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Rod Bearing Clearances


Jedi Warrior
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I'm wondering if anyone has any idea why there are different rod bearing clearances listed for the 100-6 engine and the 3000 engine?

Cranks are interchangeable with same the diameters, both standard and undersize, and the stroke is the same.
Bearing shells are the same part number and sizes.

AFAIK, the only real different between 100-6 and 3000 is the bore (the early 100-6 had clamped wrist pins as opposed to floating pins, and the gallery head, but still the same rod bearings & crank).

The Bentley Book, Haynes, and Autopress all agree:

Main bearing clearances are the same for both engines at .0013 to .0028

But rod bearing clearances are different;
100-6 - .0005 to .002
3000 - .002 to .0035

My machinist says, "You'll be fine if they are around .002"

OK, but any idea why the specs are so different?



Jedi Hopeful
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Just to add to the above (can't seem to find the edit button) I have looked in the workshop manuals for the BMC sedan models which used the 3L C-series.

The first, Wolseley/Austin (6/99 /A-99), shows the same clearance as the 2.6L motors ie .0005 to .002 [mains .0013 to .0028]

The later models (6/110/A-110) show .001 to .0027 [mains .002 to .0037]

What to make of that who knows!



Jedi Warrior
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Thanks Danny, I have no experience with the sedans, but in the case of the Healeys, I was wondering if the difference could be due to the lower red-line and the rotary vane oil pump the 100-6 used (higher volume output at lower speeds)? Doesn't explain why the mains are the same for both.

BTW, for both motors, cam bearing clearances are the same at .001-.002 and end float is the same at .003-.006.

Crankshaft end float is also the same at .0025-.0055. So the only clearance spec I can find that is different is the rods.

Reading on line, several sites say to use the formula of .001 clearance for every 1" in diameter. So for the rods at 2" dia. would be .002 clearance and mains at 2.4 dia. would be .0024.

I've talked to a couple of other machine shops and they seem to agree that Mains around .003 and Rods around .002 are good targets (they add a bit to the mains clearance to account for the flex of the long and heavy crank with only 4 mains). I even talked to a polymeric bearing coating company and they said the same. It appears to me these shops use their experience and understanding of modern oil formulations more than the 60 year old "book" in most cases. Mind you, no one was offering me any guarantees......



Jedi Hopeful
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I tend to think that the real reason for the differences is just the usual inconsistancies and mistakes/misprints among BMC's printed publications and data.

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