Handy stuff about engine builders

As we left off, I was in the middle of explaining the aspects of the mysteries of blueprinting, handy stuff to communicate to your machinist, or to school to know-it-all at your next party. In continuance of last month's discussion on blueprinting, we are still discussing setting piston-to-wall clearance.

With aftermarket racing pistons, you do not have this select fit option so the machine shop must match the bores to the pistons, not a big deal for a high quality machine shop. The piston-to-wall clearances should all be matched to within 0.0001-0.0002" of each other by the machine shop. This match should be confirmed when you assemble the motor as shops have been known to mess up. With aftermarket pistons you are usually going for a significant overbore and to get some more displacement. Most Jap engines can be bored 0.040" or one millimeter oversize with no problem. Forged aftermarket pistons run looser piston-to-wall clearances so you will want to use the clearances specified by the pistons maker, not the ones in the factory service manual.

With aftermarket pistons, slightly oversize rings can be bought so the end gaps can be controlled by filing the ends of the rings so every bore will have exactly the same ring end gap. Usually you will want to make the gap on the first compression ring slightly wider as this ring runs hotter and expands more under use. Many engine builders are running some of the newer gapless compression rings where the number-two compression ring has a staggered gap. Correct cylinder wall finish is also critical and is a point where many engine builders mess up. Modern rings found on import motors are usually low tension and chrome faced. This type of ring requires a very smooth cylinder wall finish, usually 600 grit followed by a plateau hone to make the finish even smoother. Many old school engine builders are not aware of this and make the cylinders finish too rough. Quick wear, poor seal and oil burning result.

Making sure that the combustion chamber volumes of every cylinder are equal is a blueprinting job given to the cylinder head porter. Usually a good head person will try to get all the chamber volumes to be within 0.5cc of each other so all the cylinders will have pretty close to the same compression ratio.

Every engine should be balanced. By balancing the engine's internal parts, vibratory stress on the engine is greatly reduced. An out of balance condition of a few grams can result in an unwanted load of many pounds in the wrong place. If these imbalance loads are added up, they can result in quite a bit more stress on an engine's critical internal parts. Balancing means dynamically balancing the crank, equalizing the weights of the pistons and the big and small ends of the connecting rods. The weight variation between all moving parts in the motor should all be less than one gram for best results.

This is accomplished by drilling holes or grinding the counterweights of the crank. For connecting rods, sanding the balancing boss on the bottom of the rod caps and the small end wristpin boss on a belt sander works. For pistons you carefully remove metal from around the sides and upper corners of the pin boss and the underside of the dome on pistons. With pistons you have to be careful not to make the dome thinner that 0.200" or to weaken the pin boss.

One of the trends that many builders of Jap motors are starting to follow as a fad is excessive lightening and knife edging of the crank. I don't believe in doing this for two reasons. It is important to maintain about a 50% overbalance of the crank when the counterweights of the crank are around 50% heavier than the reciprocating mass of the motor. This helps the engine work smoother on the compression stroke. Pairing down the counterweights to almost nothing and reducing the overbalance may introduce additional stress introduced into the crankshaft. This is a definite no-no for engines that must last a while like street or road racing cars. I question it for even short duration races like drag racing. Hopefully this article will help you understand blueprinting and help you communicate with your engine builder or catch a bogus engine builder that is trying to BS you.

Well that's it for this episode feel free to e-mail me on plasmaim@yahoo.com and I will do my very best to answer your queries.
Happy Racing and Keep it Safe!