replacing bolts

[lowgearfarm]

I get occasionally get tech-email from hydraulicsupermarket.com and I found this morning's post relevant enough to share. Full disclosure: I've edited the original for brevity
Unlike bolts used on engine heads, most of the bolts used to assemble hydraulic components and port flanges are not 'torqued to yield'. In other words, bolts used on engine heads are torqued beyond their elastic limit and therefore using them a second time is fraught with danger. This is generally not the case with bolts used to assemble hydraulic components. In a review of a large body of hydraulic component manufacturers repair instructions covering pumps, motors and cylinders none of the instructions specify that pump rear cover or cylinder head bolts be replaced with new. This is not to say that all bolts can go round and round indefinitely. Therefore, in practice, as in a repair shop situation, there is no way of knowing for sure how many times a set of bolts has been used and it may be prudent to crack check bolts using dye penetrant.
There is another reason for NOT replacing bolts unless absolutely necessary. High tensile bolts and cap screws are not cheap, so, like a lot of industrial parts these days, there is the possibility of counterfeiting between grades. Do you know for sure that a 10.9 grade cap screw is not 8.8 (or lower) being passed off as 10.9? So, if you're going to throw perfectly good bolts in the scrap bin, you'd better make sure the bolts you replace them with are the genuine article from a reliable source.

[BigBill]

As an 7 year old kid I changed the bolts on my bicycle front end a jc Higgins with a front spring to softer bolts not knowing about bolt hardness. I got 52 stitches in the head for that mistake plus a education about bolt hardness from the oldman.

[inairam]

I was in aerospace for 10 years. People whet to jail for passing off inferior bolts to the military as high grade bolts.

In a high cycle stress applications like a cylinder head, the main reason for the torque is not clamping force but to put the bolt under a stress higher then what the cycle stress the bolt will see ( cylinders firing) and this will extend the life for the bolt due to increase the fatigue life of a bolt.

Torque to yield bolts are a relatively new idea. I do not think there were around in the 1940s when the C60 was designed. I think there were introduced for the application using aluminum heads on cast iron blocks which is not the case of C60.

With a cast iron head and block, a strong bolt torqued correctly is the way to go.

[BigBill]

Remember the new Mack trucks years ago had the wrong grade bolt on there steering pitman arms. Not good when your rolling 75 k pounds