By: Carl Amundsen Date: March 20, 2000
Evidently a lot of people have read and paid attention to our editorial " Oil Clearance is Not a Myth But a Calculation ". We appreciate all the calls and hope it has helped save some engines.
A constant question that keeps coming up is about the bore size on the stroker rod for the .875 (7/8) journal crankshaft.
Why are the rod bores on some brands of stroker rods as much as .004" out of round, right out of the box?
These rods all have angled or what is termed "splayed rod bolts". This is a necessary design because of the amount of stroke in the crankshaft and the need for clearance at the camshaft and inside the block.
It is easy to see from the illustration that the seat area under the rod bolt and the seat area of the cap will be at an angle to the parting surfaces of the rod assembly. This is where the problem begins.
During the manufacturing process the rod and cap are assembled together and the bolts are tightened to a specific torque value. The final machining of the bore now takes place, and comes out on size and round. The next thing that happens is that the rod bolts are removed and new ones are installed. Here is your problem. Because of the adverse angle at which the bolts enter the seam of the rod, they pull the bore out of round. It will always come out egg shaped, and the more you play with the torque values the worse it gets. This is not a one out of ten problem this will happen 100 times out of 100 times. It can be bad enough to lock the motor up. Bet on it. THE PROBLEM IS IN THE DESIGN.
To add insult to injury, some rods are designed with the rod bolts exposed to the bore. This just reduces the integrity of the bore and weakens the seam area. If that's not enough, you must grind some clearance into the back side of the bearings to accommodate the rod bolts.
THE FOLLOWING IS AN ILLUSTRATION OF THE ARC DESIGNED STROKER ROD, NOTICE THAT THE BOLTS AND THEIR PARTING EDGES ARE PERPENDICULAR TO ONE ANOTHER. THIS DESIGN ELIMINATES THE PROBLEMS CAUSED BY THE ANGLED BOLT TO PARTING EDGE SITUATION. OUR WAY STILL ISN'T PERFECT, BUT IT IS A HUGE STEP IN THE RIGHT DIRECTION.