWIW, my first job out of college was developing torquing specs for the assembly of jet engines. This assignment taught me a lot about threaded fasteners. Here are a few things to keep in mind;

1. All fasterners have some amount of elasticity and is refered to as modulus of elasticity. As you tighten a bolt it stretches. If you do not exceed the yield strength of the bolt it will return to it's orignal length when the torque is released.

2. A properly designed and tightened threaded fastener will apply more clamping force than there is force to seperate the joint (combustion pressure, thermal expansion, dynamic forces such as the piston stopping and changing direction at BDC and TDC). What this means is the bolt does not stretch during normal use. And this is also why an over heated (abnormal conditions) engine quite often blows the head gasket.

3. There are two types of fastners used in automotive engines. Torque to yield and non torque to yield. Torque to yield has come into favor because using a torque wrench to tighten a bolt is only slightly more accurate than doing it by feel. The point at which the metal yields in a fastener is very consistent and once that happens you can no longer increase the clamping force. This makes for very even and consistent clamping force across a bolted surface such as a head. This is much more important in a clamped joint made from dissimilar materials such as aluminum heads and iron blocks.

3. A torque to yield bolt will always have a shank that is slightly smaller than the root diameter of the threaded section so the yield will occur in the shank and not in the threads. This also contributes to consistency in the clamping force between fasteners.

4. Torque to yield bolts are a one time use item. The reason being is that each time they are tigthened the bolt stretches and takes a permanent set which also results in a reduction is diameter of the bolt. Even though the material on subsequent tighteneing cycles has the same strength in pounds per square inches there is less square inches so you have less pounds of clamping force.

5. A non torque to yield bolt operates below the yield point so it never stretches and takes a permanent set so it can always provide the same clamping force. As stated in #2 a properly tightened joint there is no motion so there will not be any fatiguing of the fastener over time. So in theory there is no reason to change a bolt that has not been over tightened, over heated or otherwise subjected to anything that can compromise the characteristics of the bolt. The question is has anything happened to the fastener in it's life time?

6. Another consideration; High quality fasteners from outfits like ARP will roll form the threads verse cutting them on a less costly fastener. Disregarding the superior material these fasteners are made from the rolled thread has less friction associated with it so you will receive higher and more consistent clamping force with the same torque. If you use the lubricant they provide you get even higher and more consistent clamping force which now takes advantage of the superior material. Down side of this is that the clamping forces increase to a point that it starts to distort the parts being fastened. That is why mains and rods need to be resized (or at least checked) when stepping up to these types of fasteners.