This may seem a bit arcane but in the process determining what to do with the crankshaft, I measured all main journal diameters and compared the measurements to the spec values as listed in the manual. The factory specs call for slightly larger journal diameters on the front, center, and rear journals than the diameters for the intermediate journals. Now, the values have very tight tolerances (+/- 0.00025 inch) and the allowable size range is such that the smallest allowed diameter for the front, center, and rear journals is the same as the largest allowed size for the intermediate journals.
So more out of academic curiosity than any thing else I’m wondering why Jaguar called out these specs. And on a more practical note has anyone actually paid attention to these specs or is it good enough to simply turn all journals to the same diameter? Three journals being 0.0005 in. larger than the others seems to me to be ‘in the mud’. The allowable wear before turning the journals and replacing bearings is 0.003 in., far greater than the close tolerances specified.
5 tenth’s difference in size but clearance from .0025 to .0042, strange. I realize that measuring with plastigauge is not exactly to aero-space tolerances but the last crank that I had turned, years ago, the clearances were all right at .003.
Bob
889076
Plymouth, Mi.
I like Plastigauge and have used it since I was a teenager working on my Model A Ford more years ago than I care to admit to myself;-). I think you are right, measurement of the final bearing to journal clearance is the acceptance criteria. In fact the crank isn’t really out of spec right now but there is some scoring and wear patterns that need to be removed so I’ll have it turned down to 0.020 in. under and install it with the correct bearings.
The XK120 Manual says all crank journals are 2.7495 to 2.75".
So do three non-factory manuals that cover 3.8 litre engines.
Like you, I can’t see any sense in a .0005 difference when up to .040 undersize is allowed on regrinds.
I wonder if there could be a misprint in yours?
The weak spot for 4.2 cranks is a harmonics issue around the center main due to the non fully-counterweighted crank and altered cylunder spacing. I could see tighter specs in the middle and two end mains being helpful in combatting that ‘problem’ .
The 2.4 and 3.4 MK1, the 3.8 MK2 and the E-Type Factory Service Manuals
under “Crankshaft” say:
Journal Diameter…Front, Centre, Rear 2.750" to 2.7505"
Intermediate 2.7495" to 2.750"
The same information is repeated in the Scientific Publications (Australia),
Heynes, Autobooks and Intereurope service manuals for those cars.
It’s very specific and not a misprint. It does, however, seem sort of silly
since the bearing shells are all the same diameter. The journals in 4.2
Litre engines are all the same size.
Mike Eck
New Jersey, USA www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2
The XK120 Manual says all crank journals are 2.7495 to 2.75".
So do three non-factory manuals that cover 3.8 litre engines.
Like you, I can’t see any sense in a .0005 difference when up to .040
undersize is allowed on regrinds.
I wonder if there could be a misprint in yours?
Yes, the journal diameters and tolerance ranges as quoted by Mike Eck are the same that I was alluding to in my original post. So when I take the crank to the machine shop I will give them the exact diameters with tolerances and ask them to grind as specified. However I’m wondering if they can even hold those tolerances or perhaps they might simply ignore the small diametrical differences and grind them all to the same diameters. Looking forward I would probably accept it even if they ground all 7 journals to the same size.
If you’re trying to hold such tolerances you really need to provide the rods and shells and torque specs for rods. Rod big ends vary and the combined tolerance of crank and rods can lead to a disappointment.
This is a follow-up to my post about the crankshaft main journal diameters. The service manual calls for the front, center, and rear journals to be of slightly different diameters than the 4 intermediate journals.
I have since found a service bulletin that says “commencing with 3.8 engine numbers LC3498 for the Mk2, R6724 for the E-Type, and ZA2574 for the Mk10 all main bearing journals are now of 2.750 to 2.7505 in.”