I say live with it.The roughly 18 degree temperature difference will only cause the journal to appear to be .0001 smaller than it actually would be if measured at the correct temperature. Actually, if the measuring equipment and its calibrating gage blocks were at a similar temperature they would also be that much smaller so the reading you got would be still be accurate. As I mentioned before, the .0003 tolerance (1.0000-.9997) is for a brand new journal size. It is anticipated by the designer that there will be some wear during the course of the journal's life and these hardened steel journals can not be "built up" to enlarge them.For that reason a limit lower than the original new size is allowed before scrapping the crankshaft would be required.The service limit of .9994 I suggested in my previous post will effect the radial clearance in the bearing assembly by allowing it to be .00015 looser than a brand new journals lower limit. The oil film between the rollers and the journal will eliminate any perceptible play in the bearing even with this added clearance.I'm assuming the journal that measured .9991 diameter is on the lightly loaded side of the crankshaft? If so, it should still work for many miles as long as the engine is not run hard.As a side note, if this wasn't a surface with rolling element bearings running against it, it could be repaired by "thin dense chrome" or "electroless nickel" plating. These repairs could be used on a shaft that was going to have a full bearings inner ring pressed onto it, for instance.These methods of repair cannot be used with your engines design because the direct contact between the rollers and the journal would rapidly chip off any plating that was applied to the crankshaft.As I said, live with what you've got. Run the engine easy and it should last for many years.
55firearrowI've been doing a bit more reading and if you really have a 1955 250cc firearrow it should be basically the same lower end as the old 350 bullet and the transmission is mounted on the rear of the engine in a similar manner.I'm not sure where I got the idea that the bearing rollers were running directly on the crankshaft but according to what I've read, the bearings have a hardened inner race which is pressed onto the crankshaft. Of course, I could be wrong about this.If the bearings have inner races that are just pressed onto the crankshaft journals, you could have the one(s) that are undersize plated to bring it up to spec.The processes I'm familiar with are called "electroless nickle plate" and "thin dense chrome plateing".Both of these plating processes can be applied so they will only increase the steel or iron journal a tenth of a thousandth of an inch or two in the area that is undersize.Because it is a slow process where the plating fluid is applied directly to the metal with a soaked wand or stylus, there is no machining involved or required. (Actually, these plating processes can add up to .0005 per surface without a problem).The difficult part is finding a plating company who can do the process and will work for someone who does not work for a large company that will give them a production order.If you are beyond this level in your rebuild, don't worry about it.As long as the bearings inner race is a press fit on the crankshaft journals you have no problem.If one of the bearings inner races is a slip fit, not requireing a press to install it you can clean the surfaces of all oils or preservatives and apply locking compound specially made for this type of application. If you can't find any of this, good old Blue Loc-Tite will work.