Diagnosing a possible spun rod bearing
01-02-2009, 07:23 PM
#1
Racer
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Diagnosing a possible spun rod bearing
'72 small block with hookers, sidepipes, Edelbrock heads, Weiand intake. There is a dull thunk sound coming from the lower vicinity of the engine. I was over at Lar's Viking Bar and his theory is a spun bearing. I have no reason to doubt Lars but was wondering if anyone has a suggestion for verifying this without pulling the oil pan or just revving it up until a rod pushes through the oil pan. 
Any other armchair theories?
Any other armchair theories?
01-02-2009, 07:50 PM
#2
Race Director
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With the engine running, pull the plug wire at the cap of the suspected cylinder and the heavy knocking noise should diminish if a rod bearing is suspect. Otherwise pull the oil pan.
With the engine running, pull the plug wire at the cap of the suspected cylinder and the heavy knocking noise should diminish if a rod bearing is suspect. Otherwise pull the oil pan.
01-02-2009, 08:22 PM
#4
spun bearing
I went through this about 2 years ago with a '73. The definitive test is to pull the pan and check the rods one at a time. Before you do that, take a close look at the oil pressure. You have a mechanical gauge that reads pressure dierctly from the oil system via a capillary tube. If you know where the oil pressure typically was with your car, at idle and when crusing at a steady speed, you see a noticible drop in pressure if you have in fact lost a bearing. Mine was down 20 psi. I ended up dropping the pan, and found the culprit on the second rod cap I pulled. There was a piece of silicone rubber in the oil hole feeding that bearing from the crank that had blocked flow and starved that bearing, courtesy of some previous owner who was a little careless with the sealant. Pulling the pan is not that hard to do. Remove the two bolts holding the idler arm to the right frame rail and just let the steering linkage sag, then the pan will come right out.
01-02-2009, 08:29 PM
#5
Burning Brakes
Probably not a spun bearing but rather a rod ready to let go.
A spun bearing will generally make a lighter tick, tack sound initially.
I would most certainly not be running the engine RPM's up at all. It is gonna let go, it's just a matter of when...
I agree with the the above mentioned pull individual plug wires test. That's a good way to ascertain if it's a rod knock.
A spun bearing will generally make a lighter tick, tack sound initially.
I would most certainly not be running the engine RPM's up at all. It is gonna let go, it's just a matter of when...
I agree with the the above mentioned pull individual plug wires test. That's a good way to ascertain if it's a rod knock.
01-02-2009, 08:35 PM
#6
Melting Slicks
There are no other ways. It it is a rod knock, as you already know it will increase with engine speed. You can rev it a little. You will have to pull the pan to take a look. No two ways around it.
