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Old 07-08-2009, 03:10 PM   #13
Scooby Specialist
Member#: 65711
Join Date: Jul 2004
Chapter/Region: Tri-State
Location: NJ
2004 WRB WRX Wagon
2.5 Hybrid STI Conversion


So, some of you may be curious as to my mechanic's theory. I have quoted his email below.

I have a theory as to why it failed, but its hard to be sure. I checked the clearance of one of the other rods that had a bearing that was fine; unscratched. The clearance measured by plastigage was .004in, the wear limit called out by our AllData program was .04mm, which works out to .0015in. There could have been a mistake by the previous machinist, or, as it appeared to me, the crank was cut. Since the bearings were of Standard size, the wrong bearings may have been installed into the engine when it was rebuilt last.... Its hard to tell because I was only able to measure one other rod bearing.

To further my theory, most American engines run clearances of .0025 to .004in so the clearances wouldn't seem strange to a machinist who builds V8's or conventional older engines. However, when you get into higher specific output engines, the bearing clearance has to decrease so that the hydrostatic pressure of the oil will withstand the load without metal to metal contact. The stress of the motor finally making power brought out problems that weren't apparent under normal driving conditions (vacuum). Or, it could be just that one specific bearing was screwed up from the beginning and it was a matter of time before it broke.
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