Ironhead Engine Build
Text and Pictures by Mark Trotta
Unlike today's models, Ironhead Sportster engines were designed to be rebuilt. Among the many serviceable parts are the main bearing races, which can be honed oversize and fitted with new oversize bearing rollers. If your Ironhead cylinders are original (not aftermarket) they have enough material to be bored safely up to .060" oversize.
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 Engine Build Total 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 in labor (parts additional) to $3,100 in labor (parts additional). The customer also has to pay shipping, so factor in another $350-$500 (there and back).
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).
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.
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 rebuilding tools such as a piston ring installer, torque wrench, dial caliper, and feeler gauge. There are also quite a few specialty tools needed, although many of them can be fabricated.
Read: Ironhead Special Tools
Bottom line - it's your bike, you decide. Remember that you are the best mechanic for your bike, and no shop employee could possibly care as much as you do. 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.
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.
Read: Remove and Install Ironhead Clutch
Most Sportster engines have taken a lot of abuse, so having a pair of unmolested cases is a wonderful thing. Engine cases are the most important component of an Ironhead engine rebuild.
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 remember that you're dealing with a 50 year-old machine. It may be prudent to do a complete engine rebuild from the flywheels up.
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
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.
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
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.
Read: DIY vs Machine Shop
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.
Read: Ironhead Flywheel Rebuild
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 to be trued, or engine vibration will not only numb your hands and feet, it may cause internal engine damage.
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: DIY Motorcycle Cylinder Boring
The cylinders you see in the pictures are from an early Ironhead Sportster. They are painted the original factory silver.
Read: Motorcycle Valve Job
Read: What Is A Three-Angle Valve Job ?
Read: How To Remove Valve Guides
Read: How To Install Valve Guides
Read: Ironhead Rocker Box Assembly
Swap Sportster Cams
Ironhead cam removal is easier when the four push rods are removed. 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. If you swap your cam cover out with a new one, or change your cams, always check the clearance. The factory service manual calls for .0005" to .002" minimum clearance. Maximum cam end play should be between .001" to .006".