Oil Bypass Plug on an Early Hemi

For anyone who has read my previous tech articles I typically open things up by describing how I am building a traditionally built hot rod the way it would have been done in the early 50's. I was not alive during what I think of as “the golden age of hot rods” but I have a great reverence for that time in history. I am on a small budget working out of a two car garage above which my two young children are sleeping most of the hours I devote to working on my car. Like most hot rods mine is a conglomeration of parts scavenged and horse traded over many years and every piece has a story.

I like to use vintage parts when possible to stay true to tradition and period correctness. There are a few parts of the car however where modern components make sense and this article documents two little pieces which will help my antique 241 Dodge Hemi stay healthier over the long haul. In this tech article I will cover my use and installation of an Oil Bypass Plug.

Early Hemi engines were truly state of the art when they arrived on the scene in the early 1950's. The technology of the hemispherical combustion chamber was adopted when its performance was proven during the Second World War in aircraft engines. Chrysler, Dodge, and Desoto would set out to create Hemi engines for its new production cars aiming to please the American consumer who, because of the healthy postwar economy, could afford and expect more from a new car. The Hemis from the 50's did not disappoint and made their mark, but looking back sixty years later it is easy to critique the design.

One design element which was similar in all early Hemis relates to the oiling system. Each engine utilized a canister type oil filter assembly. During each oil change you simply removed one bolt to pull the top of the canister off and replace the paper filter element. Many modern European cars use a similar design but the size of the canister and its location on these old Hemis is typically not friendly to a hot rod chassis. In conjunction to the awkward filter canister, inside the block is an oil bypass which was thought of as a safety precaution in the event one of the paper element would become clogged. If oil for some reason could not pass through the filter it would be cycled back through the engine directly without passing through a filter. Over the years hot rodders have figured out a slick way to fix these issues with two handy little parts.

One of those is an oil bypass plug which installs under the rear main cap and replaces a small ball check valve (the oil diverter valve). During an engine rebuild one simply removes this original check valve and replaces it with the bypass plug. Dodge, Desoto, and Chrysler Hemis have a slightly different design, so you will want to purchase the correct piece for your engine. P/N 910-11048 from Speedway Motors will take care of you if you are in need of one for your Chrysler 331 354 or 392 engines. If like me you are working on a Dodge Red Ram, Super Red Ram, or one of the Desoto engines like the Fire Dome 8, you will want to source one through a reputable company like HotHeads Hemi Research.

With mine in hand I took it to the machine shop where they were able to install it prior to dropping in the crank and bolting down the main caps. It fit like a glove. It is important to note that you do not absolutely need to run this plug to also use a spin-on filter adapter. I have read instances where people have rebuilt their engines and forgotten to install this upgrade bypass plug. If that is the case no need to worry. As Hot Rodders we typical change our oil often, so it is not likely we will ever get things dirty enough inside to cause a new style filter to plug. Some users of this part claim it improves oil pressure but that is hard to prove.