Boring and honing Harley-Davidson cylinders Part 3

Powerglides - Harley-Davidson Technical Articles







Honing Harley-Davidson cylinders

Honing, like grinding, uses abrasives. Whereas grinding uses an abrasive wheel or disc, honing uses abrasive sticks glued to a carrier. They are made by a vitrification process that fixes abrasive particles into a filler known as a bond. There are different types of abrasives, as well as different particle sizes. This is known as the grit size. The bond is also made in different strengths. Different materials require a different abrasive. Hard steel (such as bearing races), for instance, is honed with aluminium oxide, whilst cast iron is honed with silicon carbide. The coarser the grit, the faster the stock removal, but the rougher the finish. The process works by the cutting action of the grit against the work piece.

As the grit is sharp, it removes minute amounts of material from the component being honed, but as it does this it becomes blunt and needs to be replenished. The strength of the bond affects how this works. A strong, or Hard bond, keeps hold of the abrasive particles for longer, but requires a greater cutting pressure to keep them working. A weak, or Soft bond, releases the dull abrasive easier and requires a lower cutting pressure. A Hard bond also increases the heat generated, but the stones last longer and hold their shape well. Conversely, a Soft bond produces less heat, cuts easier, but wears out faster. Soft stones are also prone to uneven wear and care is needed to avoid tapering of the cylinder.

Most automotive cylinder honing is done with fairly hard stones, as the cylinder blocks are very ridged and a high cutting pressure doesn't really cause any issues, plus stone life is very good. However, Harley cylinders have a relatively thin, unsupported spigot protruding from the cylinder base. The relatively high cutting pressures used with a hard stone cause this area to deflect away from the pressure of the cut, and the bottom of the liner can then taper inwards. Also the heat generated can cause enough cylinder expansion to make the bore measurements inaccurate. We use relatively soft stones as their lower cutting pressure requirement makes honing easier.

Honing also requires a specifically developed honing oil. This oil helps control stone wear, the consistency of the surface finish, carries away the debris created, and greatly reduces the heat build up. We use 15 gallons in a pumped and filtered recirculating system and flood the cylinders with it to help control the heat build up.

We start with coarse stones to remove the bulk of the material left after boring, the idea being to remove all traces of the boring cutter finish as well as the work hardened material just beneath the surface. This is why it is considered good practice to leave around .005" to be removed by honing. Next, we swap to the finishing stones which remove the surface left by the coarse stones. Under a microscope, however, this finish looks like a mountain range. The valleys are necessary to retain oil for the benefit of lubricating the pistons and rings, but the peaks of the mountains need to be "plateaued" to prevent them becoming micro welded to the piston rings on initial start up. So finally, we swap the finishing stones for plateau honing "stones". These are not like conventional stones as they do not really cut, rather they just remove the peaks from the surface finish. However, over plateauing can prevent the rings from bedding in, so getting the balance right is essential.

The stones are both rotated and reciprocated during honing and this leaves a cross hatch finish. The angle of this cross hatch is important, too shallow (common with hand honing) allows too much oil to be retained on the cylinder wall and high oil consumption is likely. Too steep, and lubrication can suffer, resulting in premature wear. We have found a 60 degree pattern to be the optimum for Harley-Davidson cylinders.

honing stones

Each set of stones consists of two stones and two guide shoes. The shoes help keep the bore round and straight. In the middle are the roughing stones, to the left are the finishing stones, and to the right are the plateau "stones". These do not have guides, as they do not remove any appreciable material.

Honing head

The stones and guide shoes are installed in the honing head. The carriers have rack teeth machined into them, and a pinion gear inside the head can be rotated to expand or contract the stones to apply or release the cutting action.

Honing machine

The honing head is attached via a drive shaft to our Delapena Auto Stroking Vertical Honing Machine. This both rotates and reciprocates the honing head automatically, reversing the reciprocation at the end of each stroke. Both of these functions are infinitely variable to suit the material being honed and also to control the cross hatch angle generated. The aluminium wheels at the top of the drive shaft are known as a Friction Feed, and contain a planetary gear system that allows stone expansion and contraction whilst the machine is running. We can also short stroke the machine at either end of the adjustable stroke, or dwell anywhere within the cylinder. This gives us total control over the honing process which is why we can work to .0001". A very accurate machine in the hands of a skilled operator.

Honing twin cam cylinders

The soft bond roughing stones give rapid stock removal on these freshly bored Twin Cam cylinders.

Finished cylinder

These finished Harley-Davidson Twin Cam cylinders will now be used on one of our performance upgrades with Head Quarters pistons. Notice the 60 degree cross hatch pattern conducive to good cylinder and piston lubrication with minimal oil consumption.

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