Checking Rod Bore with ARP Bolts

[2jmotorsports]

Checking the rod length itself is a little tedious for the average DIY guy. You need to bore gauge the two bores, take half of them, then measure the rod length from "apex" to apex with something like a 0.001" caliper. Add that length to your two half bores and you'll get your center to center length. Not sure how accurate that is, but that's how i've done it and got reasonably repeatable measurements.

A-ha! Thanks for this! I was scratching my head trying to figure out how to measure between 2 theoretical centers reliably. I dont exactly have a CMM in my garage.

I was going to stick a wrist pin in the small end, gently clamp a rigid straight edge on the big end and put some calipers between the bottom of the pin and the straight edge and add half the pin diameter. I like your method better since I already measured the bore of the large ends. If there is any error in this at least you can still check consistency between rod lengths.

[m42racer]

Originally Posted by KTL

To m42racer's point about same CR number in each cylinder, also worth noting are the cylinder heads themselves. The heads as cast from the factory are not really consistent in terms of chamber volume. So what you end up doing, from an economical/simple standpoint, is taking the head that has the greatest volume and using that for your baseline. The other heads can be massaged around the spark plug port (ports if twin plugged) to increase their volumes to match the baseline head.

Checking the rod length itself is a little tedious for the average DIY guy. You need to bore gauge the two bores, take half of them, then measure the rod length from "apex" to apex with something like a 0.001" caliper. Add that length to your two half bores and you'll get your center to center length. Not sure how accurate that is, but that's how i've done it and got reasonably repeatable measurements.

Good point. It cements the idea that every part needs to be checked and measured.

If the engine case is flat and parallel to the main housing, the crank is measured the same for stroke on each rod journal, the base gaskets are all the same thickness, the cylinders are the same height, the piston comp heights are all the same, any change in piston deck height is rod length error. A lot of work, but this is how its done in the pro world. Often they are up against a CR regulation and they run right on the number along with piston to head clearances so close only the carbon separates the two.