One of our members wrote me this question:

“In hydraulic systems and in hydraulic components in particular, bolts are everywhere. Should we change bolts every time we open them, for example, on a hydraulic cylinder head joint? If not, how many times can bolts be re-torqued?”

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.

I worked in hydraulic repair shops for a lot of years, and I can’t remember replacing a single set of bolts on a pump, motor or cylinder as a matter of routine. But just in case I may have missed something, in preparing to answer this question I reviewed a large body of hydraulic component manufacturers repair instructions covering pumps, motors and cylinders. And none of these instructions specify that rear cover or head bolts be replaced with new.

This is not to say that all bolts can go round and round indefinitely. But in practice, in a repair shop situation, there is no way of knowing for sure how many times a set of bolts has been used. And with this in mind, I have certainly crack checked a lot of bolts with dye penetrant over the years. Particularly large hydraulic cylinder head retaining bolts-the subject of this member’s question.

Given all of the above, there is a case for not replacing bolts unless absolutely necessary. Reason being, high tensile bolts and cap screws are not cheap. Which means like a lot of industrial parts these days, there is the possibility of counterfeiting between grades. How do you know for sure that a 10.9 grade cap screw is not 8.8 (or lower) being passed off as 10.9? The point being 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. In other words, discarding perfectly serviceable bolts can turn out to be a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.