Hello Derek
When checked running on the main bearing journals in an ideal world you should see no more than 1 thou (0.025mm) runout when clocking the ends. By the ends I would mean the primary cover bearing diameter and the bevel gear diameter. Clocking the bevel gear diameter is best checked by sliding a 6202 bearing on it and clocking the middle, this avoids the keyway. Always check that the keyway is not burred up at the edges, carefully stone off any raised areas if necessary. Clocking the oil bush nose (as mentioned) is largely a waste of time as it will almost certainly be worn (and not concentrically). You can sometimes check this surface but only right next to the thread where it hasn't been in the bush. I would recommend slipping a pair of new main bearings on the crank, to sit on the vee blocks for a smoother check. Be aware the main bearing journals may well be worn (in particular on the drive side). Positioning the main bearing on the small unworn area will identify wear and again this is generally not concentric as wear occurs on one side. Bad wear will have to be rectified by building up and regrinding. That's another whole rabbit hole to enter due to the cranks never running true to the inner centres.
You can slip a 6203 bearing on the primary cover bearing diameter (along with the 6202 on the bevel gear diameter) and run the crank on vee blocks on those bearings. That is about as close to running the crank between centres as you will get. When clocking the main bearing journals like this I look for less than 8 tenths (0.02mm) runout. Any wear on the main bearing journals will become apparent when comparing the worn and unworn areas of the journal. Actually running between centres in my opinion is not ideal as invariably the centres are damaged from abuse and even a very light end load will push the crank out of true.
Ideally I like to see the crank within a few tenths or better still perfect, but the later seldom happens. In the real world I will accept maybe one and a half thou (0.038mm) at one end. I'm generally unhappy with any runout of 2 thou (0.05mm) and above, but sometimes cranks are just wrong and there is not much economically you can do about it.
All this said, cranks often move in use. You will often find wheels spread in use, meaning the high spots are at the top when the pin is at the top. keeping crank end float to zero, will limit this.
Unfortunately some cranks are difficult/impossible to true, there are a few reasons for this:
Damage to pin bores from previous poor workmanship.
Issues due to manufacture, these cranks were ground as assemblies and some cranks when split refuse to true. This is invariably due to the cranks being built with the pin hard up against the step of the pin on the drive side. Which coupled with side thrust faces which are wedged from poor grinding, means the pin was not true to the bore when ground. This shows up when you don't shoulder pins and used parallel pins, which is the best way to build a crank especially for race use to avoid broken pins. For a road bike, shouldering a stepped pin is the only way you will get some cranks to run true.
Correcting any manufacture issues, damage and wear with these cranks is very time consuming and seldom cost effective compared with a good used crank if available.
Hopefully this will help with some basic info. Work on these cranks is not often easy and often from my experience is poorly performed even by so called "specialists". I've built 100's of Ducati Bevel single and twin cranks over the years and occasionally it feels like trying to make a silk purse out of a sows ear
Good Luck
Best Wishes Nigel
