Crankshaft Balancing

I looked at my spare crankshaft last night. It's hard for me to believe that any engine maker could forge and grind out a crank that is so perfect it doesn't need ANY final hand balancing. So Tony, you seemed to be basing your comment on having seen a large number of cranks. Can I ask, exactly what were you expecting to see and apparently did NOT see on the cranks to lead you to believe they were not balanced? Did you expect to see some large diameter drilled holes in the counterweights opposite to the connecting rod journals? Some engine makers do that, but it appears that Jaguar doesn't do it this way. Instead they skim off the extreme end of the counterweights. That's what they did on mine. Did you mean they were not balanced together with the damper and flywheel? If so, we may have been talking to crossed purposes here. I think that's where this thread started, wasn't it? Martin's damper. The Autocar photo in "Jaguar Sports" shows the crank with the flywheel on but no damper. The workman seems to be drilling the flywheel. This looks like it's the machine balancing operation mentioned in the article, at least I think I see the cutting tool which would skim the counterweights. The hand balancing on parallels would be something similar to page E22 in the manual. - Rob Reilly - XK120 FHC 679187

Rob: I am not sure I am following you, are you saying that the crank is balanced with the flywheel on? If so, if we change out a flywheel do we need to re-balance the crank? - Skip Smith

I had the crankshaft assy in my 420 balanced when I rebuilt it a few years ago. I feel it was somewhat smoother at highway speeds than other XK engines I have rebuilt. The cost was also rather low making it a good investment. It isn't hard to believe Jaguar balanced all cranks and that somewhere along the line, replacement clutches, crank regrinds, and piston replacements have defeated that balance. Don't forget the morale of union labor in England back then. I have seen signs of balancing on cranks I have taken out but how much time production workers put into each one is not clear. - Art Ford

Sorry, we were not expecting to see anything in particular from a marking point of view, rather we found the amount of wear and general overall condition to be symptomatic of an unbalanced crank. We also took apart one of our engines with the ballanced crank having some 35,000 miles on it and examined the crank to find it and all its components in fine order. I appreciate your point and certaily can not argue with it except to say perhaps they did not do a very good job of it? - Tony Parkinson, Specialist Jaguar Restoration

Skip, the Autocar article says the crank is first balanced on a machine without the flywheel, the flywheel is balanced separately, then the two are balanced together. The photo looks like it's depicting this third step. Then the final hand rolling on parallels step must be with the two together, but the article doesn't specifically say so. The article doesn't give a lot of detail, and I hope I'm interpreting it correctly, and I hope the journalist interpreted what he saw in his shop visit correctly. Yes, if you are swapping flywheels and wanted your balance to be good down to a gnat's eyelash you would do them together, but if you don't, how much are you off is difficult to say. Flywheels get worn on the teeth and sometimes unevenly, which would be absolutely the worst place to lose mass, the worst for getting out of balance. But how much is too much? Maybe paying the balance guy $40 an hour buys you peace of mind. Tony, your point is well taken that poor workmanship is always a possibility. The Autocar article describes ample evidence of "espri de corps" existing in 1958, but that sort of thing can vary from year to year, and there's always one disgruntled worker in every shop. And of course as Art says, the wear on used cranks of unknown maintenance histories can be interpreted a lot of ways. - Rob Reilly

If you have any questions or comments send e-mail to: ted@jag-lovers.org
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