Classic Motorcycle Build

Motorcycle 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.

motorcycle engine build

Shipping A Motorcycle Engine

There will always be shops that work on vintage bikes, I'm sure there's dozens of reputable shops in the U.S. alone. But will they be in driving distance of your home? The shop that I have known and trusted for 20 years is now five states away, and shipping the cases with flywheels would be expensive. It also exposes them to damage.

Machine Shop Or DIY

Should you farm out your rebuild to a local shop or do it yourself? I have several issues with machine shops.

Machining costs always seem reasonable at first, but are likely to change. They usually quote you low to get the job in the shop, then find other stuff that needs to be done along the way. Some of this is honest but some of it is not.

Second, consider their precision tools that they are using day in and day out, are they ever checked and recalibrated? The difference of a thousandth of an inch on lower-end clearance is crucial. It could mean the difference between a 50k mile engine or a 10k mile engine.

Third, they may have little or no experience with your vintage bike. So, if you have past mechanical experience and are willing to devote the time, rebuilding the engine yourself may be the better choice.

Motorcycle Engine Tools

The money you save by doing it yourself gives you a bigger budget to buy tools you need, and rebuilding a motorcycle engine will require specialty tools. If you don't have the required tools, you can buy them new or used, but many of them you can make yourself. Depending on how creative you are, many can be fabricated. Here's a few of the tools I've made to keep my old bikes on the road.

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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.

Cracked Engine Cases

Once engine cases are repaired, they are as good as new - provided the repair was done right. Don't entrust this job to someone whose only qualifications are that they own a welding machine. 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.

Repair Cracked Cases

read Repair Cracked Cases

Engine Disassembly

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, as the engine may be ruined.

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.

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Motorcycle Engine Building Tips

Before you start your motorcycle engine build, get a notebook and a camera. During disassembly, write down/document everything, especially in what order things came apart.

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 cases, make sure there's no grit left. Clean them thoroughly, two or three times if necessary.

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.

Consider investing in a motorcycle work stand. This puts you level with the work.

Motorcycle Engine Build

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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. Invest in a quality micrometer.

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.

Re-assembly is always harder. 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.

top end rebuild

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.

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.

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

How To Lap Valves

If the valve seats and faces were ground correctly, the lapping process should take a minute or less for each valve. It actually takes longer to clean, prepare, and round up the supplies than to actually lap the valves. You'll need a tube of valve-grinding compound and a valve lapping tool, which is basically a wooden dowel with a suction-cup on the end.

motorcycle engine build

Place the cylinder so the head of the valve is facing up. A couple of pieces of wooden 2x4's works well. Next, wet the suction cup on the lapping tool and stick it on the valve face. Once the lapper is grabbing the valve, begin the lapping process. Place the lapper between both hands, and using light pressure, rotate the tool back and forth at a moderate pace.

Lift the valve up periodically and rotate it 180 degrees to ensure the grinding compound is getting evenly spread. Watch the progress - look for a thin but even grey ring on the valve and valve seat with no breaks or high spots. Do not over-do lapping the valves.

Harley-Davidson 45 valve assembly

When you can feel and hear the compound losing its cutting ability, wipe off and check. Both faces should be a dull grey when you're done, with an even width across both faces. It does not have to be in the middle of the valve, just an even ring around the valve. You will see the same consistent gray line on the valve and the seat.

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.

Sealing Engine Cases

There are dozens of good products to seal motorcycle engine cases. I have used Permatex Ultra Black with good results. Other good products are Yamabond 4 and 5, which have been popular for years, but may now be discontinued. Three-Bond offers their #1194 sealant, which is an improved version of their #1104. Before applying the sealant, make sure the engine cases are very clean.

Transmission Rebuild

Many classic bikes have unit transmissions, while all Big-Twin Harleys have separate transmissions. Rebuilding a motorcycle transmission includes removing/inspecting/replacing bearings, shift forks, gears, seals, and gaskets. Lots of careful measuring and remeasuring is required. Transmission work is tedious and also requires specialty tools.

motorcycle engine build

read Harley 45 Engine Build

Harley 45 flathead engine build

I've built a few Ironhead Sportsters in my day. Although they're still fairly inexpensive to buy, they're quite expensive to have properly rebuilt. Before you start an Ironhead engine build, read this article.

read Ironhead Engine Build

Tools for Ironhead Sportster engine build

read Ironhead Special Tools

Rebuilding a classic bike motor could (and often does) become long and complex, 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.

Buying Aftermarket Parts

Factory parts are almost always better than aftermarket parts, but when original parts aren't available or stupidly expensive, aftermarket will be the only option.

Several U.S. companies make quality Harley-Davidson replacement parts. A company named Superior offers Japanese-made parts, which are also a good quality. Chinese and Taiwan-made aftermarket parts are of low quality. Beware.

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