Rod Knock twice!

[DrJay]

Ok here's an odd one. I have a '93 Pontiac Bonneville SSEI that had a complete rebuild done on it about 200miles ago now. After about 172miles it developed rod knock and the mechanic that built my engine was a jerk and fanagled his way out of the repair by filing for bankruptcy so I was stuck footing the bill for the repair. It just got finished today after replacing all the bearings and a handful of other things, crank finishing and whatnot. I paid the mechanic and took it just as he was closing for the weekend. 25miles went by and I heard a light rapping in the upper rpms and noticed the oil pressure was a little lower, nothing to worry about but lower. I immediately parked it and lightly revved the engine and sure as hell the knock is back. Prior to this the engine had no driveability issues. Idles great, accelerates ok, no hesitation or anything. What the heck could cause me to go through 2 sets of bearings in 200miles?!? I baby the heck out of it. Never went above 40, never lugged the engine, nothing. I learned alot about breaking an engine in before I got the car the first time and after dealing with the lengthy rebuild and costs the second time I hit the books again. I assure you I did nothing that would constitute abuse nor did I sway from normal breakin procedure. I just don't understand.

[jkinney]

Do you know what your bearing clearances were set at? And are you positively sure your harmonic balancer is secure, and in good shape?

[DrJay]

Not sure about either one of those honestly...wouldn't it shake noticeably if the balancer was off? How can I test? It looks fine.

[DrJay]

Sorry, i wasn't thinking I realize it can shake at a harmonic high enough t hide the shaking. The only thing I can debate with that is that the main bearings would be the first to go because they're the ones holding the crank in position. Its obviously rod knock.

[jkinney]

Well, the inertia ring may have broken loose from the bonded rubber that attaches it to the hub, usually you can tell by looking closely and seeing if the rubber has a continious crack around the whole balancer. It happened alot to these Bonne's (and mine), The junk yards were empty of balancers and I did the best thing by purchasing a new one. Also, I would think the balancer is dampening the harmonic forces generated by the (piston/rod) rotating and distributing energy, so I could see the rods coming into play as well, but the weakest link will go first, rod or main bearings. The real tell tale would be your oil pressure and how much psi is a "little lower". A little history on rebuild...Did the old bearings look spun? Was the tab rolled off of them (would think so), was lubricant put on the back side of them when installed on fresh crank (shouldnt have) Did the crank get turned undersized, and the right bearings get installed for undersize? Alot of places to look to see where something is wrong, but I would start with looking at balancer, and even checking the keyway on it to make sure the keyway itself is not burgered up, or the key itself, before I started on round three of the bottom end. Best of luck!