Hmmm...
So now I'm wondering what else could be wrong. Earlier I noticed that the small end of the rods seemed like they had a lot of play in them (side-to-side parallel to the crank) even though they are bolted on the crank. Again, I'm not sure what is normal.
I just hooked up a dial/drop indicator the top of the the number 4 rod and pushed it back and forth (parallel to the crank) and I got a total deflection of 0.03" (or approximately 0.015" in each direction). Just to clarify, I'm not measuring the large end sliding back and forth between the weights, but rather the side-to-side defection at the top of the small end.
Is this too much play, and if so, how does one correct this? (are there bushings in the large end of the rod?)
First things: use Plastigage on all of these bearings. It will tell the real story of clearances, where "rod wiggle" sure won't tell you the truth. That method was reserved for pressed-together (aka "built-up") crankshafts with roller bearings. Those systems have no other reliable method besides a jigged-up pull-push test for their clearance, and most folks don't have those tools.
The "scratched" center journal: check the divot on the crank with your fingernail, see if it is an 'outie', or if that part wore off already. If it is still raised, some fine crocus cloth or 600 grit emery paper will smooth it off: don't be too aggressive, but start right on the chip, with your fingertip. That may be enough, often is, to smooth it back down.
The main thing to remember about the 750 bottom end: if you compare it so, say, a modern Hyabusa bottom end (the highest HP kind of sportbike, 3x the 750), their bearings are not even this wide. These babes are seriously overbuilt. I have seen folks simply groove out the bearing where such a scratch was found, so it would not generate more debris and scratches, and ride on for many, many miles. I have even done it myself, to a lesser degree than they did.
If this bearing and journal were approximately 55% of the present width, and the engine was an early sandcast at 67 HP output, this would still be a 100,000 mile bearing. That's approximately how strong these are. Russ Collins used to have a lot to say about it, as his bottom ends were not terribly exotic for all the power he built in these Fours.
If you're real concerned, then do 2 things: 1. Remove any raised portion of the divot and the lines leading up to it with 600/800/1000 grit emery (or crocus cloth and patience), and install a new bearing shell pair. Plastigage it to make sure it is at least .0010"to a max of 0.0025", and ride onto the sunset.
