Engine Balancing

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Overhauling your classic’s engine during its winter lay up? Before reassembling his engine, Rob Marshall added a little extra to his budget and now shares his experiences of the balancing process

How’s your washing machine. Does it carry out its final spin smoothly as it dries your clothes, or more likely rumble and grumble by vibrating itself, almost to pieces as it gets older and the drum wears? The problem is balance or, rather, a lack of it and engines are much the same.

As the resulting forces, generated at a lowly 1000rpm, are sufficient to propel a concrete- ballasted domestic washer across the room, imagine what damage could be caused at engine speeds that are between two and six times faster. Trevor Wilkinson, a dynamic balancing specialist, with over half- a-century’s personal experience in the field, and proprietor of Andrews Precision Engineering in Worcestershire (01905 754513), advises that, when the speed doubles, the forces quadruple. He also commented that many classic enthusiasts do not appreciate that imbalance can waste a significant amount of energy and that a balanced engine, even one that is not intended for competition use, will run smoother, last longer, realise more power and become more frugal.

Whether, or not, you notice these benefits depends on how out-of-balance your engine’s components are, a factor that is influenced by not only basic design but also how each unit was assembled, when new. Expecting a balanced, but otherwise stock, Ford V4 to match a hand-built Jaguar V12 racing engine for smoothness is unrealistic but there should be a noticeable improvement on the standard motor’s behaviour, especially at high RPMs, where the undesirable imbalance forces increase rapidly. Should your engine be high-revving but its internals are out-of-balance, the benefits are more likely to be felt. Yet, if the motor is rarely extended beyond 3000rpm, the advantages of balancing will be less obvious.

HOW IT IS DONE

Most of the rotational imbalance emanates from the crankshaft, flywheel, end pulley and clutch pressure plate but, ideally, the pistons and connecting rods’ individual weights should be equal. Although most mass-produced classic car engines were balanced at the factory, inevitably their tolerances were far less rigorous, in contrast with today’s more highly stressed power-plants.

Engines can also go out-of- balance naturally, as heat, pressure and age affects the rotating parts. Fitting new pistons, clutches, or even having the crankshaft reground and its flywheel refaced, will also affect balance, as will missing dowels and unequal-length pressure plate bolts.

Brian Hart, who sadly died earlier last year, was a renowned race engine builder. He also redeveloped the V8 engine for the Triumph’s much-criticised Stag V8. One of his primary solutions was rebalancing and a number of those Hart-modified units continue to run faultlessly today.

While balancing cannot be carried out in a typical domestic workshop, the process is not ruinously expensive and refining the pictured Triumph 1500cc components cost me £144.00, including VAT.

As the long-stroke, vibration- prone unit tends to knock out its crankshaft bearings with alarming regularity, especially when revved hard, I figured that, while balancing would not make the motor able to rev much more beyond its safe limits, it would help the maligned engine last longer, especially as I envisage a continuous 4000rpm motorway cruising speed, owing to my car’s lack of overdrive and a lowly axle ratio.

Trevor started work, by separating the pistons from their connecting rods and weighing them. The lightest one was taken as a benchmark and the other, heavier pistons were lightened, by having excess aluminium removed from their undersides, using a lathe. The connecting rods were placed on a bespoke scale, which weighs both the little and big ends. Again, the lightest part was taken as a yardstick and material was ground from the heavier con-rods, until their mass was equalised.

Balancing my in-line engine’s crankshaft was straightforward but you can expect to pay a small premium for V-configuration units, because setting them up on the balancing machine is more complicated, because counterweights are needed. Once weighed and mounted on the apparatus, the bare crankshaft was spun and the rate of imbalance was indicated on a screen, which displayed not only the degree of imbalance but also the point at which it occurred, in a similar manner to that of a tyre balancer.

Trevor calculated that Triumph would have balanced my crankshaft to a tolerance of 80.5gm mm @ 6000rpm but the actual imbalance was found to be 175gm mm at its nose and a whopping 490gm mm at the clutch end. Post balancing, the imbalance had been reduced to 34 and 43gm mm respectively, which is half of British Leyland’s original tolerance specification.

Next up was the flywheel. After being weighed, a rusty drill mark revealed that balancing had been carried out previously. Trevor’s calculations established that its limit was set at 75.6gm mm but its actual figure was almost ten times that. Instead of grinding the material away, the flywheel was drilled at the points indicated by the balancing machine’s readout; engineer experience dictates how much material is removed, without inducing structural weaknesses.

Surprisingly, Trevor recommends that new clutch pressure plates should be balanced, mainly because their quality varies so much. Sure enough, the new part that I was going to install showed no signs of having been balanced before and the figure of 610gm mm, which reduced to 15gm mm afterwards, confirmed it. Finally, the crankshaft pulley was weighed and its balance checked, prior to being drilled and corrected.

Before I departed Trevor’s workshop, he made sure that each piston was refitted to the corresponding connecting rod and the bearing caps were not mixed-up. As each component was balanced on the crankshaft, I shall have to fit the flywheel, clutch and pulley in exactly the same positions, when reassembling the engine – otherwise the precision balancing would have been in vain.

ON THE FLY (WHEEL)

While you are having your vehicle’s engine balanced, you can add a bit more zest to it at the same time by having the flywheel lightened at the same time. By necessity it’s a pretty weighty affair to help balance the engine, which hinders throttle response. By removing some of the metal this improves (by a considerable margin on some engines) pick up but the trade off is usually a lack of smoothness and a lumpier tickover. Speak to your specialist or engine tuner on what’s the best compromise.

Balancing engineers do not refer to the force in Newtons but express imbalance instead by multiplying the weight of a component by the radius from its pivot. Therefore, a flywheel that has 10 grammes (gm) of imbalance, mounted at a radius of 50mm will give an imbalance of 500gm mm.

As my old washing machine demonstrated, the forces that imbalance promotes can be considerable. If a flywheel, balanced to 105gm mm, was offset on the crankshaft by only 0.05mm, the resulting imbalance of 500gm mm would generate a hefty 20kg of force at 6000rpm!

COSTS

Sample prices are exclusive of VAT and provided by Andrews Precision Engineering, Unit 86g, Blackpole Trading Estate West, Worcester, WR3 8TJ. The procedure includes crank, front pulley, flywheel and pressure plate balancing.

* 4-cylinder in-line: £120.00

* 6-cylinder in-line: £160.00

* V6: £200.00w

* V12 – £300.00