Pistons — Dos and Don'ts

For the pistons that we regularly offer in our online catalog, we indicate the diameters that we have measured ourselves. So not only the nominal bore diameter. For this, we randomly measure the delivered goods.

What can this information tell you? How can you use it?

It can serve as a guide if you are not sure which piston to choose. The diameter + the piston clearance suitable for this piston type gives the diameter that the cylinder must have. The fact that this may not correspond exactly to a specific oversize bore is of secondary importance. Oversizes are nominal sizes and tend to be a rough guide. Whether the calculated dimension from diameter and running clearance fits your cylinders is something you then have to determine and evaluate based on the condition of your cylinders. Or you can have a trustworthy workshop have a look at it.

What does this information not tell you? What should you not do under any circumstances?

The specified diameters are not absolutely guaranteed. Yes, we are responsible for our information. But, when measuring, there are tolerances, also depending on the environment temperature. Therefore, you will not have your cylinders drilled and honed in advance. Doing so will go wrong in most of the case. Do it the old-fashioned way: measure the actual piston yourself after purchase, add the desired running clearance according to the manufacturer's instructions or the Harley manual, and not only tell the workshop this measurement, but also take the actual piston with the actual cylinder to them so that they have the piston as a sample for their work.