Cylinder Bore & Hone - HRBW Tech

The cylinders have about 60k on them. Visually they look to be in good shape; this owner followed the maintenance schedule religiously.

Overall rating: +++

So you're ready to freshen your motor and bump up the displacement with some bigger pistons. You know your barrels need to be bored and honed, but don't know exactly what that means or how it's done. After taking the upper end off of your bike, it's time for a trip to the machine shop. This could be your local H-D dealer or an aftermarket shop. But what happens after you leave? The shop seems like a black hole that swallows your parts, then spits them out with a bill. Let's see if we can remove some of the mystery.

To get a level set on sizes, let's review measurements used. The average clearance of piston to cylinder wall is .001-.002. This is one to two thousandths of an inch. The human hair is about .003, or three thousandths of an inch. As you can see, it is a very small amount. Most shops operate at tolerances of plus or minus .0002. That's two ten-thousandths of an inch, the same as cutting a hair into thirty pieces and stacking two of them on top of each other.

Step one is to clean the head and base surfaces with a gasket scraper.

The requirements for your cylinder bores are that they be square, straight, and round. To get there we bore them, hone them, and put in the proper crosshatch or surface finish.

The bore must be square to the base surface. The mating of the base to the crankcase sets the path the piston follows. If the bore is at slight angle, the piston will not run true. This will cause loss of power, and short ring and piston life.

Into the wash tank to remove any loose gasket material, oil, and grease.

The bore must be straight. Taper will cause the rings to flex in and out to meet the cylinder wall. This constant flexing will reduce their life expectancy and lead to loss of power.

The bore must be round or you'll get ring flex and piston noise. All of these will shorten the life of the pistons, rings and the cylinder bore itself.