That engine has replaceable bearing shells, it doesn't have an aluminium connecting rod that serves directly as a bearing material. Presumably the connecting rod is steel. Most likely you do not need to replace the rod, just the bearing shells. The upper shell looks as if it has worn through its babbit or trimetal layer, in the center.
I can't judge so far whether you have a problem with the condition of the crank pin. There seems to be some scratching in the center, in the area that has been in contact with the backing metal of the bearing shell, but I can't see whether it is serious. If you can feel any roughness or striations around it, or detect any circumferential scratches, it would be better to use one of the other crankshafts.
How many hours has the engine run? So far it looks more like a worn out bearing than an abused one.
The cause of the knock from a big end bearing is slack. The most accurate way to measure the slack is with a product called Plastigage. It is just a piece of small diameter plasticene-like material. You put a bit of it in between the connecting rod and the crankpin, and assemble the big end, torquing the big end bolts to the correct tension. Then you disassemble the bearing and check the width of the squashed piece of plasticene against the gauge that comes with the kit. It tells you how many thousandths of an inch of clearance the bearing had. From memory (big end bearings seldom wear out these days) you can hear annoying bearing rattle at 0.003" clearance, and if you have much more than that it sounds as if it is about to break something. It is impossible to maintain a proper oil film in a loose journal bearing, especially with splash lubrication.
Incidentally I see the engine has a pair of counter-rotating balance shafts, to reduce vibration. The larger single cylinder Briggs engines tend to have either a reciprocating weight, or a pair of shafts, to do the same job. Large single cylinder engines produce considerable vibration unless you do something to counteract it.
Last edited by grumpy; 11/08/12 04:08 PM. Reason: Add detail
