Classic Motorcycle Build

Ironhead Flywheel Rebuild

Article by Mark Trotta

All pre-2000 Sportsters have taper-shaft flywheels that were designed to be rebuilt. The assembly consists of left and right flywheels, crank pin, sprocket shaft, pinion shaft, male and female connecting rods, roller bearings in cages, and thrust washers.

Ironhead flywheel rebuild

Rebuilding an Ironhead flywheel will include disassembly, cleaning, and inspection of all components. Roller bearings and thrust washers will likely need to be replaced.

Crank Pin Lock Plates

From 1957 through early 1978, Sportster crank pins were fitted with lock-plates and screw-head bolts. Thereafter, they were deemed unnecessary by the Motor Company. If yours has them, re-use them.

Ironhead flywheel rebuild

Inner Sprocket Shaft Bearing

Once the flywheel assembly is out of the cases, check the inner and outer sprocket shaft bearings for wear and excessive play. The inner bearing is a press fit and you'll need a puller to pull it off the shaft.

Ironhead flywheel rebuild

I fabricated the tool below to remove the inner sprocket shaft bearing. The Harley tool number is 96015-52 which you can buy or make. You can also modify a two-jaw puller to remove it.

Ironhead flywheel rebuild

Read: Ironhead Special Tools

To remove and install the crank pin you will need an 1-5/16" socket or wrench. The sprocket shaft and pinion shaft nuts require an 1-3/16" socket or wrench. You can buy a Harley wrench for these (H-D part number 94546-41), or use impact sockets.

Ironhead Sportster remove and install crank pin

There are at least two ways to hold the flywheel while loosening and tightening the nuts. A shop press is easiest.

Ironhead flywheel rebuild

As an alternative, you can buy or make a simple holding jig.

flywheel holding jig

While removing the connecting rods from the crank pin, be careful of the roller bearings and cages. The bearings are loose and will go all over the place.

Ironhead Flywheels

There is a left and a right flywheel. The one stamped "A" has two keys and has the timing mark on it. The "B" flywheel has one key.

Ironhead flywheel rebuild

Ironhead flywheels are made from forged steel and extremely durable. I have seen at least four different casting numbers, there may be more.

Commonized Flywheels And Shafts

From 1957 through mid-1981, Sportster pinion shafts and sprocket shafts had an end taper of 8 degrees. Beginning in late 1981, these were changed to a 6 degree taper. The female taper in the flywheels was changed as well.

These "commonized" engine shafts and flywheels were a running change at the factory. Old inventory was used up first, so an early 1982 Sportster could very will have the older style flywheel shafts. Assembly and disassembly is the same for either type of flywheel, but the torque values are different.


Are Ironhead Flywheels Interchangeable?

Early and late 1000cc flywheels and shafts do not interchange with each other, not only because of the slight angle difference, but because the oil holes of the crank pin are in different positions.

All 900cc sprocket shaft, pinion shaft, and flywheels are interchangeable.

Ironhead Crank Pin

The crank pin is one of the most highly stressed parts in the engine, and has been known to fail if over-stressed. Examine the crank pin for wear, grooves, pitting, and out-of-round.

Ironhead flywheel rebuild

If the surface is worn at all, replace it. This is not a place to cut costs - it will cause a lot damage if it fails!

Crank Pin Replacement

Early Harley-Davidson Sportsters (1957 through mid 1981) used a three-hole crank pin (HD #23960-54). The part number for late 1981 through 1985 models is H-D #23960-80A. Early and late crank pins are similar but do not interchange.

Ironhead flywheel rebuild

Aftermarket crank pins are offered in standard and oversize O.D. They usually come supplied with new heat-treated nuts as well.

Flywheel Thrust Washers

While checking both flywheels for cracks, examine the flywheel thrust washers carefully. If they are worn or grooved, replace them.

Early Sportster thrust washers were made of steel, later ones were made of bronze. Steel washers should be used with steel roller bearing cages. Bronze washers should be used with aluminum roller bearing cages.

Replacing Thrust Washers

Removing thrust washers usually requires drilling a small hole (1/8" or smaller) slightly deeper than the thickness of the washer at the outer edge of the washer. This will allow you to stick a pointed tool underneath and pry it out. Don't remove more metal than necessary.

