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The balance shaft gear bolt holes should be in the middle as shown below in the yellow box highlight. They should not be right on the edge of the holes.


Place the intermediate gear on and test fit it. As you you slide it into place, the balance shaft gear bolt holes should stay about in the middle. Again, note the dog eared washer underneath. With the gear in place but not tight, tighten the 4x 10mm balance shaft gear bolts in place to 15 ft-lb (20 Nm) but don't add the final 1/4 turn yet. I suggest not adding the final turn so you can check for play. Once the bolts are tightened to their final torque they cannot be reused. Remove the balance shaft lock.

Note: the intermediate gear coating should be uniform around the gear teeth. This coating will wear away with use and this sets the correct gear lash or play. If the gear has a painted dot on the face and the coating is only at one spot, install the gear so that the coating is touching the other gears. When you tighten the intermediate gear it's normal for the balance shaft and gear to slightly rotate.

Use a piece of wood to push hard on the intermediate gear in about the direction of the blue arrow (between the crankshaft and balance shaft gears). While pushing on the intermediate gear, tighten the 14mm triple square intermediate gear bolt to 15 ft-lb (20 Nm) but do not add the final 1/4 turn, the final torque. Again, you should have loosened it (it's lightly installed from the factory) enough so that it can slide when you press on it but have it tight enough so that it won't be lift and be crooked.

The purpose is to test fit the gears and check for gear backlash and gear motion. The gears should have no backlash (play when turning) and should move smoothly and be tight. The crankshaft lock should be at TDC and the balance shaft lock should slide in and out. Double check that the dog eared washer under the intermediate gear isn't crooked and pushing the gear out. The gears should be relatively flush and it should look like this from underneath:


If it passes these tests, add the final 1/4 turn to the 14mm triple square intermediate bear bolt and then to the 4x 10mm balance gear bolts. When you tighten the intermediate gear, push hard on the intermediate gear as before so that it won't move at all. The front flange is aluminum and can't counterhold using the timing belt crank tool (the black thing with the handle and teeth) so I suggest using the crank yank to counterhold the crankshaft sprocket when tightening the bolts. Again, there should be no backlash in the gears when done.

 

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In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced (for example, most four-cylinder engines).

Overview

Balance shafts are most common in inline four-cylinder engines which, due to the asymmetry of their design, have an inherent second order vibration (vibrating at twice the engine RPM) which cannot be eliminated no matter how well the internal components are balanced. This vibration is generated because the movement of the connecting rods in an even-firing four-cylinder inline engine is not symmetrical throughout the crankshaft rotation; thus during a given period of crankshaft rotation, the descending and ascending pistons are not always completely opposed in their acceleration, giving rise to a net vertical inertial force twice in each revolution whose intensity increases quadratically with RPM, no matter how closely the components are matched for weight.^[2]

Four-cylinder flat engines in the boxer configuration have their pistons horizontally opposed, so they are naturally balanced and do not incur the extra complexity, cost or frictional losses associated with balance shafts (though the slight offset of the pistons introduces a rocking couple).

The problem increases with larger engine displacements, since one way to achieve a larger displacement is with a longer piston stroke, increasing the difference in acceleration or by utilizing a larger bore thereby increasing the mass of the pistons. One can utilize both techniques in order to maximize possible engine displacement. In all cases, the magnitude of the inertial vibration increases. For many years, two litres was viewed as the 'unofficial' displacement limit for a production inline four-cylinder engine with acceptable noise, vibration, and harshness (NVH) characteristics.

The basic concept has a pair of balance shafts rotating in opposite directions at twice the engine speed. Equally sized eccentric weights on these shafts are sized and phased so that the inertial reaction to their counter-rotation cancels out in the horizontal plane, but adds in the vertical plane, giving a net force equal to but 180 degrees out-of-phase with the undesired second-order vibration of the basic engine, thereby canceling it. The actual implementation of the concept, however, is concrete enough to be patented. The basic problem presented by the concept is adequately supporting and lubricating a part rotating at twice engine speed where the second order vibration becomes unacceptable.

There is some debate as to how much power the twin balance shafts cost the engine. The basic figure given is usually around 15 hp (11 kW), but this may be excessive for pure friction losses. It is possible that this is a miscalculation derived from the common use of an inertial dynamometer, which calculates power from angular acceleration rather than actual measurement of steady state torque. The 15 hp (11 kW), then, includes both the actual frictional loss as well as the increase in angular inertia of the rapidly rotating shafts, which would not be a factor at steady speed. Nevertheless, some owners modify their engines by removing the balance shafts, both to reclaim some of this power and to reduce complexity and potential areas of breakage for high-performance and racing use, as it is commonly (but falsely) believed that the smoothness provided by the balance shafts can be attained after their removal by careful balancing of the reciprocating components of the engine.^{[citation needed]}

[edit] Four-cylinder applications

 

Mitsubishi Motors pioneered the design in the modern era with its "Silent Shaft" Astron engines in 1975, with balance shafts located low on the side of the engine block and driven by chains from the oil pump, and they subsequently licensed the patent to Fiat, Saab and Porsche.^[1]

Saab has further refined the balance shaft principle to overcome second harmonic sideways vibrations (due to the same basic asymmetry in engine design, but much smaller in magnitude) by locating the balance shafts with lateral symmetry but at different heights above the crankshaft, thereby introducing a torque which counteracts the sideways vibrations at double engine RPM, resulting in the exceptionally smooth B234 engine.

[edit] Six-cylinder applications

 

Due to the odd number of cylinders in each bank, V6 designs are inherently unbalanced, regardless of their V-angle. All straight engines with an odd number of cylinders suffer from primary dynamic imbalance, which causes an end-to-end rocking motion. Each cylinder bank in a V6 has an odd number of cylinders, so the V6 also suffers from the same problem unless steps are taken to mitigate it. In the horizontally opposed flat-6 layout, the rocking motions of the two straight cylinder banks offset each other, while in the inline-6 layout, the two ends of engine are mirror images of each other and compensate every rocking motion. Concentrating on the first order rocking motion, the V6 can be assumed to consist of two separate straight-3 where counterweights on the crankshaft and a counter rotating balance shaft compensate the first order rocking motion.

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