Got the Engine Running and Driving back and forth in the driveway after cleaning the carburetor.
Pretty sure i have a Rod Knock going on now though.
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Posted 23 December 2016 - 02:43 PM
Rod knock is when the bearing clearance is excessive for several reasons. With too much clearance it's difficult to contain an oil film between the rod bearing and the crank. Without oil there is no hydraulic 'cushion' when the piston is driven down suddenly when the cylinder fires. The rod and bearing slam into the crank and there is a definite knock sound like a muffled hammer blow on solid metal.
Generally louder under load and gets softer, even silent when you let off of the gas.
In early stages much quieter progressing to MUCH louder. This can take from seconds to months depending how hard it is driven.
Once the rod begins to knock it never gets better. You can somewhat reduce the noise by running very thick oil but it will get worse.The bearing shell can wear down or just like forging metal can be pounded flat to razor thinness. When there is enough clearance, one bearing shell can ride up over or under the other and block the oil supply. If driving slow enough you will hear a squealing sound and shortly after the engine will just seize and stop. The heat of friction (from lack of oil) melts and wends the rod to the crank. Naturally at high speeds, high RPM, this can happen so fast the rod will weld to the crank but the crank cannot stop so the rod snaps off, rotates around and punches a hole in the block or pan. Sometimes the crank will break.
If caught soon enough, I mean really soon you might get away by replacing the rod bearing. This will reduce the clearance back close to normal. If nothing else you can save the rod and the crank.
Now for the good news. There's a good chance that you are mistaking excessive valve clearance for this. Valve noise is more of a loud clicking sound that a knocking sound. Here's the difference... valve clearance noise is the same volume no matter how hard you are driving it, even at idle. It will increase with engine speed but not get louder or softer.
Posted 26 December 2016 - 10:50 AM
Good little topic, except for some poor grammar
I own a z20 powered car, appreciate the info. I know d*** about the engine but seems like the short 'road' could cause the bearing to get beat up..?
"Rod" I replaced it with a z20s rod.