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A Primer on ENGINE BALANCING -- EB-101

When balancing an engine, you may hear that you have to supply all the moving parts to the shop doing the work, crankshaft, harmonic balancer (front pulley), flywheel, clutch cover, clutch disk, bearings, connecting rods, pistons, wrist pins, piston rings. This may lead you to believe that they balance all the parts together. This is usually not so, and for an MG engine in particular this is not necessary. The key factor here is that the MG crankshaft is symmetrical when viewed from the front, so can be balanced alone with no other parts attached. To state that differently, #1 and #4 rod journals are symmetrically opposed by #2 and #3 rod journals, and everything else on the crankshaft is round and concentric to the main axis of rotation. All other parts are either also concentric to the main axis, or come in pairs offset by 180 degrees on the crankshaft to counterbalance each other.

All that means that the rotating parts can be balanced individually, and the reciprocating parts can be balanced as sets. In this case it is also best to do it that way. If you balanced the whole assembly but ignored the individual parts, then changing any one part later could unbalance the whole system.

Balancing engine parts differs from balancing road wheels in that weight is not added to the light side of the parts, but is removed from the heavy side. For the crankshaft, the factory finds it quick to do this by drilling holes in the outer edges of flanges and counterweights. It can also be done by grinding off a bit of any non-machined surface at some distance from the center of rotation, again usually at the extremities of the counterweight areas.

Since the crankshaft is not symmetrical from front to back, it cannot be statically balanced like bubble balancing of a road wheel. The crankshaft needs to be spun in a machine to produce a vibration. The location and direction of the vibration is indicated by the machine, and the operator can then remove weight from the appropriate location on the crankshaft to reduce the vibration to some specified acceptable maximum.

Main bearings are non-moving parts, so are not required to be balanced. Rod bearings are machine made, nearly identical, and small, so are usually well within 1/2 gram of each other as shipped, so are usually ignored. If you were to carefully check the rod bearing shells and find them not matched to your satisfaction, weight can be removed from a bearing shell by grinding a bit off of the non-contact surfaces. That would mean only at the sides, not on the ID or OD and not where the flat ends come together.

The front pulley on an MGA is not a harmonic balancer type (except very late production MK-II 1622 engines), but is an assembly of two pieces of stamped sheet metal riveted to an iron hub. Here it is impractical to add or remove weight from the sheet metal pulley, so if balancing is required, a bit of material can be removed from the edge of the iron hub. This type of pulley is usually quite symmetrical and has low mass at the rim, so generally does not upset the balance significantly and is usually ignored.

Later MG engines used harmonic balancer type front pulleys, these having a heavy iron rim section attached to an iron hub with a vulcanized rubber joint. These pulleys do have enough material to balance at the rim, usually done by drilling into the face of the rim section. These pulleys also have significant mass at the rim by design, and should be checked for balance. Spin balancing is the preferred method here.

The flywheel is by far the most massive moving part of your engine, so the most critical to balancing. Even if you use a light flywheel that weighs less than the crankshaft, the bulk of the mass of the flywheel is much farther from the center of rotation and will affect the balance proportionately. Spin balancing is the preferred method here as well, with material being removed from the non-contact face at the rim. The factory always balances flywheels (within reason), so you will generally find some drill holes in the front face of the outer rim just inside of the ring gear.

The factory balancing of the flywheel may not be close enough to satisfy some people (especially for a race engine). With considerable age, warping by thermal stress or wear on the friction surface may require refacing of the flywheel. It is a good time to have it re-balanced if any machining is done. Also, while changing a ring gear should not significantly affect the balance of a flywheel, if you're superstitious or critical or if it's a race engine, this is also cause for rechecking the balance.

The clutch cover (pressure plate assembly) and clutch disk are usually well balanced as shipped from the factory, and can generally be ignored. However, if you're concerned, or if it's a race engine, the clutch cover can be mounted to the flywheel and balanced after the flywheel has been balanced alone. The MGB diaphragm type pressure plate assembly is not likely to loose the balance over time. The MGA three-arm pressure plate assembly is more likely to get out of balance if somehow misadjusted, especially if it is disassembled for reconditioning. After reconditioning and reassembly the MGA pressure plate should always be re-balanced.

The connecting rods are the next critical item to balancing an engine. These are statically balanced on weighing scales. Two scales are used, placed under the extreme opposite ends of the rods (one rod at a time), and carefully located in the same position for each rod. The method here is to weigh all the rods first, find the rod with the lightest big end, also find the rod with the lightest small end. Then you remove weight from the ends of the heavier parts until the big ends all weigh the same and the small ends all weigh the same. Again material is removed from the non-contact surfaces. Maximum imbalance here is usually specified as all parts to be equal within 1/2 gram.

As an alternative for balancing con-rods, begin by weighing all of the small ends first. Then remove material from the heavier ones until all of the small ends have the same weight. Then you can weigh the entire connecting rod, all of them in turn, to determine which one is lightest. Then remove material from the big end of each until they all match the weight of the lightest one. There is a good YouTube video of this process

As a side note, for racing engines an effort is often made to considerably lighten the big end of the connecting rods to reduce bearing loads and allow higher engine speeds. When carried to extreme, some material is removed from the bosses with the tapped holes and the outer shoulders are rounded off. The seating surface for the bolt heads is sometimes cut thinner or counter-bored and shorter bolts with smaller heads can be used. Much material is sometimes removed from the big end bearing cap. Do not try this at home if you value your engine, it is not a procedure for the faint of heart.

Pistons are also critical to good balance, and the specification here is also usually all weighing the same within 1/2 gram. Material can be removed from the bottom edge of the piston skirt, or from the interior surfaces around the large boss that holds the wrist pin. I would not recommend removing any material from the interior in the vicinity of the ring grooves, as I have personally witnessed a piston failure because of this.

As another side note, many racing pistons can be lightened considerably by removing material around the wrist pin bosses. Forged pistons especially can be lightened by cutting away material between the wrist pin boss and the inside of the crown. Removing only a minimal amount of material for balancing is easily done. However, if you have in mind to remove considerable material for lightening the pistons, especially if you want the lightest parts possible for a race engine, you had best give this job to someone with a great amount of experience. When it comes to high stressed and high speed internal engine parts, it's not nice to fool mother nature.

For my MGA 1500 I just bought a new set of +.020" pistons. These are Aerolite 4-ring and came from Moss Motors inventory. Inside the cover of the box is printed the proclamation that these pistons are weight matched within 1/2 gram. My engine guru just smiled and said not to bother bringing these into his shop, that's as good as it gets.

Now, my new spare MGA 1500 engine has the original style sheet metal front pulley, I don't have a flywheel for it yet, and the pistons are already balanced. So I took in just the crankshaft and connecting rods. For this I was quoted a price of $35 for balancing. Now I've always thought that balancing was a waste of money for a street engine in normal service, but I autocross a lot and often run the engine to 7000 rpm, so for the price I couldn't resist balancing this one. For the curious, SCCA does allow engine balancing in all classes, but lightening of the internal engine parts puts you squarely into Modified class.

Barney Gaylord

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