[bowtie1Z28]

I think I have a crankshaft main bearing going out but not exactly sure how to diagnose or pinpoint it. The knock is not noticeable at idle and is more pronuonced when the engine is cold. The intensity of the knock reduces significantly when the engine gets to operating temperatures. It is mainly heard when you wing the throttle. Has anyone ever replaced one or more main bearings while the motor is still in the car? I would like to try to get this motor to live through the summer as I have plans on building a new short block for next year. Am thinking of a destroked 400 using a large journal 302 steel crank, long rods. Will probably need custome pistons too. With a .030 overbore should come out @ 325 cu. in.

Any help on the main bearing issue would be appreciated!

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1980 Z28 Dk. Blue
'65 327/350
ST-10 4-speed
3:90 w Auburn posi

[BillK]

bt,
If the sound is heard when you "wing" the motor, it "sounds" more like a rod bearing, or maybe a wrist pin, to me. If I were you, I would try to find someone locally to listen to it and give you a second opinion.
Hope this helps,

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Bill Koustenis
Owner
Advanced Automotive Machine
Waldorf Md
1971 Chevelle "Heavy Chevy" original owner
1973 Z-28 ..one family car...Brother bought it new in 73

[bowtie1Z28]

Thx BillK!I had kind of ruled out rod bearings because I would think you would always hear one knocking no natter the rpm but I was thinking wrist pin in the past. This motor had been overrevved by a previous owner requiring the replacement of #2 and #4 pistons and I think maybe the rods and bearings. I know a guy here locally I will have him take a listen to it and get his opinion.

[Milan]

Is it possible that you are hearing piston slap?

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67 prostreet BB Camaro
71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.

[rlrr]

Sounds like my TRW Forged Pistons....."clank a clank a" when they are cold-almost like a diesel. Most of the noise stops when it is warm.

It's a nitrous motor and was put together "loose" for N2O expansion.



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Mike Steinkamp
69 Camaro w/ 383 Edelbrock RPM System (including heads), Nitrous, 350 Turbo, 3:42 gears
13.09 on motor, 12.12 with 125 hp shot of nitrous

[bowtie1Z28]

I suppose it could be piston slap. Just replaced the heads, cam and lifters. Seems to be a little noisier noisier now with the extra compression generated by the small chamber heads. Also realy sounds like one or maybe two cylinders are making the noise and the noise seeems to be coming from the pass side where the replaced pistons are on the drivers side. The other six pistons are the originals and as far as I know in the original untouched bores. All seem to have excess play in the bores.

No signs of any metal particles visible to the eye in the oil at the last oil change so maybe I am jumping to the wrong conclusion with the bearings?