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Ironhead Flywheel Rebuild

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

Ironhead flywheel rebuild

Once the flywheel assembly is out of the cases, measure side-play between flywheels and connecting rods with a ribbon-type feeler gauge. Also check trueness and mark flywheels accordingly. Do this before disassembly, as it will ease re-assembly.

An Ironhead flywheel rebuild includes disassembly, cleaning, inspection, and replacing roller bearings and thrust washers if needed. The crank pin needs to be inspected and may need replacing, and the sprocket shaft and pinion shaft should be checked as well.

Tools Needed

To remove and install the crank pin nuts 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 the proper wrench for these (H-D part number 94546-41), or buy 1/2" or 3/4" drive impact sockets.

Ironhead Sportster engine tools

If a shop press is not available, a sturdy workbench with a good-sized vice will work. If the bearing on the sprocket shaft needs to be removed, you can use a shop press, buy a tool, or make a tool.

Ironhead special tools

R&R Sportster Flywheel Assembly

Several methods can be used to hold the flywheel while loosening and tightening the nuts. If you don't have access to a shop press, you can buy or make a simple holding jig.

Sportster flywheel holding jig

read Ironhead Special Tools

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 Crank Pin

The crank pin is one of the most highly stressed parts of the Sportster engine, and has been known to fail if over-stressed. Examine the crank pin for wear, grooves, pitting, and out-of-round. If the surface is worn at all, don't take a chance, replace. This is not a place to cut costs - it will cause a lot damage if it fails!

Ironhead flywheel rebuild

Early Sportster models 1957 through early 1981 used a three-hole crank pin (OE #23960-54). Late 1981 through 1985 models had a two-hole crank-pin (OE #23960-80A). Due to slight differences, early and late crank pins do not interchange.

Aftermarket crank pins are available for both the 57-81E and 81L-85 versions, and are offered in standard and oversize diameters. They usually come supplied with new heat-treated nuts as well.

Ironhead Sportster Flywheels

Made from forged steel, Ironhead flywheels are extremely durable. There is a left and a right - the one stamped "A" has two keys and has the timing mark on it. The "B" flywheel has one key. I have seen at least four different casting numbers, there may be more. All 900cc flywheels are interchangeable 1957-1970.

Ironhead Sportster flywheel rebuild

Check both flywheels for cracks and examine the flywheel thrust washers. Early Sportster thrust washers were steel, later ones were bronze. Steel washers should be used with steel roller bearing cages, and bronze washers should be used with aluminum roller bearing cages.

Replace Flywheel Thrust Washers

If either washer is worn or grooved, remove and replace both. Removing them 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.

Once you get the washer out, carefully clean the recess so there is a good flush fit. A scribe or awl works good for this. A flush fit is important - if the washer does not seat fully, the female connecting rod may not give enough clearance for side-play. Measure the washer thickness and replace with the same size - several companies offer oversized washers if needed.

Ironhead Thrust Washer

Once the recess is cleaned and 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.

Roller Bearings And Cages

If connecting rod play was acceptable before disassembly, you're probably OK to re-use the existing roller bearings. If there was too much play or you have to recondition the rods, refer to the H-D manual for measuring and selecting oversize roller bearings.

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

Ironhead roller bearing cages

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 before re-installing.

Sprocket Shaft run-out

Sportster Flywheel Assembly

Although the flywheel assembly can only go together one way, re-assembly takes patience and attention to detail. Before assembling, clean and wash all parts thoroughly, then dry with compressed air. Blow compressed air into the pinion shaft hole to make sure the oil passage is not clogged.

The pinion shaft and sprocket shaft nuts get torqued to 100-110 ft/lbs. After 50 ft/lbs, tighten it in stages, 10 ft/lbs at a time. Double-check your final torque.

Ironhead Sportster Lower End

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 Truing

After your Sportster flywheel assembly is back together, it needs to be trued before being put back in the cases. For this, you'll need either a lathe or a flywheel truing stand. A $750 truing stand is a nice luxury, but homemade units work just as well. True the flywheel assembly so run-out is within 0.001".

harley flywheel truing stand

read DIY Truing Stand

Flywheel Torque Specs

While truing, tighten in stages, 10 ft/lbs at a time. Final crank pin torque is 150-175 ft/lbs. 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.

Lock Plates

Sportsters from 1957 through early 1978 left the factory with lock-plates and screw-head bolts. Thereafter, they were deemed unnecessary by the Motor Company. Never loosen nuts to fit the lock-plate bolts, tighten a little more until they line up.

Flywheel Side-play

Side-play between flywheels and cases can be measured with a ribbon-type feeler gauge. The H-D manual recommends .006" to .010" for 1957 to 1969 models. Side-play for 1970 to 1978 models is .005" to .015". Side-play for 1979 to 1985 models is .005" to .025".

feeler-gauge to measure Sportster flywheel sideplay

If sideplay is too loose, tighten crank pin nuts. If sideplay is too tight, check that thrust washers are seated properly,

Ironhead flywheel rebuild

If installing will not be immediate, put the crank assembly in a plastic bag and keep it out of harm's way.

Ironhead flywheel rebuild

read Sportster History

read Ironhead Engine Build

tools and supplies

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