I made up some expanding spacers to fit in the big end journal space and taking very small cuts turned the mains to final size, next were the front and rear mains turned down for the 8x16x4 ball races, it all looked quite good. I mounted the crank in the block and it spun over quite nicely, however I noticed what looked like run out on the 3 internal mains, must be an optical illusion I thought, they were turned between centres and I did check them with a DTI, they had no run out. Just to be sure while the crank was mounted in the block I placed a DTI over each main and checked it - almost 3 thou run out ! checking with my calipers two of the 3 mains were tapered by around 2thou I was stumped. No option but to put it back in between centres and rectify the run out and taper, this time I double checked - no run out and no taper.
Once again back into the block and again I'm sure I can see run out in the mains when the crank is spun over, I check with a DTI and yes there it is, the run out varies from from 2-3thou on each of the 3 mains, the front and rear are fine, time to go inside, have a coffee and give it some thought. I come to the conclusion that in applying some pressure/preload on the crank from the running centre the crank is flexing - perhaps only a thou or so and once machined and the preload is removed it flexes back and we have run out, to check on this I remount the crank and with a DTI on one of the mains I gradually put some preload on the crank, sure enough I get a reading of 2 - 3thou, this is with the adjustable spacers in place, not sure what to do I decided to put minimal preload on the crank so I can cleanup the mains and eliminate the run out, while I did have a hint of chatter before which was most evident if I let the cutting tool rest in the one place, with less preload the chatter was constant and unacceptable making it impossible to get a good finish, I increase the preload bit by bit until I find a satisfactory setting where I can machine out the chatter marks.
Job done I once again set it in the block and you guessed it there is run out but only around 1.5thou, I was hoping to get it less than 1/2 thou so there would be no tight spots, then I thought that 1.5 thou is not that much and to get it to 1/2 thou I only need to remove 1 thou from 1 side so with a felt tip marker I marked the high side and with the crank mounted in the vice used some 600 emery on the high side gradually working around with less pressure, bit by bit I managed to get it down to around 1/2 thou, I know it's probably not perfectly round anymore but it is round to within 1/2thou. I do the same to the other 2 mains, the good thing is I have all of them with in the spec I wanted but the bad thing is they are now under size which can be compensated for when I make the three split bearings but worse they are all a different size now which means I'll have to number them. I know my method is not exactly precision machining, maybe if I had used a different material rather than an unknown I may not have had these issues.