Can anyone identify the crankshaft main bearing sizes I need from these markings? My calipers are not accurate enough to be sure of sizes.
It doesnt seem to show any offset size markings so is probably stock size. But from the look of the inside face there may be some wear on the crank, to match the scoring you can see just above the machined groove. If you do as I did just slap on another set of 0.00/0.00 stock size bearings, on a crank with wear, your oil pressure will start low and go lower again. Even the £10 Chinese calipers can tell the difference between the 0.25mm size increments, in new parts, but it gets quite tricky when things are wearing. Its all a million miles from the Samba where some of them will end up in a twitching heap if you dont know to the nearest wavelength of (deep infrared) light what the measurements are .
Thanks guys. The journals themselves look ok to my eye, and I cant feel any roughness with my fingernails, as per Tom Wilson's book. I have used my cheapo calipers and got the following results: 1st and 2nd main bearing journals: both measure 54.75mm 2nd main bearing shell measures 55.25 internal 65.96mm external 3rd main bearing measures 65.56mm external. The case is getting blasted so cant measure it until next week. The engine was rebuilt in 2003 by Laurie Pettitt and the invoice shows "regrind, mains, ends, cam bearings". Does this suggest that the crank journals have been ground by .25mm and therefore I need .25mm undersize bearings?
They will be 0.25 oversize bearings. Check your cam bearing measurements also as you mentioned them being reground also.
You say that, but everyone keeps saying .25 oversize, but if my measurements are correct then the crank has been ground and I need .25 UNDERSIZE?
the crank has been ground smaller so you need thicker bearings (oversize) to take up the now bigger space between the crank and the case.
My understanding was that an increase in the OD of the bearing was oversize (I.e. bored case) and smaller ID was undersize (I.e. ground crank). Is this definitely incorrect?
When something is reground, the bearing required to take up the slack ( material ground off ) will by definition have to be bigger, i.e. oversize.
Sure, but how then do you differentiate between material added to the inside or the outside of the bearing? Just want to make sure I order the correct thing.
Someone like @mikedjames @79westy will be along to give a definite answer. I haven't rebuilt an engine since my Hillman Imp in the 80's, but equally I've never heard of a undersized bearing, at least not in normal engine building terms. Fear not, the answer is out there!
This is my understanding too.. based on a Google definition The outside diameter of the bearing is what fits into the engine block, and the crankshaft fits in the inside diameter. An oversized bearing has a bit more material on the outside diameter so it bigger (case re-bore). An undersized bearing has more on the inside, smaller hole in the middle (crank regrind....). So it can be both undersize and oversize at the same time... and then there's saddle width!
From memory, there are two markings on the bearing. Poss more for the end thrust bearing. Inner over size and outer oversize if the case has been line-bored. Another on the thrust bearing if the end web has been narrowed. I’ll dig out one of my old shells to refresh my memory. There are umpteen variations of the things.
