The anatomy of a spun bearing??
 

There has been some talk of spun earings lately, what exactly is a "spun bearing"? I am guessing when a cam, crank, or rod bearing spins loose inside its housing thus wearing it out????
Also what causes this to happen and are there any early warning signs and what can you do to prevent it?

Mmmmm, Johno, you want to answer this question?:( :eek: :confused:

When a bearing is installed it will have a certain amount of "crush" that holds it in place. It is crucial that it stays in place so the oiling hole stays where it is supposed to. Also it has to stay in the rod or the crank cap so it doesn't fall out. It can spin for a few different reasons. Either the bearing was too loose when installed(usually due to improper machining or wrong part), it was run low on lube which caused it to scuff and drag on the crankshaft, it got wore out due to age and the clearance allowed the oil to bypass and lost lube, also creating scuffing.

In some instances there may be a short loss of oil pressure as the bearing spins but there really are no other symptoms until it starts to knock.

As I remember Kieth Eickert placed the blame for spun bearings in healthy engines on improper warm-up.

When the bearing spins it wipes metal out of the crank and rod or block. A rod can be reconditioned if it is not too bad. A block has to be align honed to fix. In either case the cap mating surfaces have to be machined so the oversized hole can be restored to specifications.

If you get really lucky like I did once, a rod bearing will stack, which means one half of the bearing gets under the other half and the piston kisses the cylinder head....

I just spun a bearing this last week. Motor had 1 hour on it. Followed all procedures, priming oil and waiting for oil to warm e.t.c. Had 60' oil pressure and slowly it started dropping & then the infamous knocking sound. Trust me it just sucks. I will have the machine shop go over everything & hopefully I have not destroyed the crank or the piston. Hopefully I will have the questions answered this week.
John