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.
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.
R&R Flywheel Assembly
Once the flywheel assembly is out of the cases, the bearing on the sprocket shaft needs to be removed. You can use a shop press, buy a tool, or make a tool.
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 the proper wrench for these (H-D part number 94546-41), or buy 1/2" or 3/4" drive impact sockets.
Several methods can be used to hold the flywheel while loosening and tightening the nuts. If you have a shop press, that's great, if not you can buy or make a simple holding jig.
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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 in the 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!
Early Harley-Davidson Sportsters (1957 through early 1981) used a three-hole crank pin (OEM #23960-54). Late 1981 through 1985 models had a two-hole crank-pin (H-D #23960-80A). Due to slight differences, early and late crank pins do not interchange.
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Aftermarket crank pins are available for both the 57-81E and 81L-85 Sportsters, and offered in standard and oversize O.D. 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.
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 cages, and bronze washers should be used with aluminum cages.
Replace Flywheel Thrust Washers
If either washer is worn and 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 - they do make oversized washers if needed.
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
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).
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.
Sportster Flywheel Assembly
Although the flywheel assembly can only go together one way, re-assembly takes patience and attention to detail. Before reassembling, clean and wash 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.
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 Torque Specs
Place the crank pin in the flywheel and tighten to 150-175 ft lbs. After 50 ft lbs, tighten it in stages, 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.
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-plates, tighten a dot more until they line up.
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.
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.
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".
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If installing the crank assembly will not be immediate, oil it up and put in a plastic bag. This will keep it protected from dirt and dust.