Carl, there have been many schools of thought on this topic over the years. And whether increasing the rod length enhances the scavenging of those things is often debatable. I think initially back in the early days of the Stovebolt's popularity, enthusiasts did if for different reasons than they do today. First, aftermarket rods were still 25+ years away from being commonly available back in the 40's, and they no doubt saw the svelte GMC rod as a great upgrade in strength over the "pinchbolt" Stovebolt rods, plus it was an easy upgrade to do and inexpensive to find GMC rods. The GMC rods offered inserted bearings compared to the babbited Chevy rods. Second, they were longer than the Chevy rods. At 7.000" compared to the Chevy's 6.812", they provided the advantage of being able to use a lighter piston because of that fact. The GMC rods also had a full floating wrist pin in its S/E with a bushing, instead of the Chevy's "pinchbolt", and it was able to provide full oil pressure to the wrist pin via an oil hole that ran the entire length of the rod beam from the housing bore where the bearings are installed all the way up. And it even sprayed oil to the underside of the piston crown. Fast forward 60+ years, several aftermarket racing connecting rod companies are now today offering their rods with this feature as an upgrade for extreme racing conditions like NASCAR and other severe duty racing environments. And many high boost turbo and supercharger guys are benefiting from the ability to cool the underside of their pistons in these often high combustion chamber temps and ultra high chamber pressures that occur with these engine mods. Talk about being ahead of your time in innovation, GMC was on that one. So whether or not this rod swap gave them any true increases in performance or any other advantages, it definately gave them a peace of mind.
Increasing the stroke (Stroking),
although a viable method on the Ford Flathead, is not recommended for the Chevy 235. Internal clearances between rod and cylinder base greatly restrict stroking. It has been done but is quite expensive and limits reliability. The small gain in CID just isn’t worth it. However, since the CID is proportional to the mathematical square of the bore, increasing the bore (Boring) yields a much higher gain in CID than stroking and is highly recommended. So how much bigger can we make the bore? The block can be over- bored 1/8" to a diameter of 3-11/16" increasing the displacement to 255 CID. However, the block must be closely inspected for bore alignment with the casting. Detroit gets a little lax when casting and machining the blocks so if you want to bore this big, start with a well aligned block. Otherwise you may make the cylinder walls too thin. Also keep in mind that you have to find pistons of this size to fit the bores. Good luck! A good over-bore is .060". This raises the displacement to 244 CID and makes the engine competitive with the Watertown and Waterloo 260 Flatheads of the era and still retains reliability. These .060-over pistons are still available as stock replacement parts and special racing equipment.
article by Larry Witter
