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samstrader

Posted 2020-11-02 1:52 AM (#605201)
Subject: Engine Knock Troubleshooting Summary - 1955 259 V8

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Posts: 443
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Location: Beaumont TX

Thanks to everyone for the help you have given me on this. I just wanted to make a summary of my problem and what has been learned for future reference for others.

Problem:
* I have a knock in the bottom of my engine. This knock is not present when the engine and oil is cold. But when the engine is warm and the oil is warm, there is a knock at idle speed. It also knocks when I accelerate strongly.

Troubleshooting:
* All the work has been done with the engine in the car. It is desirable to get to an acceptable operation without pulling the engine.
* When idling, if I pull the number 5 spark plug wire, the knock stops.
* My engine was completely rebuilt and has less than 1000 miles. There have been many problems since the rebuild. Cam bearings misaligned, oil non return valve installed backwards for example.
* I am using Mobil 1 15W50 Oil. I tried 10W30 but the knock was bad enough that I was afraid to operate the engine.
* The service manual describes engine noises on page 200; copy attached. My noise is not exactly as they describe a rod noise or a main bearing noise but it is closer to the description of a main bearing noise.
* I plastigaged rod bearings. All 8 rods had .0015 inches clearance. The spec is .0005 to .0015 inches. Rod bearings are .010 currently oversize. The appearance of the rod bearings is good; see pictures.
* I plastigaged main bearings. 4 main bearings had .003 inches clearance and the rear main had .0015 inches clearance. The spec is .0005 to .0015 inches. The crank has been ground twice and is .020+ oversize. The crank was not supported when the main bearings were plastigaged so the clearance on the main bearings is likely bigger than .003. The appearance of the main bearings is good; see pictures. Lesson learned is that I did not specify to the machinist the size of the main bearing grinding and should have been very specific about this.
* The side clearance of the rods is between .021 and .025 for all 4 rod pairs. The spec is .006 to .014 inches so there is too much side clearance.
* The crankshaft thrust clearance is .007 and the spec is .002 to .007.
* Oil pressure is OK. With the engine hot, just above the low normal mark when at idle and just below the high normal mark when running 50 MPH.
* Piston skirts look good and cylinder bores look good. All pistons replaced during rebuild.

Current Thinking:
* Could be a main bearing knock. -- Main bearing clearance is too great because the main bearings were over-ground. The clearance on the main bearings is too big and out of spec.
* Could be a rod side clearance knock -- The side clearance is above spec.
* Probably not a rod knock -- Rod bearing clearances are within spec and bearings look good. It also doesn't sound like the description of a rod knock but will test again for this when the engine is back together.
* Could be slack in the rod to wrist pin connection. Have not checked this.

Additional problem...
Oil flow is from pump to main bearings then to rod berarings and cam bearings in parallel. A big gap in the main bearing clearance will lower the oil pressure for the main bearings and also the oil pressure supply to the rod bearings and main bearings.

Next steps:
* Install a high volume oil pump to try to overcome the problem caused by the over ground crank. Higher volume and higher pressure will hopefully get more oil to cam and rod bearings.
* Do not install new rod bearings at this time since they are within specifications.
* Try straight 30W oil instead of the Mobil 1 15W30 oil to see if this helps stop the knock. (may not know since doing oil change and oil pump at same time)

Next next steps:
* If acceptable operation not achieved, pull the engine and crank and grind the main bearings to .030 and the rod bearings to .020 and replace everything. Will still have too much side clearance on rods but don't know how to fix this. Is it possible to fix this?

MAHLE Bearing Brochure; https://www.mahle-aftermarket.com/media/local-media-north-america/pd...

Edited by samstrader 2020-11-02 1:56 AM