Bought my Fiero about a week ago and got it running, drove it about 60 miles and started to hear knocking. Some research suggested it was spun connecting rod bearings so we dropped the oil pan out and lo-and-behold, the bearings show sign of wear. Not entirely sure if any of the bearings spun, but they definitely had signs of wear. Already replaced the bearings, just wondering how likely it is that the crankshaft is not going to function properly. Is there a way to check without dropping the entire engine out?

just use a micrometer and check the journal roundness it should not be any more than .0005 out of round

quote Originally posted by P-Dub: Not entirely sure if any of the bearings spun, but they definitely had signs of wear.

A spun bearing will be missing that little alignment tab on the end. The tab sheers off when it spins. If one spun, there will be damage to the crank and the connecting rod.

Yea, if it spun a bearing you could tell on the old bearing and also on the inside of the Rod being damaged

quote Originally posted by 1fatcat: A spun bearing will be missing that little alignment tab on the end. The tab sheers off when it spins. If one spun, there will be damage to the crank and the connecting rod.

Then they didn't spin, because the alignment tabs were attached on all of them.

DO NOT just replace only the rod bearings ! Replace the Mains also, or you will be doing it again . How Many MILES are on this engine ????

Actually, the mains are probably fine, though somewhat worn. You should measure the rod journals as has been noted, and check the rod end for being out of round, however, that second one I don't think can be done with the crank in the motor. Unless you've got the time and money to do an engine rebuild shortly I'd go ahead and just install the new bearings and reassemble everything. You've got really nothing to lose. If you do want to pull the motor you can replace the rods and mains, but leave the pistons in the block if the cylinders pass a compression test.

Maybe bad pistion/rod pins? If you take any more rod caps off, push the rod up a bit and then see if there is any play between the piston and rod...

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87 Formula Clone, 3800SC, 3.4 pulley, 4T65E-HD, HP Tuners, 12.98 at 103.4 1/4 mile, Regal GS Gauge Cluster.

you are likely fine. I've done the same - replaced worn bearings - and it worked out great.

I am sure you would have seen the damage had the bearing actually "spun".

Replaced the bearings, changed the oil, replaced the catalytic while we were down there because it was cracked lengthwise, started it up...

*knock knock knock*

We are just about 100% that it isn't a connecting rod, it isn't the crankshaft, it isn't the connecting rod bearings...

This car likely sat for 10 years, is it possible that it's a buildup of carbon, a sticking lifter, a bent pushrod, what else could it be? The sound is coming from the top part of the motor, trunk-end.

[This message has been edited by P-Dub (edited 10-13-2011).]

It can be hard to diagnose without actually hearing it, but most rod/main bearing knock is fairly loud and has more of a bass sound (like a drummer in a band hitting the low toms or kick drum). If it's more of a metallic tapping noise, it may be a collapsed/stuck lifter.

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Rod Schneider, Woodstock, Ga.
"You can't have too many toys!"
1988 Fiero GT
1987 Porsche 944S
1985 Porsche 944
1966 Porsche 911
Van's RV-6 airplane-at the airport, nearly ready to fly

quote Originally posted by Rodrv6: It can be hard to diagnose without actually hearing it, but most rod/main bearing knock is fairly loud and has more of a bass sound (like a drummer in a band hitting the low toms or kick drum). If it's more of a metallic tapping noise, it may be a collapsed/stuck lifter.

Our consensus thus far is a sticking/collapsed lifter.