Harley-Davidson Crankcase Boring For Big Bore Conversion Kits

The installation of most bigger bore aftermarket cylinders requires the Harley crankcases to be be machined to accommodate the larger diameter cylinder spigots. Additionally, the top centre case bolt hole, and the area surrounding it, must be modified. The Screaming Eagle 110 Twin Cam cylinders also need the cylinder base O- ring counter bores remachined.

These critical operations are done using a special machining fixture, designed and built here, on our large milling machine. It is not necessary to remove the cylinder studs, even on Evolutions and Twin Cams.

Crankcase reboring at Powerglides

Boring twin Cam 96 cases to take the SE 110 conversion cylinders. This is accurately carried out on our milling machine utilising a fixture built here. Because our mill is large enough, we do not have to remove the cylinder studs.

This service is available for all Harley Big Twin and Evolution Sportster/Buell crankcases, including aftermarket cases from S&S, STD and Delkron.

Please note that the OEM Evo Big Twin cases from 1990 through 1995 are not suitable for these conversions due to inherent structural weaknesses.

Crankcase reboring

Part of the case boring process also involves modifying the top centre case bolt area. Different crankcases, engine type, and cylinders used, all require different techniques. In this instance we use a ball nose cutter to remove superfluous material without causing any stress raisers.

Boring Sportster crankcases

Boring Harley-Davidson Evolution Sportster crankcases to accept Axtell 88" big bore conversion cylinder and piston kit.

Crankcase Breather Bore machining

Prior to the introduction of the Twin Cam, all OHV Big Twins used a rotary breather valve to help control pressures and oil flow inside the crankcase. Debris from other component failure frequently gets caught here, and causes damage to the bore that the breather valve sits in.

The late Shovelhead and all Evolution breather valves are manufactured from an engineering grade of plastic that can also cause unacceptable wear to this breather valve bore. Unless this condition is corrected, excessive oil carryover from the breather can result.

We can bore this breather valve cavity larger to suit a .030" oversize breather valve. This service is available for Knucklehead, Panhead, Shovelhead, and Evolution crankcases. We can do this with the OEM Harley-Davidson crankcases, as well as those from S&S, Delkron, and STD.

We can machine the breather bore on all Harley-Davidson, S&S, Delkron, and STD single cam Big Twin crankcases

We can machine the breather bore on all Harley-Davidson, S&S, Delkron, and STD single cam Big Twin crankcases, to suit oversize breather valves.

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