Engine Tolerances in WWII.

This makes complete sense.  Tight fitting demands a top professional, not so readily available.

Yet a fighter pilot wants that extra five percent of performance in a dog fight.  Whether that could be then achieved in the field is doubtful.

This is an issue that constrained war production far more than likely appreciated and that included Germany which had a huge number of skilled machinists.  This also explains why their preference for tanks that were over engineered was so constraining.  Interchangable parts are so obvious it appears odd when it is not used.  .

Is it true Ford of England refused an air ministry request to build Rolls Royce Merlin engines under license because the tolerances Rolls Royce allowed were looser than Ford was willing to work with?

Is it true Ford of England refused an air ministry request to build Rolls Royce Merlin engines under license because the tolerances Rolls Royce allowed were looser than Ford was willing to work with?

NOT EXACTLY

I have worked in engine overhaul shops and overhauled engines since the 1970s.

It seems the original RR Merlins were balanced and blueprinted type engines. I have worked on factory Jaguar engines which had multiple different fit pistons in cylinders that were all standard bore. They were not interchangeable. I have balanced and blueprinted engines fitting parts to the minimum clearance. Pistons were knurled for the best fit to a particular cylinder. They were not interchangeable. Rod and main bearings fit to between .0005- .001 inch clearance. Such was the case with original RR Merlins. The precise machining of parts eliminates the need for gaskets or makes gaskets very thin sometimes just a piece of silk thread or paper.

Standard assembly for mass production requires looser tolerances for assembly. Any piston goes in any hole. Bearing clearances become .001-.003 inches so they all work no special fitting to tighter tolerances. Crankshafts are then ground to a size that allows this. Instead of tighter tolerances there are actually looser tolerances for assembly but tighter tolerances for acceptance of parts which allows bigger clearances. These tighter tolerance for acceptance are to make parts more interchangeable by making them a looser fit. Gaskets become more common and thicker so less precise machining is required to prevent oil leaks. Thicker Paper and Cork gaskets become common.

So yes they had to re design the specs to tighter tolerances. Instead of having a crank journal that could measure as example 2.1245–2.1285 +-0.0005. Mechanic assembles to 0.0005–0.001 clearance for bearing.

They tightened up the specs to 2.126–2.128 inches +-0.001 now everything is interchangeable and can be assembled by anybody. Assemblyman assembles to 0.0005–0.0035 clearance for bearing. No fitting allowed.

We lose a bit of power, smoothness and life. However we can now crank out way more engines with unskilled labor.

It would be interesting if somebody had the original specs from RR and the revised specs for the other companies so they could be compared.