Ironhead Thrust Washer

Once you get the washer out, Measure the washer thickness and replace with the same size - they do make oversized washers if needed.

A flush fit is important - Clean the recess out with a scribe or awl. If the washer does not seat fully, the female connecting rod may not give enough clearance for side play.

Once you have a flush fit, peen the new washers into position at several points. This is an easy process that distorts the metal just enough to hold it in place.

Sprocket Shaft and Pinion Shaft

The sprocket shaft and pinion shaft will most likely be re-usable, but a good engine builder never assumes anything. Check both shafts before assembling. I modified my homemade truing stand and did a quick check on both sprocket shaft and pinion shaft run-out.

Sprocket Shaft run-out

Roller Bearings And Cages

Early Ironheads left the factory with steel roller bearing cages, which Harley-Davidson later updated to aluminum cages. Again, steel thrust washers should be used with steel cages, and bronze thrust washers should be used with aluminum cages.

Ironhead roller bearing cages

Most builders upgrade to the bronze washers/aluminum cages when rebuilding early engines (yours may have already been updated).

Connecting Rods

If your rods are worn and need replacing, consider buying a complete kit, which includes new rods, cages, and roller bearings. The imported kits made in Japan are a good quality. I've built two motors with them without issues.

Ironhead connecting rod replacement

Flywheel Assembly

Although the flywheel assembly can only go together one way, re-assembly takes patience and attention to detail. Blow compressed air into the small oil holes to make sure the oil passages are not clogged.

Tools for Ironhead Sportster engine build

Clean all components thoroughly and dry with compressed air.

Ironhead Thrust Washer


Flywheel Centering Jig

After the flywheel assembly is back together, it needs to be trued before being put back in the cases. A flywheel centering jig will help get into initial truing.

flywheel centering jig

Read: DIY Motorcycle Tools

flywheel centering tool

If you haven't already done so, now is the time to press on the inner sprocket shaft bearing. Do this before you start the flywheel truing process.


Flywheel Truing

For this, you'll need either a lathe or a flywheel truing stand. A truing stand is a nice luxury, but homemade units work just as well. True the flywheel assembly so run-out is within 0.001".

truing sportster flywheels

Read: DIY Flywheel Truing Stand

Truing vs Balancing

Truing is about how the two flywheels mate to each other. Balancing is about the actual weight of the flywheels, spoken in terms of grams. Truing is done at the factory, as is balancing, but not to the same degree of precision as would be required for racing.

Flywheel Balancing

Are you doing a stock rebuild with factory parts or a performance upgrade using aftermarket parts? If you're sure you're using original factory parts, the balance is probably OK, but it's a good idea to check that your piston/ring/rod weight isn't way off. If you're re-using original components, your engine should run like it did when it left the factory.

Ironhead flywheel rebuild

Truing is mandatory, balancing is optional.

Balancing the flywheel assembly components (pistons, rods and other rotating items) will make your engine run smoother and allow higher revs. It is a mandatory step when seeking more horsepower. Balancing can be done either statically or dynamically, and balancing kits are available from S&S for Harley flywheels with tapered crank pins.


Flywheel Torque Specs

Different years Sportster flywheels have different torque ratings. On 900cc models, the crank pin gets tightened to 150-175 ft lbs. Check the factory manual for the correct 1000cc torque specs.

The pinion shaft and sprocket shaft nuts on 900cc engines get torqued to 100-110 ft lbs. Check the factory manual for the correct 1000cc torque specs.

After 50 ft lbs, tighten the nut in increments, 10 ft/lbs at a time. Double-check your final torque. Be careful not to over-torque, or you may widen the taper to the point where the flywheel must be replaced.

Installing Lock Plates

On flywheels with lock plates, when re-installing, do not loosen the nuts to make them fit. Tighten them a little more until they do line up.

If installing the flywheel assembly back into the motor will not be immediate, oil it up and put in a plastic bag. This will keep it protected from dirt and dust.


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DIY flywheel truing stand

Read: DIY Flywheel Truing Stand