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Bearings and compression

312 Views 9 Replies 5 Participants Last post by  Hybrid77Z
I searched and couldn't find anything definitive. A friend and I were talking the other day about the effect on compression with a bad rod bearing. With the extra play in the travel of the piston, I would think it would cause a decrease in compression, but would it be noticeable? For example, he's working on a motor that has an audible knock which he thought might be a rod bearing, but the compression was 180 (plus or minus 3psi) across the board. Cutting spark to the cylinder reduced the noise somewhat, but not eliminated it. The plug shows a healthy burn and is identical to the others, and their is no noticeable metal bits in the oil (no lab analysis done though). Regardless, my question is simply, will a bad rod bearing noticeably effect compression readings? What do ya'll think?
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The procedure followed was correct:
Remove spark from the suspect cylinder to see if the knock goes away while engine running. Goes away, rod.
Two adjacent grounded plugs and goes away, main.

I never heard of compression checking for diagnosis of rod knock.

REaching a bit tooooo far into the theoretical there.
You will get a knock with a few THOUSANDTHS of excess clearance letting the oil wedge dissipate.
Only when the bearing is totally disintigrated will you get 1 or 2mm clearance, and one heluva knock. By then it's so obvious something is wrong, pulling the plug is thenext step, and then shutting it down till repair is completed.

Not taking a compression check.
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I have to agree,,, if it is knocking,,, quit driving it before it detonates.
Adjust the valves and see if the noise goes away. It may just be clattering valves.

Wow, where did that come from?

Valve noise that goes away when you remove combustion from the cylinder...

Care to expound on that one. Never heard that symptom before...
Re: Wow, where did that come from?

Yeah, don't think it's the valves..............Also, I would think if you put a stethoscope on the oil pan directly underneath the suspect cylinder, the knock should be quite distinct, right? On this motor, it's not.
Sure, He said that the noise did not go away, just reduced it "somewhat".
Depending on the level of just what "somewhat" is, it's hard to diagnose. Since this is 3rd hand information about some friend, I do not even know if it's a Datsun or not. Could be a sticky hydraulic lifter, who knows.

What cylinder ? I'd bet its #1 ... Either now drive short distances with plenty of oil cans or pull engine (maybe not) and install new connecting rod bearings and clean up your piston tops and hone your cylinder bore walls and FIX your problem. I once drove across country in a 240 with a really BAD knock due to a broken bottom rod connecting bearing.... Had to use a case of oil over 3500miles of driving. Goodluck to you. Have fun.
Seen that too!

#1 bearing big end totally GONE and the enigne RAN...

A sticking lifter or valve would have NO difference when the power stroke was eliminated AT ALL---therefore I still don't see how a knock that diminishes with the removal from spark to the cylinder would be anything other than rod knock...

If there are other noises, pull more cylinders and see if it diminishes. The main is right nexct to the rod, and if particles flowed or flung into the next place.......

But springs, vlaves, lifters don't care about spark being present in the cylinder, and if he noticed ANY reduction I would rule out them as anything to worry about at this juncture.

Thatit decreased AT ALL when the plug wire was pulled is a BAD sign, and one of IMPENDING BEARING DOOM!

The more reduction in noise / knock you get when you pull the plug wire, the WROSE SHAPE the bearing (and journals) will be in at teardown!
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Or a misfiring ignition system will react the same way....
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