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[ultrafxr]

Exactly right, djkak.

Let me try and give an example that may make this easier to understand.  Think of the head studs in the motor case.  There is a tolerance for centerline of the head stud tapped mounting hole in the motor case - For example, let's say the tolerance for the centerline of the diameter of the head stud tapped hole at full depth as compared to the centerline of the head stud tapped hole at the face of the case is .002.  Now, insert the head stud.  If you could measure the centerline of the furthest end of the stud in relation to the centerline of the tapped hole at full depth, it may be .012.  The result of .012 would be fine.  It is simply the projection of the .002 error at the exit of the head stud tapped hole projected 6 inches further away, at the far end of the threaded stud.  The hole at full depth compared to the hole at the head face is .002.  Project this .002 error out to the end of the threaded stud and the error is .012.

The MOCO spec. of .003-.004 for crank runout in vee blocks or a truing stand projects to a larger number when measured with the crank installed in the motor cases, when measuring at the gearshaft.

I am not commenting on whether any of these numbers are acceptable nor am I trying to defend the MOCO in any way.  I am only trying to explain that the .003 spec as measured in a truing stand does not compare to the .012 spec when measuring in the motor case.  As Djak said - apples and oranges.

(Reading what I just wrote does not convince me I made this easier to understand 

Scott :

Made sense to me.

[grc]

Let me preface this with the fact that I have never seen a Twin Cam crank in a checking fixture or truing stand.  I have seen a couple of the old cranks (with the crank pin nuts rather than a press fit) being trued in a stand, but I understand that those old stands aren't used with the TC cranks.  The stand I saw supported the crankshaft between centers, just like a lathe, and readings were taken at the flywheel rims and the main bearing journals.  Now, since the outboard ends of the shafts were supported by centers, they were at zero by definition.  If you were to take that same crank after truing and support it at the main bearing journals, then measure runout at the outer ends of the shafts, the runout should in theory measure the same as the result you got when measuring the main journals while supporting the shaft ends.  So I guess what I need to know is just how the TC cranks are supported when they are measured in a checking fixture that would make this different.  I fully understand how any variation from true at the crankpin will be magnified at the end of the pinion or drive  shafts.  What I don't understand is how the old tech cranks could consistently be trued such that the runout at the shaft ends could be maintained at less than .003" (and if you remember, they had gear drive cams so this shaft runout was fairly critical), but now with modern engineering and tooling H-D changes the spec first from .003" to .004", and then tells us that a wobble at the end of the pinion shaft of .012" is acceptable.  It isn't acceptable with a gear drive, and it isn't going to help the wear on the oil pump and cam plate bushing either.

Maybe if someone has an actual photo of a modern crank in a fixture it would be helpful.  Anyone???

Jerry

[djkak]

....I understand that those old stands aren't used with the TC cranks.  The stand I saw supported the crankshaft between centers, just like a lathe, and readings were taken at the flywheel rims and the main bearing journals.  Now, since the outboard ends of the shafts were supported by centers, they were at zero by definition.  If you were to take that same crank after truing and support it at the main bearing journals, then measure runout at the outer ends of the shafts, the runout should in theory measure the same as the result you got when measuring the main journals while supporting the shaft ends.  So I guess what I need to know is just how the TC cranks are supported when they are measured in a checking fixture that would make this different.  I fully understand how any variation from true at the crankpin will be magnified at the end of the pinion or drive  shafts.  What I don't understand is how the old tech cranks could consistently be trued such that the runout at the shaft ends could be maintained at less than .003" (and if you remember, they had gear drive cams so this shaft runout was fairly critical), but now with modern engineering and tooling H-D changes the spec first from .003" to .004", and then tells us that a wobble at the end of the pinion shaft of .012" is acceptable.  It isn't acceptable with a gear drive, and it isn't going to help the wear on the oil pump and cam plate bushing either.

Maybe if someone has an actual photo of a modern crank in a fixture it would be helpful.  Anyone???

Jerry

There is no difference in the method used to check crankshaft runout between the old multi piece, bolt together wheels and the Twin Cam press together crank; correcting runout issues between the early and late cranks is a different story.

You don’t have to rely on theory to measure the difference between a crank supported on center or by its main bearings in a crankcase. A shaft offset of half a thou (0.0005”) will result in somewhere near one and a half thou (0.0015”) of runout at the shaft ends when measured in an assembled crankcase; early or late style.

In my experience H-D consistently true’s shaft offset to less than one thou (0.001”) in both the early and late crank assemblies. I agree that the service wear limit of 0.004” and 0.012” is extreme relative to actual conditions experienced in a normal engine.  I can’t explain H-D’s rationale for this without resorting to pure speculation.

The basic construction of the early bolt together cranks remained somewhat consistent from 1941 through 1984. These early cranks were extremely flexible relative to today’s press together units. This was evident in the high polish present on the tapers of the flywheels and shafts of early, high mileage cranks. Flexibility and severe flywheel shift was not at all uncommon in the early cranks, yet cam or pinion gear tooth breakage was virtually unheard of over the many decades that these engines were built. I believe that the reliability of the gearcase in these early machines is a testament to the metallurgy and effective gear tooth design of these components rather than the stable nature of the crank assembly.

djkak