Classic Motorcycle Build

Ironhead Engine Build

Text and Pictures by Mark Trotta

Unlike today's models, Ironhead Sportster engines were designed to be rebuilt. Among the serviceable parts are the main bearings - the races can be honed oversize and fitted with new oversize roller bearings. Original Ironhead cylinders (not aftermarket) have enough material to be safely bored up to .060" oversize.

Ironhead engine build

A vintage Sportster engine requires special skills and tools to rebuild properly. And it seems that precious few shops actually rebuild Ironheads in-house any longer. Finding a good mechanic that will work on older bikes gets harder with every passing year. So, should you rebuild an Ironhead engine yourself?

Ironhead Rebuild Cost

In 2016, I contacted five motorcycle shops in my area (Raleigh, NC) to get an idea of what a total Ironhead engine build would cost. The local Harley Dealer said they couldn't help me, as did two of the local Indie shops. One shop offered to ship my motor from their shop to a place in New York. Another offered to ship my motor from their shop to a place in Wisconsin.

Estimates for a complete engine build ran from $2,400 to $3,100 in labor (parts additional). The customer (you) also has to pay any shipping, so factor in another $350-$500 (there and back).

DIY Ironhead engine build

Should I Rebuild A Sportster Engine Myself?

There are two big negatives for first-time Sportster engine builders. First, special skills are required, which are no longer being taught in today's vocational schools. These include flywheel rebuilding, flywheel truing, and honing main bearings (see below).

The second big negative is the cost of the tools. If you don't already have them, they may cost you more than having someone do the work for you. Hopefully you won't need every engine tool listed in the factory manual.

Tools Needed To Rebuild Ironhead Motor

Aside from basic hand tools, you'll need basic engine building tools such as a piston ring installer, torque wrench, digital caliper, a micrometer or two, and a feeler gauge. There are also quite a few specialty tools needed, although many of those can be fabricated.

Tools for Ironhead Sportster engine build

Read: Ironhead Special Tools

On the plus side, every part is available (with few exceptions) for Ironhead Sportsters, either used, N.O.S., or reproduction. Also, any information you may need is easily found on internet forums, Youtube videos, or the good old factory manual.

900cc vs 1000cc

There are far more 1000cc Ironhead Sportsters than 900's, and not a lot of parts interchange. Other difficulties you will encounter rebuilding an early motor is limited parts availability, as well as several other specialty tools needed.

900cc Ironhead engine build

Read: 900cc Ironhead Engine Build

Planning An Engine Build

An Ironhead engine build can be broken down into several categories. These include the top end, bottom end, transmission, clutch, carb/fuel, oil pump, and electrics (distributor, generator). Because no two engine builds are alike, you won't know what tools and skills you'll need until a complete disassembly and careful inspection of parts.

Check Bottom End Play

After the cylinder jugs are removed, check the connecting rods for up and down play. Some side to side play is normal and actually needed for when the engine gets hot and metals expand.

On average, a Sportster top end will get rebuilt twice before a bottom end rebuild is needed. But when dealing with a 50 year-old machine you don't have a history of, it would be wise to consider doing a complete engine rebuild from the flywheels up.

Unit Transmission

Unlike Harley Big-Twins, you can't ignore the transmission when doing bottom end work on a Sportster engine. The transmission is housed inside the engine cases, so at a minimum you'll at least need to know how to remove and install it.

ironhead-transmission remove and install

Read: Ironhead Sportster Transmission

Most Sportster engines have taken a lot of abuse, so having a pair of unmolested cases is a rare and wonderful thing. Engine cases are the most important component of an Ironhead engine rebuild.

Inspect The Cases Carefully

Always check carefully for cracks upon disassembly. Aluminum engine cases are repairable and can be made as good as new, provided the repair was done right.

Read: Repair Cracked Cases

Ironhead Cracked Cases

Mixing Ironhead Cases

If your Sportster engine was already running, the cases may be okay, but if they weren't, you won't know what problems you'll have until you put two mismatched cases together.

Ironhead Sportster Project

Read: Ironhead Engine Cases

Ironhead Bottom End

The most time-consuming part of an Ironhead engine build is the bottom end. Repairing the lower end requires specialty tools and skills, and can take considerable time to properly measure and rebuild. Aside from the labor, factor in the cost of parts and special tools needed.

Read: Ironhead Bottom End Build (1957-1976)

Read: Ironhead Engine Assembly (1977-1985)

Line Boring vs Line Honing

Line boring is the process of centering and re-fitting the bearing races in one or both cases. The objective is to have the crank assembly sitting parallel in the cases. In some cases, the races may need to be replaced.

Whereas line boring uses cutters with a boring bar, line honing uses abrasive stones, which removes less metal and leaves a smoother finish. Neither line-boring nor line-honing may be needed, but careful inspection and measurement is critical.

Truing Flywheels

Due to their 45-degree configuration, V-twin motors such as Harley Sportsters are inherently out of balance to begin with. It is essential that the flywheel assembly be properly rebuilt and trued, or engine vibration will not only numb your hands and feet, it may cause internal engine damage.

Read: Ironhead Flywheel Rebuild

truing sportster flywheels

Read: DIY Truing Stand

To true a flywheel, 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".

Truing vs Balancing

Balancing is about the weight of the pistons, rods and other rotating/ reciprocating items. Truing is done at the factory, balance is not. Balancing may make it run smoother, but if the flywheels are trued and the run-out is within specs, re-using your original parts will have the engine running as it did when it left the factory.

Ironhead Top End

Compared to the bottom end, rebuilding the top end of an Ironhead requires less time and less special tools. The two main obstacles will be if the cylinder jugs need boring and if the valve seats need to be cut. This is where careful inspection and measurement pays off. If you're lucky, you may get away with just honing the jugs and installing new piston rings.

Read: Ironhead Top End Rebuild

Read: Ironhead Sportster Cylinders

Read: How To Adjust Ironhead Valves

Sportster Gear Case

Although the Sportster four-cam design is derived from the Harley 45 flathead, it's still pretty complex. During installation, if the breather valve is not correctly timed, the gear case does not get properly lubricated. This can have disastrous results at higher RPMs.

early Sportster pinion gear

Read: Ironhead Sportster Pinion Gear (Remove and Install)

Once the breather is timed correctly, there is no need to reference the flywheel timing mark when installing the cams. The cams get referenced off the notch on the pinion gear.

Sportster Cam Bushings

cam cover bushing install

Read: Remove/Install Sportster Cam Bushings

Sportster Cams

If you swap your cam cover with a new one, or change your cams, always check the clearances. The factory service manual calls for .0005" to .002" minimum clearance. Maximum cam end play is not as important (.001" to .006").

Ironhead Sportster cam set

Read: Install Sportster Cams

Ironhead cam removal is easier when the four push rods are removed, although the tappets can just be loosened to remove them. Old Sportsters do not have as close tolerances as modern engines have, but cam clearance should be set to factory specs if not a tad bit looser.


Engine Assembly Lube

I pour a bit of engine assembly lube inside the connecting rods and anywhere else it can reach. The pinion shaft bearing geta lubricated with engine oil.

motorcycle engine assembly lube

Read: Best Engine Assembly Lube



Should you attempt an Ironhead rebuild yourself? Bottom line--it's your bike, you decide if you're up for the challenge (and a few headaches). But if you don't feel comfortable in your abilities, consider farming out some of the repairs and stick with the ones you do feel comfortable with.

Ironhead engine build


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