Friday, January 26, 2007

Look Sharp

This post has to do with some of the details that can make or break an engine. Sharp edges. Rarely, if ever, are sharp edges a good thing inside your motor. Of course the time to address this issue is during the engine build. I wouldn't tear down an engine just to look for a "potential" problem like this unless I had a pretty good idea that the problem is likely to be there.

Starting in the middle (more or less) we find the most obvious and in many ways most egregious offenders. That would be the pistons. It is common knowledge that sharp edges in the combustion chamber are a good source of pre-ignition. The piston domes are one of the areas that nearly always needs some attention. Many pistons come out of the box with sharp machined edges, particularly on the edges of the valve reliefs. A few years back one of the aftermarket Harley engine manufacturers even came out with instructions that if the engine was to be used in hot climates that these edges should be rounded to help prevent pinging. All that is really needed is a some medium grit sandpaper to solve this potential problem. I usually use either a de-burring tool (or Buck knife if the de-burring tool is on the other end of the shop) and then finish with sandcloth. So now your beautiful shiny machined pistons domes aren't quite as photogenic, but they are more functionally attractive. Even the generic cast pistons we use in our Knucks, Pans, and Shovels should be inspected. Often they will have a small raised spot at the tip of the dome that has a sharp edge.
But you are not done with the piston yet! Now look at the bottom edge of the skirt. There should be a slight bevel on that lower edge. If there isn't one, imagine that piston on its downstroke with just a little cocking of the piston on the wrist pin. Yeah, I can picture the sharp leading edge of that piston scraping all of the oil right off the cylinder wall! By the way, credit for that tip goes to Axtell Sales in Des Moines (the people who are famous for their drag racing cylinders), who have a note in one of their piston instruction sheets to that effect. While you are in the area, make sure that any reliefs in the cylinder spigot have a rounded edge for the same reasons. If you have added the reliefs for a stroker motor, be sure you don't leave them sharp. Aftermarket cylinders will likely come with sharp edges here. Stock cylinders are usually ok from the factory but what about after you bore them a few times?

Working our way down the engine, the outside edges of the female connecting rod where they come into contact with the flywheel trust washers should be inspected. If you are running an early roller bearing Pan or Knuckle, then attention should be paid to the edges of the case races as well as the flywheel end play adjusting washers. In the same vein, take a close look at those case races. I have seen some new ones that were rough enough on the inner end that they would work like a grinding wheel on the end play washers. Within a couple thousand miles you will have double (or more) the end play that you set it up with!

Moving over to the gearcase, some of the aftermarket tappets (lifters) that I have seen have such sharp edges that you are in danger of cutting yourself just handling them! Obviously with the designed-in side loading on these parts in most of Harley's engine layouts, what I said about sharp edges on the lower edges of piston skirts also applies to both the top and bottom edges of the tappets.
Back up top, everything that I said about the piston domes in regards to pre-ignition also applies to the combustion chamber.
You're probably starting to get the idea by now. Just examine each part before you install it, and consider how any sharp edges you find might effect the engine. Remember, look sharp . . . but don't leave it sharp. A happy Harley engine is one that looks sharp on the outside, but isn't sharp on the inside.

No comments: