JOHN HARMON  120 c.i. Shovelhead Engine 

History & Rebuild

Classic editorials were many through the 1980s covering John Harmon project designs. Street Chopper mag had printed various full page spreads on Harmon engines by Bob Clark. Below, brand new Harmon 120 cubic inch cylinders and heads. This kit was sitting under a workshop bench in Sweden for over a decade untouched, It was one of two Harmon engines I had managed to purchase in Scandinavia.     

Size does matter - John Harmon flat top 4-300" Venolia pistons were used for 3 years with no issues. 

The Biggest John Harmon Built

The largest Harmon ever built and known, which I've personally seen that ran like a rocket was Jims custom 3 litre engine in Denmark. An original Harmon crankcase was used with one-off huge billet CNC machined cylinders and heads. The 3000cc engine displacement was also a kick start only. Image - HD Denmark  

The Beginning

Firstly I’d like to dedicate this page to the late John Harmon. His brake away engine design was the beginning of a new era of big bore kits that went ballistic within the biker world after his passing. The John Harmon engine kits weren't flawless. There were several mistakes and mis concepts with the design even before any assembly begins. The cylinder stud holes were not CNC cantered aligned. This can give issues when using other aftermarket cylinders. Harmon heads were finished without having steel inserts for head bolts. The addition of adding steel threaded inserts into the heads is recommended. The breather case oil hole also requires alignment trimming, and the extended pinion shafts that come with engine kits should be replaced with a Jims pinion shaft. Crankcase bearing inserts on some kits are known to be slightly over machined. A fastidious construction process is required when rebuilding these engines, which includes several minor changes and modifications to make any Harmon Big Twin reliable. 

Replacement Cylinders & Pistons

Replacing pistons is recommended due to the large cut-out on the original rear piston skirt. Quality made aftermarket suppliers like Hyperformance in the U.S. can reproduce billet CNC ductile iron cylinders and custom CNC machined pistons for John Harmon engines. Hyperformance front and rear pistons are the same weight by keeping a section of the cut-out at the original piston skirt length. This eradicates unbalanced issues, resulting in less vibration and pistons stable under thrust.      

Images showing the size of a Harmon piston compared to stock Big Twin Shovelhead pistons

The first thing that needs replacing are the original Harmon pistons with other aftermarket brands that are machined made with the same weight. Hyperformance pistons are recommended. Below, Hyperformance billet machined ductile cylinders are good recommended replacements when original Harmon jugs are worn out.

Bottom End

A set of reputable aftermarket flywheels, rod, and pins is essential to keep Harmon engines together. This one received on its second rebuild a set of S&S flywheels, Carillo rods, 3 hole Jims crankpin, and Jims pinion and sprocket shafts. The previous owner had built the engine with OEM shovel flywheels and rods which were shoddily put together causing engine failure. The inner crankcase sprocket shaft bearings had disintegrated and disappeared somewhere. This caused the left flywheel to crack, tilting the front rod which seized the piston rings with irreversible damage to the front piston and cylinder. This is what usually happens to any engine when taking cheap short-cuts.   

Crankcase Breather 

Harmon breather cavities need modifying for maximum return oil flow from the crankcase. The breather hole should be squared-up with the breather. Removing the breather screen will also give less restricted flow. Any metal or other bottom end pieces entering through the crankcase vent hole will normally mean the engine has already blown to bits, so removing a screen is relatively safe. Adding a machined billet made scoop fitted in to the breather itself will ensure fast maximum oil flow from the crankcase. 

To be continued through November 2019