Classic Motorcycle Build

Harley 45 Engine Build (Bottom End)

Text and Pictures by Mark Trotta

After engine teardown and cleaning, inspection included checking the engine cases for cracks and broken edges, and looking for and repairing stripped threads.

Harley Davidson 45 flathead engine assembly

Two common problem areas on Harley 45 cases are where the gear cover and oil feed pump bolt on, and where the timing plug and drain plug are.

Harley 45 Flywheel Assembly

The flathead 45 flywheel assembly consists of right and left flywheels, sprocket shaft, pinion shaft, male and female connecting rods, roller bearings in cages, and thrust washers.

Harley 45 flywheel assembly

The flywheel assembly was mounted and spun on a homemade flywheel truing stand, and showed to be within .001" in tolerance.

Install Flywheel Assembly

With the right-side case laying flat on it's side, install the right-side bearing washer (the one with the tab) into the right case, then drop in the right-side roller bearings (the two-piece cage). Next, prop up or hold up the right-side case and carefully install the flywheel assembly into it.

Harley 45 engine repair

Install the left-side roller bearings (the two one-piece cages) and bearing washer on the left-side (threaded) sprocket. At this point the left case should easily fit on top of the right case.

Connecting Rod Placement

On 1939 and later engines, the forked rod is at the rear. On earlier engines, the forked rod was the front rod.

Trial-fit Engine Cases

While fitting the flywheel assembly to the cases, you'll probably have to separate the two halves several times to get the clearance right.

Without gasket sealer, join the engine cases with a minimum of two cap screws at the top, and two studs with nuts at the bottom.

Once these are tightened, flywheel end-play can be checked. The Harley service manual recommends .012" to .014". The crank should rest centered in the cases. It should not run to one side or the other when rotated.

Harley 45 engine build

The factory manual also states that the two thrust washers should be "approximately the same thickness". Place one each on both sprocket and pinion shafts.

Crankshaft Thrust Washer Clearance

Having proper crankshaft end-play is important. Too much play affects the stability of the piston in the cylinder. Not enough crankshaft end play will cause the bearings to overheat. It's better to have a little more play than too little play.

NOTE: If you need to purchase thrust washers, you may find that they're only available in a set of eleven (one of each size).

After installing the flywheel assembly into the right-side engine case check that the crank is centered in the cases. When you're satisfied with the fit, it's time to seal the cases.


Sealing The Cases

There are several good products to seal motorcycle engine cases. For this engine, I chose Permatex Ultra Grey.

Before applying the sealant, get the engine cases as clean as possible. Brake parts cleaner works well and leaves no residue. A good way to make sure all oil and grease residue is removed is to apply a little heat to the cases.

Harley 45 engine assembly

After applying to both engine cases, I allowed the sealant get a little tacky (5-10 minutes) before final assembly. The two bolts and four studs were then tightened.


Harley 45 Engine Stand

Harley engine stand

I welded up a simple engine stand, which made working on the motor much easier. It also gave me something to mount the dial indicator to check for crankshaft end play.

Read: DIY Harley Engine Stand

checking thrust clearance Harley 45

If you don't have a stand, you can lay the engine down on a couple of 4x4 wood blocks.


A Harley 45 flathead has two separate oil pumps, and each has a different function.

Oil Feed Pump

Harley-Davidson 45 flathead oil feed pump

The "feed" pump (pictured above) is mounted to the right-side of the engine on the cam cover. It is a vane-style pump which is gravity-fed oil from the tank, then delivers it to the engine under pressure.

Oil Scavenger Pump

The lubrication cycle is completed by the "scavenger" oil pump. This is a gear-type pump, and mounts to the bottom of the right-side engine case with four studs and nuts. It's job is to return engine oil back to the oil tank.

Harley 45 engine assembly

Read: Harley 45 Scavenger Oil Pump Repair


Related Articles:

Harley 45 Engine Teardown and Inspection
Repair Stripped Threads In Aluminum
Install Harley 45 Cams
Assemble Harley 45 Cylinders
Harley 45 Drivetrain
Harley 45 Frame Choices
digital caliper for engine building

Read: Best Digital Caliper For Engine Building