F300 rebuild sizing bearing journals

[choogenboom]

I am in the midst of a 2012 F300 4.2L rebuild and the service manual is vague on how to size the crankshaft journal and crankpin bearing shells. Specifically the service manual has a chart for each of the journal and crankpin that relies on markings from the crankshaft to lookup the correct bearing shell "colors" and note that two different colors can be combined and used on one journal or crankpin. More on that later. The chart seems to be useful only when building a new engine. The service manual also helpfully has a section on checking oil clearances using plastigauges which is useful when rebuilding a used engine. What it lacks is what do you do when the result of the plastigauge is out of spec? Since I am rebuilding a used motor with 1200 hours the journal and crankpin bearings are likely worn so I am assuming that the ONLY way to properly choose bearing shells is by the results of the plastigauge. Missing information needed to solve this riddle is what are the critical dimensions of the various bearing shells? Does anyone have a source for the shell critical dimensions? In other words if if I test with plastigauges using "green" shells and the gauge results say I need to add 0.0005 to the green shell thickness which color does that?

Next grey area. The look up tables require the use of different "color" bearing shells for the "upper" and "lower" shells. I take that to mean different thickness bearing shells. Use of 2 different thickness shells will offset the center axis of rotation which I get will not be a problem for the crank pins but this seems like a _very_real problem for the crankshaft which has 4 main journal bearings all of which would of course like to rotate about the same axis. If you introduce different thickness shells for a given journal it will offset _that_ journal's axis of rotation. Do that across all 4 journals and you could get a crank that will not rotate freely when the crankcase halves are torqued up. Can anyone solve this riddle? Seems like you should plastigauge BOTH bearig shells for each journal and correct each shell half individually using the gauge data. But the ship manual instructs to use only one gauge and has no help on how to correct for results that are out of spec.

Next question. The meaning of the words "upper" and "lower" that are used for describing the bearing shells are not defined in the shop manual. Fore and Aft might make sense, or upper and lower could apply to the thrust bearings but they seem like very ambiguous words to use to describe the two bearing shell halves for vertical shafts. Does anyone know what is meant by "upper" and "lower"?

[boscoe99]

I am not following you when you say that the bearing selection for the crank pin is for a new motor.

[choogenboom]

The shop manual uses clearances engraved on the crankshaft and the the block to do a lookup in a chart in the manual for the correct bearing shells. Since these are stamped at the time of manufacture they are based on the dimensions of the crank and crankcase when new. Once the motor has 1200 hours like mine does the selection of the bearing shells should be based on the current dimensions _as measured_ in the block in its current worn state. Unless one assumes that nothing has worn in the last 1200 hours. Granted the block has no wearing surfaces but the crank does, especially mine since it was run with less than an ideal amount of oil. I measured all of the bearing journals on the crank and there are a few that are worn so that means that it will need bearing shells that are larger than what the motor came with.

[choogenboom]

For reference see the service manual pages 7-86 through 7-89.

[99yam40]

If the crank has damage, then it would need to be welded, turned, and polished back to standard specs, or replace with new

[boscoe99]

The shop manual uses clearances engraved on the crankshaft and the the block to do a lookup in a chart in the manual for the correct bearing shells. Since these are stamped at the time of manufacture they are based on the dimensions of the crank and crankcase when new. Once the motor has 1200 hours like mine does the selection of the bearing shells should be based on the current dimensions _as measured_ in the block in its current worn state. Unless one assumes that nothing has worn in the last 1200 hours. Granted the block has no wearing surfaces but the crank does, especially mine since it was run with less than an ideal amount of oil. I measured all of the bearing journals on the crank and there are a few that are worn so that means that it will need bearing shells that are larger than what the motor came with.

I will be the first to admit that Yamaha's methodology is very strange when it comes to bearing selections. The procedure has the rod big end ID measured so as to be able to select the correct bearing color. You then use the mark on the crank shaft, but as you note, maybe the crank pin journal has worn. I would try the bearing colors suggested and then use plastigage to see if the clearance is correct. If not, then make an adjustment by trying some different bearing colors.

[boscoe99]

You might want to get a later model service manual. Much better explanation as to bearing selections. Different colors are used and it tells you which side each color goes on.

[choogenboom]

Thanks. I have a 1st edition 2010 service manual. What is the current version and where do you recommend I get it?