Hi "swarf". Just a reminder... so you do not drift into bad practices..
This thread is all about Model Engine Machining. So really you should machine precisely to the right size, then instead of the approach of "a tight fit and wear it in" you start at "the right fit - first time" - which curiously lasts much longer than the "worn-in" method. - Which is why "Industry" has spent a small fortune working to finer and finer tolerances so your car (and other modern "necessities") has become relatively cheap, Incredibly durable, and maintenance free for a long-enough life.
When I started (around 1966) in a machine shop doing re-bores and crank regrinds, the NORM was for every petrol engine to need new rings or small re-bore after 10,000miles, but a larger re-bore if it had been run for 20,000 miles... Cranks lasted 20.000 miles before a re0-grind to first oversize. - NOW it is 150~250.000 miles before any sign of significant wear on the bore, and more of wear on the crank.
All this achieved by making parts true, and closely fitted "as new".
SO I suggest that having reamed the bearings all in one in-line, there is actually a difference between the size of reamer and crankshaft main bearing journals. - Or possibly some misalignment of the crank main journals. (because the set-up can distort the crank.. leading to journals machined on that set-up to become out-of-alignment when the set-up stresses are released).
If you reset the crank in the lathe, between centres, then put a DTI along the journals, you may spot the "bend" of the crank between journals. BUT you will need to pack between the flanks of the crank in order to NOT distort the crank when you fit the tailstock centre. use Engineer's blue (of felt-pen marker) on the journals and recheck the "touch" point of the lathe tool on all journals. It should be at EXACTLY the same setting on the lathe dials. Then when you are absolutely sure that the crank is NOT distorted in any way by the set-up, just "touch" any high-spots of the crank main journals with the tool while the part is rotating, to "true" the journals to a sizes that is less than 0.001" than the micrometer measures the reamer you have used. - Remember if it is a tapered reamer, the first 1/3rd will be under sized as it has a taper for starting the reaming process. Making a true and straight shaft the fits the true and straight bearings (reamed in line) is the key to a smooth running - from NEW. The adage "Wearing -in" is truly "ACCELERATED WEARING-OUT" the engine, I.E. giving the finished engine a short life from a worn-out condition. It is really a bodge to overcome deficiencies in the manufacturing process.
However, there is a process of "LAPPING" - This uses VERY fine abrasives, to "grind-off" the peaks of machines surfaces, - but not remove more than the amount necessary to make a finer finish than the machining tool can do. This is best described by the HONING process on bores, where a precision set grind-stone, of grit much finer than the cutting pitch of the tool that was used for boring, is used to take off the "Peaks" from the machined surface to leave 50 or so as a rubbing" surface, and 50% or so for "storage of oil". (Not exactly true but the simplest explanations are often not quite true).
I hope this sketch helps?
As long as the crank measures "true" (straight and parallel at the journals with the DTI) and "SIZE" (same as the reamer/bearings) then you can remove tightness by linishing "high-spots" where the bearings can wipe-off the marking blue, when assembled and rotated.. But check it is crank or bearings that need the linishing! Use the shank of a drill, reamer, length of silver-steel or equivalent test bar, at the size of the crank journal, fit it into the bearings, - duly tightened - and rotate to the bearings, and rotate to see if the bearings are "wiped" all the way around and all the way along. If you get a line "top and bottom" of the split bearings, this indicates a bearing that is not round when clamped. If you get a wiped circle at one end or the other of the bearing, then it suggests the bearings are tapered. If you get odd, diagonally opposite wiped zones, the bearings are not aligned. Only when the bearings are good with a test piece should you look to the crank. By "Wiped" I mean the Engineers' Blue has either transferred form one surface to another, or been wiped off a surface, indicating the contact points between shaft and bearing surfaces, within the bearing.
Equally, on the test shaft - and again with the crank, - the contact point should tell the same story.
Depending of "shaft journal size" and "bearing size" you can decide if the shaft needs reducing (linishing) a tiny amount to fit the reamed bearings, or if the alignment of the bearings needs addressing. (Re-set to a true test-bar, not the reamer's cutting edges, or crank).
I hope some of this helps?
Ask where you want more explanation, as there are many more capable than I who can advise or correct any errors I have made. (I learn as much as I teach!).
Cheers!
K2
On the crank, the
until you snug up the nuts and then the crank was pretty hard to turn.
