Quote:
Originally posted by Kai-Petri:
From Death Traps by Belton Y Cooper
On Shermans
One of the most needed maintenance parts was spark plugs. Most of the M4 tanks had R975 Wright nine-cylinder air-cooled radial engines. When the engine was started the tank usually backfired with considerable noise which gave away the unitīs position.Most tank crews would idle the engines as slowly as possible when trying to maintain a defiladed position in the hedgegrows. Designed for high constant speeds in an aircraft the engine had excessive clearance between the cylinder walls and the pistons. In a tank, where the engine was run slowly, the excess clearance allowed then engine to pump oil which fouled the spark plugs.Each engine had nine cylinders and each cylinder had two spark plugs. This meant that eighteen spark plugs had to be changed every time the engine fouled.
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This is true only of the M4A1 that represents about 30 - 40% of the vehicles in Europe in early 44. By late 44 it is down to around 20%. Most US Shermans in the ETO were M4A3's with the Ford GAA engine (half a RR Merlin acutally). Lend lease vehicles were variously diesel (GM or Caterpillar) and Chrysler powered.
One advantage the Sherman (all models) had in this respect was that idling the engine when stationary and not planning to move immediately was not necessary as a small donkey engine was provided to power the radios and keep the battery charged (it also kept the crew warm in the winter to some extent as it was mounted in the bottom right rear of the crew compartment).