Harley-Davidson 124R Upgrade

Even More Power!

We begin by laying out our components and inspecting each part thoroughly. We have a nice selection of high-quality parts from Harley-Davidson, Darkhorse Crankworks, T-Man Performance, Screamin’ Eagle, Dave Mackie Engineering, Cometic, and more.

  1. Pictured here are the OEM bearings, seal and retaining clip, washers, and shims that were removed by Darkhorse when reworking our crank and case.
  1. We place the assembled bottom end in our Twin Cam engine cradle provided by JIMS USA and secure it properly.
  1. After removing the cam cover, inspect the crankshaft run-out, which DHC does a great job of making sure is well within H-D specs when it assembles every bottom end. Double-check there is no debris in the cam compartment.
  1. We begin the motor assembly by installing 120R base gaskets from Harley-Davidson. Since no case boring is required to increase the cylinder size, no special gaskets are required.
  1. Using a piston ring installer, we carefully install the piston rings onto the pistons. T-Man Performance measures ring gap prior to shipping each kit. It is always a good idea to check the ring gap to verify proper sizing.
  1. Using Red Line assembly lubricant, we generously coat the crank bearings. We then rotate the crank to evenly distribute the assembly lube. This ensures that all crank surfaces have a lubricated coating on them so there is no metal-to-metal friction upon initial start-up.
  1. We use rubber hose to cover the cylinder studs to protect the piston from being damaged upon installation. After applying an assembly lubricant coating to the pistons, we install each piston onto the connecting rod. Each piston included in the T-Man Performance 124-inch upgrade kit is clearly marked “FWD,” making it very clear which direction the piston is to be installed. Using a wrist pin clip installation tool, we install the wrist pin clips.
  1. Pictured here is our 124-inch piston resting safely against the rubber tubing covering the cylinder studs. At this point we are ready to move on to the next phase of the top-end assembly.
  1. Before installing the cylinders we first wash them with a soapy water solution then rinse. We next use brake cleaner to remove any residual residue from the cylinder walls.
  1. Using a rubber mallet we lightly tap in the dowel pins provided with the cylinder kit.
  1. Next, we install the cylinders onto the engine case. Using a ring compressor we compress the piston rings around the piston so the cylinder is able to slide over the rings effortlessly. T-Man Performance has modified its piston ring, so installing them is a simple task.
  1. Here is our 124-inch top end installed; we rotated the motor over a few times to make sure everything is in spec. We can now move on to the next step.
  1. We install 0.030 head gaskets by Cometic.
  1. Next, using the H-D Screamin’ Eagle compression release fixture, we set up to drill the heads for manual compression releases. We begin by attaching the fixture plate to the head and torqueing the bolts to spec. The jig has front and rear head positions, so be sure to attach the fixture to the appropriate head.
  1. After attaching the drill bit, the depth collar must be set to the correct measurement in order to be sure you don’t drill right through the head. Measure three times and be certain; if you can’t read a vernier caliper, stop here. Drill into the head until the collar stops you from going any deeper into the head.
  1. Attach the supplied tap and insert into the hole you just drilled. Tap the hole to create some threads for the compression release mechanism.
  1. Next, attach the supplied drill bit and pop a hole through the base of the larger hole you just drilled. This will allow compression to escape through the compression release.
  1. Pictured here is the compression release hole that was just made.
  1. After a good thorough cleaning, we can install our Dave Mackie-modified 120R/124R heads. We will be using S&S stock-length Twin Cam head bolts.
  1. We follow the instructions included with the Cometic 0.030 MLS head gaskets when torqueing the front and rear heads. They are different than using OEM ones, so be sure to follow directions accordingly
  1. Next, we set up our Screamin’ Eagle cam plate using Dave Mackie Engineering cams, SE bearings, and retaining clips. Using the JIMS USA T/C cam installation tool, we can be assured the cams press into the plate correctly with no damage to any of the components.
  1. After pressing the cams into the plate, install the retaining plate and torque to spec.
  1. Install the snap ring using snap ring pliers.
  1. We now have the cam plate assembled and can install the SE hydraulic tensioners. It’s a good idea to put a few drops of oil into the tensioner so it doesn’t have to prime as hard upon start-up.
  1. Install the oil pump onto the pinion shaft. Make sure the pump is assembled correctly prior to installing. Lubricating the oil pump gyrator gears is a good idea and can be done before putting the oil pump onto the pinion shaft.
  1. Next, we attach the cam plate using alignment dowel pins and the OEM mounting hardware. Using the H-D service manual, we follow the cam plate/oil pump installation procedure.
  1. Using the SE cam chain locking tool, we lock the cam gears to prevent them from spinning while we tighten the pinion shaft nut and cam gear retention nut to factory spec.
  1. We can now insert the lifters into the lifter bore, install the tappet covers, and install the lower rocker boxes and tighten to spec.
  1. After putting the rocker support, rocker arms, and breather setup components into their positions, we now can install the pushrods and top rocker covers and secure the bolts.
  1. Next, we adjust the pushrods to factory specs and install their covers, and we place the cam cover onto the engine case.
  1. After turning the motor over on the bench to listen for any funny sounds, clinks, or clunks, we install the motor back in the chassis. What we have now is a 124-inch monster that should make some nice power.

Horsepower and torque. These two things are desired by Harley-Davidson owners, and most will go to elaborate means to achieve what they want. When H-D introduced the 120R, it was only a matter of time before aftermarket manufacturers developed parts to make The Motor Company’s biggest motor even bigger.

After we sent our bottom end to the guys at Darkhorse Crankworks, it only seemed fitting to reassemble the motor as a 124-inch beast. To do this, we contacted TR Reiser of T-Man Performance and went over the details of the build. TR informed me that he has developed a 124-inch upgrade kit for the 120R crate motor using Nikasil cylinder technology as well as a new piston ring design. The kit is a bolt-on and requires no case boring. After we got some specs from TR and passed the info along to Darkhorse and Dave Mackie, the two were able to set up our heads and the crank accordingly and get the bottom end on its way back to the shop. TR sent us the piston and cylinder kit. We had the 120R heads worked over by Mackie, whose porting skills and finishing work are second to none. When we had received all the parts back it was time to get to work building a 124-inch monster.

Sources Darkhorse Crankworks darkhorsecrankworks.com

Dave Mackie Engineering davemackie.com

Harley-Davidson harley-davidson.com

JIMS USA jimsusa.com

San Diego Customs sandiegocustoms.com

T-Man Performance tmanperformance.com