Posted by Matt Wilson [108.162.221.53] on Monday, December 24, 2018 at 10:00:21 :
The old rule of thumb is that there should be between 0.0007 and 0.001" of clearance for every inch of crankshaft journal diameter. For the longer flatheads (251 and 265), the rod and main journal diameters are something like 2.2" and 2.5", respectively. So following the rule of thumb would dictate clearances of about 0.0015 - 0.0022 for the rods and 0.00175 - 0.0025 for the mains. So why do the factory rebuild manuals specify clearances of 0.0005 - 0.0015"? That seems to be so far off from the rule of thumb and quite frankly very tight (almost scary tight). Some guys on other Dodge/Chrysler flathead forums seem to be happy when they get 0.0015 - 0.0025" clearance. Presumably they are following the rule of thumb. I suppose their engines do ok with those clearances. But presumably, there are plenty of folks following the tighter factory specs and are also getting good results.
What comments do you guys have? What clearances have you been running, and how have your engines held up? I know this is a very complicated science with many variables, but I'd like to hear people's input.
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