Classic Motorcycle Build

Motorcycle Engine Build

(text and pictures by Mark Trotta)

So you're thinking it's time to tear down the old motor and freshen it up a bit? Two things are for certain; it's not going to be cheap and it's not going to happen fast. There's a lot to consider before you start a motorcycle engine build.

Harley 45 flathead engine build

Unlike classic car builders, who can order a crate motor and have it delivered to their door, the classic motorcycle builder has just two choices when needing an engine overhaul; rebuild the engine themselves, or find a shop that is reputable, dependable, reasonably priced, and familiar with vintage engines.

Run a few tests before dismantling the motor. If you can kick the motor over, check for compression. If your project is a non-running bike, hopefully you only paid scrap prices, because there's a good chance the engine is scrap.

A motorcycle engine build can be broken down into categories. These include top end, bottom end, transmission, clutch, carb/fuel, oil pump, and electrics (distributor, generator). Each and every step, large or small, is as important as the others.

Engine Disassembly

Before you start your disassembling your engine, get a notebook and/or take pictures. During disassembly, write down/document everything, especially in what order things came apart.

With the exception of a few specialty tools, you should be able to take your engine apart with little difficulty. Inspect and measure parts as you remove them. Also inspect the cases for stripped threads and cracks.

repair stripped threads in aluminum engine case

Read: Repair Stripped Threads In Aluminum

Cracked Engine Cases

Aluminum engine cases can be repaired and made as good as new, provided the repair was done right. Don't entrust this job to someone whose only qualification is that they own a welding machine.

Repair old motorcycle engine

Read: Repair Cracked Cases

Trying to save money here is false economy - a welder with good intentions can make the damage worse than it is. Find someone who is experienced at welding cast aluminum and have it MIG or TIG welded.

Never Take Anything For Granted

A motorcycle engine build requires measuring and re-measuring, assembly, disassembly, and reassembly. Make sure everything is right before continuing to the next step.

Bottom End Engine Build

The bottom end can be disassembled only after the cylinder head(s) have been removed. Bottom end reconditioning includes checking piston to cylinder wall clearances and cleaning the piston crowns. A cylinder hone or over-bore may be necessary.

Inspection of parts include pistons, rings, wrist pins, and bearings. If there are any doubts, replace suspect parts while the motor is apart. Oil pump and oil pressure relief valve modifications can be done at this time.

Engine Case Sealant

There are dozens of products to seal engine cases, and the ones that are sold at automotive parts stores are almost always wrong for your motorcycle.

best sealant for engine cases

Read: Anaerobic Gasket Sealant Review

Top End Engine Build

Compared to the bottom-end, a top-end rebuild is easier. Correct measurements are crucial to choose between a re-ring or re-bore. If the motor was burning oil, a re-ring is usually necessary.

Harley-Davidson Flathead Engine Build

Boring And Honing Cylinders

Boring motorcycle cylinders with a hand-held tool will take much longer than a $5,000 shop machine can do it in. But, whether cylinder boring is done in a machine shop or in your home garage, the results will be as good as the operator.

Read: DIY Motorcycle Cylinder Boring

Read: DIY Motorcycle Cylinder Honing


Proper valve seating is crucial to a strong-running engine, and the quality of any valve job will be co-dependant on valves, guides, and springs.

If you need valve guides, aftermarket ones are available for most classic bikes. Worn guides are usually pressed out and new ones pressed in.

replace valve guides

The use of three-angle seats in cylinder heads has been an industry standard for decades. This is done by having a relief angle above and below a 45 degree center contact surface.

Read: What Is A Three-Angle Valve Job ?

three angle valve job

Read: How To Lap Valves

Engine Reassembly

Re-assembly is always harder than disassembly. Everything is easier when you have the proper tools.

Transmission Rebuild

Many classic bikes have unit transmissions. Two notable exceptions are vintage Triumphs and Big-Twin Harleys, which have separate transmission housings.

When a unit transmission seizes up, it often takes the engine case with it.

Ironhead Sportster transmission

Rebuilding a motorcycle transmission will include removing/inspecting/replacing bearings, shift forks, gears, seals, and gaskets. Transmission work is tedious and requires precision measuring tools.

Motorcycle Engine Building Tips

A factory service manual for your year and model motorcycle is helpful, but I've found discrepancies in manuals numerous times. Your common sense should override anything you read.

The longer you handle the old engine components, the more time you have to notice any defects. If you just give them a quick cleaning, you may miss something.

If you decide to sandblast your engine parts, make sure there's no grit left. Clean them thoroughly, two or three times if necessary.

flywheel truing stand

Outside sources needed while restoring your engine include parts suppliers, tool suppliers, and a machine shop.

Glyptal Pros and Cons

There are some that swear this product. Personally, I never felt the need for it. Some claim it seals the inside of the block and fills in any open pores, giving the oil less resistance so it drains back to the pan quicker. A common reply is, "I've never heard of an engine locking up because all the oil was stuck to the sides of the block." Another argument in favor of Glyptal are "it keeps your engine clean", but so does changing your oil on a regular basis.

One particular concern about using Glyptal on motorcycle engine cases is, if gas leaks through the carburetor while your bike is on it's side-stand, the unburned gas would eat away the finish on the inside of the cases.

Rebuilding a motorcycle engine could (and often does) become long and tedious, particularly if parts are hard to find. But if you get stuck, there's countless forums and internet videos to help you get back on track.

Meticulous attention to detail and cleanliness are needed to build a reliable, strong-running motorcycle engine. If valves open and close at the appropriate times, maximum engine efficiency and performance will be achieved.

Is rebuilding a motorcycle engine something that can be done in your home garage? Yes, but results will vary. One thing is for sure - your results will likely be better than a machinist who doesn't care.

Read: DIY vs Machine Shop


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