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Connecting Rod Bearings

I disassembled a 3.7 V6 engine that had a rod knocking . # 4 rod bearing was shot . All the rest of the rod bearings & the main bearings looked good . Is there a likely reason one bearing would fail & all the rest are good ? Lack of oil to that one bearing , possible defective bearing , etc ?

I can’t think of any reason other than what you’ve listed. The bearing could go bad if it was improperly installed or something wrong w/the oil clearance. When you install a new bearing, give it the plasti-gauge treatment, see what it measures. Another idea for why one bearing would go bad, if the engine hydro-locked or something, and it just happened that bearing was the one that took the worst of the force. Good idea to take a look at where the rod connects to the piston on the other end too.

Were the bearings that looked good copper colored? If so, they’re badly worn.

Assuming there is no oil flow restriction on that particular rod journal and IF the other bearings have the copper underlayment showing it could be that bearing was just worn enough that a constant beating finished it off.
The others would not be far behind.

You could try blowing some compressed air into the oil filter inlet on the engine and note how much air comes out of the oil feed hole on the crank journal.

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The weakest link always goes first. I have torn down many engines with rods knocking…and it’s always been just one.

No copper color on any of the other bearings . I’m installing another crankshaft & all new bearings . I’ll also do the air test .
This engine has a bedplate that requires a special sealant where it mates to the block . Any companies make bedplate sealant besides OEM ? It’s a mopar engine & their sealant is $40.00 a tube from what I’ve read .