***THESE PISTONS ARE THE SAME SPECS AS OUR REPLACEMENT PISTONS, EXCEPT THESE WILL BE THE PISTONS YOU NEED IF YOUR BORE IS NOT PERFECT. IF YOU HAVE ALREADY BROKEN A PISTON, WE RECOMMEND THIS PISTON TO REPLACE YOUR BROKEN PISTONS. YOU WILL NEED TO HAVE YOUR CYLINDERS HONED TO .005 OVER TO MAKE THIS PISTON WORK! IF YOU ARE LOOKING FOR A PERFECT FIT PISTON WITHOUT HAVING TO HONE THE CYLINDER, PLEASE SEE OUR 4.075” 121 PISTONS!

If you own a 2024 and newer CVO ST with the 121 engine package, then you have surely heard of the failing piston problem that they have. In fact, it happens so often that we will not install an upgraded camshaft into any 121 engines simply because the added cylinder pressure can break the piston! Crazy, we know! The factory cast piston is super thin and light, however that is not a good combination when you have a cast piston! What’s the answer? Jim’s Piston kit is the answer. Designed specifically by Jim to be a direct replacement piston set for your factory 121 engine. Simply pull your old pistons, dingle berry hone the cylinders and install the new pistons! These pistons in combination with Jim’s M857E camshaft and a good exhaust will have you making 150 torque and 150 horsepower! We’ve tested that exact combination with several exhaust systems and have gotten similar numbers across the board!

CP Carillo manufactures the pistons to Jim’s specs. They are then coated by Calico Coatings with the very same coating that goes onto every piston in a NASCAR engine! After coating, each piston set is personally inspected and packaged by Jim to ensure product consistency.

Piston kit comes complete with: Pistons / Ring Set / Wrist Pins / Wrist Pin Clips / Base Gaskets / Head Gaskets

Designed for the Harley Davidson CVO ST 121 engine: 4.080” Bore & 4.625” Stroke

Maintains stock 11.4:1 compression ratio.