The Stovebolt Site Forums The Shop Area The HiPo Shop Connecting rod and main bearing diameters

The old engines we like to tinker with are the product of early 20th. Century engineering and design practices, and at the time they were on the drawing board, the practice was to over-engineer everything. That's why a stovebolt six engine weighs in at around 700 pounds. It's also why connecting rod and main bearing journals are so big- - - - -there's probably 20 pounds or more forged steel in a stovebolt crankshaft than is really necessary to handle the power these engines are capable of producing. Ditto for connecting rods, pistons, and other components. Of course, the engines were designed to be rebuilt frequently- - - -not the "One and done- - - -maybe it will make it through the warranty" mindset that came around sometime in the 1970s and continues to this day.

Those design features can work to our advantage, however, if we're willing to do a little research and think outside the box where engine parts are concerned. Why go searching for parts that went out of production 50 years ago, when a little detective work can find an "almost right" modern part that will serve the same purpose? It's also possible to put modern, lighter weight parts inside a stovebolt engine block, and get a considerable increase in cubic inches, and still keep the outward appearance of an "original" engine. By reducing the diameter of crankshaft bearing journals, there's also a noticeable reduction in rotational drag, and the less internal friction the engine has, the more power goes out the back end to drive the vehicle. Ditto for piston and connecting rod weight- - - -with a lighter assembly, less energy is consumed by the engine to keep itself running, and that gain is also translated into more usable power to the wheels. The sticking point is finding someone with the equipment and the knowledge to use it, to make those "different" parts work together. I've watched some very talented people make mismatched parts from oddball sources work together for many years, and I know what's possible. Getting laser focused on "stock" parts might be good for a museum curator, but hotrodding these old dinosaurs is a lot more fun!
Jerry

A guy over on Inliners about 12 or so years ago built a GMC 302 race engine and used rods out of a Hudson in it. He was able to used forged pistons for a SBC in it and he said the combo allowed him to reduce the overall rod, piston and wrist pin weight by almost 2 pounds per cylinder....

It looks like I can use rods for a flathead Ford 85 HP V8 and pistons for a Japanese Diesel in a 216, and end up with a 250+ cubic inch engine with a major reduction in rotating and reciprocating weight, plus having smaller rod and main bearings. The same basic concept can be applied to 235 and 261 engines, with displacement of approximately 270 and 290+ cubic inches. There's also a custom length H beam rod for a Chevy big block V8 that has some possibilities, especially in the 261 block.
Jerry

But wouldn't having Blue Oval, Nippon and Chebbie parts in the same engine cause spontaneous combustion?

More like spontaneous acceleration! I once ran a VW beetle with piston rings for a flathead Ford, and wrist pins for a Corvair. J.C. Whitney wouldn't sell me rings- - - - -they wanted me to buy a whole big bore kit when I needed to do a ring job- - - -cylinders, pistons, and all! The connecting rod bushings had enough wear that I needed oversize wrist pins- - - - -Corvair wrist pins were about .015" bigger than the VW pins. Sunnen hone to the rescue!

Apparently the 216 would stay together spinning fairly fast .
According to to my dad the hot Roders of his youth that was a Chevy guy could be heard for miles
With the 216 just screaming.
Now dad was a flathead ford guy had many a tail about them.
A 250 ci 216 is an interesting idea

Since inline six engines are naturally balanced by the degrees of rotation between the cylinder pairs, as long as the weight of the rod/piston assemblies are properly matched, they can spin to some unbelievable speeds. The limiting factor on a stovebolt engine will always be the cylinder head. Port size and shape, and the route the gases must take getting in and out of the engine will always be the bogeyman the stovebolt hotrodder has to deal with. The obvious solution is to build for low and midrange torque, not high RPM. A stroker crankshaft gives the engine a longer lever- - - -more torque, especially down low where it's useful.
Jerry