"Twin Cam Performance Guide"

Big Bore Kits

I am the first to boast of Harley Davidson's hard work at improving quality control. They are leap years ahead of where they were ten years ago and continue to strive for excellence. But the Factory has done something that simply confounds me. When purchasing the Screaming Eagle 1550cc Big Bore kit, the barrels, (cylinders), are purchased separately from the piston kits. This is a big no no in the mechanical world. Even with the best quality control procedures in the world there will eventually be a screwup in sizing with a seized piston as a result. What's more, the consumer, the Dealer, or aftermarket shop that doesn't have a tool called torque plates are unable to check the piston to cylinder wall fit to ensure proper fitment tolerances. This is a pretty expensive tool that is comprised of two heavy precisely ground flat metal plates that sit on top and bottom of the barrel. They are bolted together and torqued to actual operating pressure the same as when the barrels are installed on the engine. This is a precise method to determine the tolerance between the piston and the cylinder wall when the barrel is in a perfectly round configuration. Whoa! What the hell is that supposed to mean? Modern Evo and Twin Cam 88 barrels are honed and/or bored to fit while under actual torqued pressure. When the torque plates are removed, the cylinder relaxes into a non-performing state that may be out of round and/or tapered. So. I ask you. How does one check piston fitment without this tool? One doesn't! One more wrinkle. The Factory is presently having difficulty providing all the specialized tools to work on the Twin Cam 88. Evo torque plates do not work on Twin Cam barrels. We had to make our own torque plates in our machine shop in order to check piston fitment. (good pics page 3-71, 1999 FLT Factory Manual part #99483-99) Harley's specs for fitment are between .0006" and .0016". That's pretty tight. Six ten thousandths of an inch to 16 ten thousandths of an inch. Compare this to a human hair, which averages three thousandths of an inch, which translates into 30 ten thousandths, of an inch. The piston is worn out at .0053" which is a huge measurement compared to the fitment specs. The pistons will be a knockin' at that tolerance. Guess what? The separately purchased barrels and piston set we measured had different tolerances on each cylinder. They were within Factory specs. But they pushed the limits. One was fit at the minimum standard of .0006" while the other was near the maximum of .0016" So, what am I going to do in the future? I'm going to keep .005 oversize pistons in stock and hone the standard sized barrels out to specs. that I know are accurate if standard sized pistons aren't on the mark. It just makes me feel better. It may save a customer a lot of grief and myself a lot of warranty work. In order to do this, a shop must have some expensive machinery like a boring bar and a precision hone machine. If you are in doubt, ask to see this equipment plus tools like the torque plates. Speaking of warranty. Check this out! This is a direct quote from the Screaming Eagle Twin-Cam 88 Big Bore Cylinder Conversion Kit instruction sheet.



"Use of this cylinder conversion kit may reduce or void the Limited Warranty coverage"

Pleeeze.

Harley might not warranty it but any quality shop will. Ask for a written warranty on your work order. Any reputable shop will not have a problem with this. One last point. Early Factory instruction sheets for the 15550 cc conversion did not tell the consumer or mechanic to leave off the stock barrel top cylinder dowel o-rings. Later instruction sheets have been corrected. Continue to use the bottom cylinder oil return dowl o-rings. The 1550 cc big bore head gaskets have eliminated the need for the top o-rings that are required with the stock 1450cc head gaskets

Cam Sprocket Change

Although Harl