Race Engine Forum UK

Hello,

Engine is a Fiat 2000cc Twincam, high performance street engine, naturally aspirated.
I am baffled by scored main bearings after less then 800 miles. Previously we had an oil leak on the
dyno at the oil filter block, which cause less than ideal pressure. At that time the rod bearings
were scored, so we replaced them along with the mains. We also changed to forged pistons since
the engine was apart.
Oil pressure is good, oil pump is an original Fiat one installed new when the engine was first built
last summer, and we use Valvoline VR1 20w50 racing oil. Crankshaft was not turned, bearings are
standard. Clearances checked out when the engine was built.
There was very light (one line) scoring on the upper rod bearings. I attached a pic, but I am not
sure if it will show up. Also attached a pic of the main bearings.
Any ideas would be appreciated.
Thanks,

Csaba

Need to do a dimensional survey on the mains Csaba, looks like the journals may not be parallel, eg: worn in the middle and only contacting at the sides.

GC

First : I have to agree with Guy in that the contact areas indicate non parallel journals, main saddles or oddly manufactured bearing shells.

Second: Often an inexperienced crank polisher can induce as much as .001 error on crank journals.

Third :The contact areas are remarkably consistent on both top and bottom shells indicating that the crank may have some level of run-out. In normal situations the load side shells (cap) would show highly dominant damage resulting from contact due to lack oiling. I would recommend having run out checked also.

Fourth: The markings appear to be more of a burnish/polishing then a score an not uncommon during a some what dry start up. I doubt they would get much worse but good to find the root cause.