MG Restoration

Balanced and Blueprinted

Les Bengtson

A number of people are confused by the terms “balanced” and “blueprinted” when referring to engine rebuilding. What does it mean when someone says they have a balanced and blueprinted engine?

All parts are made to certain tolerances, meaning that there are variations on size and weight. In addition, parts that rotate will not rotate truly if one side of that part is slightly heavier than the other. It will produce runout, which is a machinist’s term for wobble. Even in an item, such as the crankshaft, which is supported by bearings along portions of its length, the rotating part will try to wobble and cause excessive bearing wear.  So, in an engine, we have two separate balance requirements—those of rotating parts and those of reciprocating parts—the pistons and connecting rods.  Both of those classifications are balanced differently.

Reciprocating parts are balanced on a spring scale—similar to a bathroom or postal scale. The pistons are all weighed and the weight marked on them. Then, the three heavier will be lightened so they weight the same as the lightest. The idea being to produce four pistons (in a four cylinder engine) of exactly the same weight. The connecting rods have two ends—the small end where the piston connects and the big end where the rod connects to the crankshaft.  Each end is weighted and the three heaviest small ends are matched in weight to the lightest small end. Then, the same process is repeated with the big ends.

With the reciprocating parts—crankshaft, harmonic balancer (crankshaft pulley) and the flywheel, the process is different. The previous parts have been balanced statically (they do not move) while the rotating parts are balanced dynamically (they are spun). The first step is to assemble the harmonic balancer to the crankshaft using the crankshaft pulley nut. Then, the crankshaft is installed in the balance machine and the flywheel is installed in it, using the flywheel bolts.  The assembly is spun on a machine that works like the dynamic balancer used for tires. It will show where either metal needs to be added to the crankshaft or removed to balance all of the rotating mass as an assembly.  Peter Burgess, well known MGB engine specialist, recommends the reciprocating parts be balanced in all engines and that the rotating parts be balanced on high performance engines.  Even a road engine, however, will benefit from being dynamically balanced, providing smoother operation and longer life.  Since the cost of having the reciprocating and rotating parts balanced was about $100, I decided to have both done on the 79 engine I am rebuilding. Actual cost was influenced by the fact that I had already had the pistons installed at another machine shop and they had to be removed, balanced and re-installed.  I took the balance work to Bailey’s Machine Shop in Mesa because the machine shop which did the majority of the work did not have a balance capability.

Blueprinting an engine simply means to build it to some specification. This could be a factory specification (for cars which must run “stock” engines), the specifications of someone’s racing engine or one of the ones listed in Peter Burgess’ “How to Power Tune MGB Four Cylinder Engines”.  In other words, it is an engine built to some plan rather than one taken in with the directions, “see how much you need to clean up the cylinders and tell me what size pistons to order”.  In its most basis sense, a blueprinted engine is any engine built to a plan. Ideally, this should be an integrated plan with all the factors of performance and use built in. You can build a fully balanced and blueprinted race engine, but it will be of little use as a daily driver.  A more modest engine specification, such as .040” over pistons, crankshaft rod and main journals turned no more than .010” and a chrome bumper cam and duplex timing gear, is also a blueprinted engine, offering street driving capablility and, probably, meeting all emissions requirements (have not put the engine together yet, so do not know).

So, the next time someone says, “I have a balanced and blueprinted engine”, smile and say, “So do I. From the factory.”

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Copyright © 2001 by Les Bengtson

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