Machining Harley-Davidson Cylinder Heads

The faces of Harley-Davidson cylinder heads, as with other engines, need to be both flat, and possess a good surface finish, in order for the head gaskets to seal correctly. This is even more important with compression increases and the use of Multi Layer Steel (MLS) gaskets. These MLS gaskets, whilst capable of withstanding very high cylinder pressures, are not very tolerant of distorted, or badly finished, surfaces. What the Americans call "decking the heads", is common practice in both engine remanufacturing, and performance upgrades, and machining larger amounts from the cylinder head "deck" is also an easy way to achieve a moderate compression increase.

Machining Harley-Davidson cylinder heads

Machining is most commonly done on a milling machine, but we do it on our lathe, as this gives a better surface finish.

Powerglides designed and built cylinder head machining fixture

Our fixture, designed and built here, positions the heads using the original factory datum locations. The head will then spin around the true centre of the combustion chamber, which then allows us to use some more creative machining techniques.

facing cylinder head

Facing the gasket surface of this Twin Cam cylinder head in the lathe allows us to ensure that the cylinder head is both flat, and has an excellent surface finish. We can also remove larger amounts of material to raise the compression ratio if desired.

Setting the quench area of the cylinder heads so that the pistons come to within .030" at Top Dead Centre (TDC) makes the engine more efficient and less prone to detonation, particularly important with higher compression ratios and the lower quality petrol that we have to endure these days. The Harley-Davidson Evolution Big Twin, Twin Cam, and the '04 up Sportster 1200 heads have a quench area designed into them. Unfortunately, these clearances are far too large on the mass produced factory engines, and aren't really effective.

What is often referred to as "squish correction" can sometimes be achieved with different thickness gaskets, or pistons (such as some of the Head Quarters ones), with a corrected compression height. However, where this cannot be done with available parts, we can face each cylinder head up to the point where the head gasket bore starts, and leave a short spigot protruding into this gasket bore. This brings the cylinder head quench area closer to the piston and allows it to do what it was designed for. Pioneered by Head Quarters in a Harley application, this is known as "Spin dropped heads".

Spin dropped Twin Cam head

Because the machining fixture allows the head to spin around the true centre of the combustion chamber, by facing the head up to the inside edge of the gasket bore, we are left with a shallow spigot that sits inside the gasket, and brings the quench area closer to the piston.

machining Buell Thunderstorm heads

Some heads, like this Buell Thunderstorm cylinder head, have an angled quench area and a corresponding dome on the piston crown. By setting the compound slide on the lathe to the appropriate angle (15 degrees in this case), we can also correct the rather generous, and somewhat ineffective, factory clearance. Some high compression aftermarket Harley-Davidson race, and "fast street" pistons have angled domes, 30 degrees being common, and these require the heads to be machined in this manner.


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