One of the most important measurements you can take when assembling an engine is the rod bolt stretch. Why is it so important? As we find out in this video from ThatRacingChannel and narrated by Real Street Performance‘s Jay Meagher, measuring rod bolt stretch is the most accurate way to determine the fastener’s clamping load.
A rod bolt can be thought of as a stiff spring, and it needs to be stretched a specific amount in order to provide the clamping force required to keep the connecting rod bolted together.
If the bolt is not stretched enough, there may not be enough clamp load to maintain the rod cap’s position, which can result in a spun bearing, or worse – a broken rod bolt. Nobody wants to deal with a broken rod bolt that can potentially destroy your brand-new engine.
Conversely, if the fastener is stretched too much, the yield strength of the fastener is compromised, with the same potentially fatal result for the engine.
As Meagher explains, the bolt should be checked in the relaxed state prior to tightening; he uses a bolt stretch gauge made expressly for this purpose by the fine fastener experts at ARP. The gauge is placed into two recesses in the fastener with one tip at each end to set the gauge’s zero point.
He makes a number of good points through the video. Most importantly, that the builder is using a known good, calibrated torque wrench, and that the fastener is torqued in one clean sweep. Each fastener will have its own stretch requirements to provide proper clamping – check with your manufacturer for the specifications.
For more information on the complete line of ARP products, check out their website.