Radial Clearances of XPAG Crankshafts.
Can anyone help? There is a minefield of varying information on the radial (or running) clearances of XPAG crankshafts.
Bearing in mind (pun intended) the thickness of a human hair is about 0.05mm, when doing a dummy assembly using brand new components on my engine, I’m outside the tolerance by one fifth of the thickness of a human hair!
The Sherrell Book quotes 0.02-0.04mm but Mike seems to throw this figure at “journals” in general not specifying whether they be Mains or Big Ends. In my experience this tolerance seems to be wholly credible and within the sort of order of magnitude that I expect for both the mains and big ends, although I’d agree that possibly the mains might have had a slightly larger tolerance. Then, working to the Sherrell figures I measured the radial clearance with “Plastigauge” on my brand new solid billet steel crank and this is when the trouble started.
My running clearances are typically 0.05mm, so I am just outside the Sherrell tolerance on both the mains and big ends by 0.01mm! A couple of journals are close to the 0.04mm end of the scale but more of them are 0.05mm than 0.04mm.
My first reaction was to question myself. Am I being too picky? Is the book correct? Is the (new) plastigauge in date? Then I measured the journals of the crank and found that, although sold as “standard size”, the journals all measure 0.02mm below the standard size! This explains my discrepancy. Had the crank been finished to 52.00mm & 45.00mm for the Mains & Big Ends respectively then my measurements would have fallen slap bang in the middle of the Sherrell tolerance and I’d have been happy. I rechecked that my micrometers “zeroed” and they all did, so I I know that my crank is finished “undersized”.
I thought that machine shops will grind and finish to up to a maximum of “Half-a-thou” (0.0005”), which is 0.0125mm! Even this would bring my radial clearance within tolerance, but 0.02mm is causing me a problem, so I’m left wondering if I’m out of touch, if I’m being too fussy or if the data I’ve found is wrong. Does anyone have experience of building several XPAG engines and could they put me on the correct track before I return the new crank to the suppliers?