Boring and honing Harley-Davidson cylinders Part 2

Powerglides - Harley-Davidson Technical Articles

Boring Harley-Davidson cylinders

In the early days of the internal combustion engine, all cylinders were turned on a lathe. In line multi cylinder engines tended to have individual cylinders, and aircraft engines carried this on for many years. Obviously, radial engines and V-twins, consist of a number of single cylinders fixed to a common crankcase, and always benefited from being produced on a lathe. Even multi cylinder in line engine blocks were produced this way, but the lathes had a massive "swing" and so eventually the use of boring machines became the normal production method.

Reconditioning used cylinders and cylinder blocks presented it's own problems however. Cast iron (and it's alloys), that has always been used for cylinders or liners, is relatively easy to machine when new but in use develops a very hard glaze due to the action of heating and cooling, the by products of combustion, and the very high loads applied. Remachining a used cylinder bore was very prone to having the cutting tool fail before the end of the cut, and it wasn't until tungsten carbide and the use of hard abrasive honing stones came into use during the 1930's that the cylinder reconditioning techniques that we now use became the norm. Prior to this many cylinders had to be reconditioned by internal grinding.

With the advent of tungsten carbide (it will scratch glass) portable boring bars came of age. These would be attached to the cylinder block by clamping into the adjoining cylinder after first being centred in the hole that was to be machined. For motorcycle cylinders, the boring bar is attached to a stand and the cylinder clamped upside down, underneath it. It's a good way of doing Harley cylinders and is still in use today. We used to use an American Kwik-Way boring bar and stand to do our cylinders, but it does have limitations, notably the size of cut that can be taken. These machines are great for boring cylinders to the next oversize, but require many cuts to remove the amount of material required in taking a 3" bore 883 Sportster cylinder out to the 3½" bore that is used in a 1200 conversion.

We've already discussed that the axis of the bore must be at a perfect right angle to the cylinder base (if it isn't, then the effect is the same as a bent connecting rod), and these portable boring bars must be well maintained, and reconditioned if required, to ensure that this "squareness" is guaranteed. Also, because the cutting head extends further from the body as it moves further down the cylinder, any wear, or excessive clearance, can produce a hole that is neither round nor straight.

There are, of course, large, fixed, boring machines used by automotive reconditioners, but these require special fixtures to accommodate Harley cylinders and have to be very accurately made to ensure the critical "sqareness" of the bore to the cylinder base.

As the bulk of our cylinder work is now Harley-Davidson big bore conversions used in our Twin Cam and Evolution Sportster performance upgrades, all of which require a fair bit of metal removal, we use our large lathe with our own fixturing and a suitable boring bar.

Harrison M400 boring a Twin cam cylinder

Our Harrison M400 lathe is very rigid, powerful (it has a 10hp motor), and very accurate. Using our own fixturing and a 2" diameter boring bar with a carbide tip makes short work of boring large amounts of material from a Harley cylinder. This lathe weighs the same as five Streetglides!

Checking cylinder fixture

We have designed and manufactured fixtures that mount directly to the lathe spindle. This Twin Cam fixture also acts as the lower torque plate. Since it has been machined "in situ", there is zero runout on the face that the cylinder mounts to. This guarantees that the axis of the cylinder bore will be perpendicular to the cylinder base, and thus fulfils one of the important criteria demanded of precise cylinder boring.

Clocking cylinder bore

Cylinder is effectively clamped between torque plates. We lightly clamp it first, and then ensure that the cylinder runs true by using a dial indicator. Then we torque the clamping bolts.

Boring a Harley-Davidson Twin Cam cylinder

After setting the cutting tool we can start machining. Digital Read Out (DRO) tells us where we are.

Boring a Harley-Davidson Twin Cam cylinder

This is a very strong and accurate way of boring Harley-Davidson cylinders. We are only limited by the strength of the cylinder itself and it's tendency to distort if the cut is too heavy. The relatively thin, and unsupported, cylinder spigot is the limiting factor, as it can deflect away from the tip if the cut is too heavy. Correcting this at the honing stage is time consuming, so it's better if we go easy at this stage. For the Twin Cam cylinders we take one roughing cut and a finishing cut. For the Evolution Sportster 883 to 1200 conversions we take two roughing cuts and a finishing cut. We leave five thou in for honing and we have perfect bore geometry.

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