They have higher load capacity, are more expensive to manufacture and create axial thrust.
3.
Double helical gears overcome the problem of axial thrust presented by single helical gears by using two sets of teeth that are set in a V shape.
4.
This arrangement cancels out the net axial thrust, since each half of the gear thrusts in the opposite direction resulting in a net axial force of zero.
5.
If the direction of rotation is reversed, the direction of the axial thrusts is also reversed, so a stable configuration becomes unstable, and " vice versa ".
6.
Several components had design changes, longer rods ( 152 mm c-c, longer ), pistons with a lower compression height, lower friction bearings ( smaller in size ), a crankshaft with 8 counterweights ( instead of 4 on the older Bxx engines ) and a heavy harmonic balancer ( aka damper ) in the crank pulley . 1989 saw an upgrade to the crankshaft, with a relocated axial thrust bearing back to the number 5 journal, bigger main bearings, rods were enlarged to from in 1990.
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