Motorcycle Engine Build
(text and pictures by Mark Trotta)
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.
A motorcycle engine build can be broken down into major and minor 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.
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.
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.
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.
Read: Motorcycle Engine Tools
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.
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.
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.
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.
Read: Best Sealant For Engine Cases
Glyptal Pros and Cons
There are some that swear this product. I've rebuilt a dozen or so motors and 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.
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.
Motorcycle Cylinder Boring
Boring motorcycle cylinders with a hand-held honing tool can certainly be done. 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
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.
Read: Motorcycle Valve Job
If you need valve guides, aftermarket ones are available for most classic bikes. Worn guides are usually pressed out and new ones pressed in.
Read: How To Remove Valve Guides
Read: How To Install 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 ?
Read: How To Lap Valves
Many classic bikes have unit transmissions, the notable exceptions being vintage Triumphs and Big-Twin Harleys, which have separate transmission housings.
Rebuilding a motorcycle transmission will include removing/inspecting/replacing bearings, shift forks, gears, seals, and gaskets. Lots of careful measuring and remeasuring is required. Transmission work is tedious and requires specialty 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.
If you decide to sandblast your engine parts, make sure there's no grit left. Clean them thoroughly, two or three times if necessary.
Before you start your disassembling your engine, get a notebook and a camera. During disassembly, write down/document everything, especially in what order things came apart.
Run a few tests before dismantling the motor. If you can kick the motor over, check for compression. If you bought a bike with a seized engine, hopefully you only paid scrap prices, because there's a good chance the engine is scrap.
Re-assembly is always harder than disassembly. Aside from basic hand tools, you'll need a piston ring installer, torque wrench, dial caliper, feeler gauge, and most likely some specialty tools. Outside sources needed while restoring your engine include parts suppliers, tool suppliers, and a machine shop.
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
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.