MEA­SUR­ING ROD-BOLT STRETCH

NZ Performance Car - - Inside Line -

Be­fore drop­ping the pis­tons into the cylin­ders, the gud­geon pins and cir­clip were in­stalled. Rich looked to be hav­ing great fun in­stalling the pins — not. Be­fore drop­ping them in, he did one fi­nal check to en­sure the rod bolts were to spec. Rod bolts are the most stressed fas­tener in the en­tire as­sem­bly and get worked every ro­ta­tion of the crank. The higher the rpm, the greater the load on the bolts, so achiev­ing the cor­rect amount of clamp­ing force is in­te­gral to en­gine re­li­a­bil­ity — in other words, to not throw­ing a rod. If you over-tighten dur­ing as­sem­bly, you risk the bolt over­stretch­ing dur­ing high rpm, when the load on the bolt is max­i­mized. Why?

Be­cause a rod bolt is a sprung piece of steel, which achieves its max­i­mum clamp­ing force when stretched close to its max­i­mum elas­tic­ity. But that does not mean you want to tighten to this point dur­ing as­sem­bly. You need to leave a buf­fer, be­cause, when un­der max­i­mum load, at high rpm, the bolt will stretch fur­ther and could stretch be­yond its max­i­mum elas­tic­ity. Once a bolt has reached this point, it will no longer re­turn to size and, in­stead, will re­main per­ma­nently stretched. It is for this rea­son that mea­sur­ing the rod­bolt stretch is pre­ferred to torque­ing the bolts, as the torque method mea­sures the fric­tion, which is af­fected by as­sem­bly lube, etc., giv­ing you no in­di­ca­tion how close the bolt is to over stretch­ing.

Rich first used a torque wrench to 94.9Nm, and then an ARP rod-bolt gauge and checked them be­fore in­stalling. It’s a good thing he did that this time, too, as he found two of the brand-new bolts from ARP to be well out­side the 0.004 to 0.007 stretch. Not want­ing to risk it, he or­dered and in­stalled a full new set of higher-grade ARP2000 bolts.

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