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I agree with Kotzy, it sounds like a wrist pin with more clearance than normal. From what I have heard, an engine will run a long time in this condition. A quart of Lucas Heavy Duty Oil Treatment (about $10) will often quiet-down wrist pin noise.
When I bought mu 51 it done the same thing. When you set in the truck it sounded like the knocking was coming from the center of my dash. When I reved the motor up the noise would quit. The guy sold it to me b/c he thought the motor had a bad main bearing or a wrist pin was lose. I got it home and we got it running at it knock like crazy when the motor was cold. Know I don't know if this is your problem or not, but I pulled my oil pan and everything to check it out. What the PO did was put a new voltage regulator on it and didn't polverise it. That was the knocking sound we heard and it sounded like it was coming from inside the motor the whole time. We polverised the old 6 volt reg. just for the heck of it b/c it is 12 v now and the knocking sound stop. I got a good deal on the truck b/c he said the motor would need to be rebuilt and he didn't want to put that much work in it. Not saying thats what is wrong with your but it might be something to check out.
Make sure your battery cable's is on and the key is turned off. Take the field wire off the regulator and touch it to the battery side of the regulator. You should see a faint blue spark, then put the field wire back on and the regulator and your whole system is polverised. Another way to tell if it is the regulator is start your truck and take the positive battery cable off. If the knocking sound quits then your voltage regulator is not polverised or it is bad. You can try this first before you take the field wire off and touch it to the battery side. My knocking sound would quit when I took my cable off. I would try this before I started taking the motor apart. I took my oil pan off and check the bottom end of my motor and it ended up being the regulator. I know it sounds funny but the regulator knocking sounded like it was right in my motor and it wasn't.
Well,I wasn't lucky enough for it to be the regulator.I even unhooked it and cranked the truck and still have the knock.I am going to pull the generator and water pumps belt off tomorrow and check. Thanks,Russell
Ok, Ryan's secound theory:
I'm gonna present the idea of the fuel pump "kick rod" The rod that comes up from the rear part of the motor and pushes the fuel pump arm.
They wear out and are a near and dear part that ma quiet down at higher revs.
Some nights you just have to realize your gonna be slaying a Dragon and not saving the Princess
Well,I found out where my knock is coming from.I took it to older gentleman that found the problem.He shorted out each plug wire while it was running until he got to number 4 cylinder and it quietened down.Well Max said he bet it was a wrist pin and sure enough he was right.He dropped the oil pan and pulled the head off and got piston and rod out.You can fell the flop in them.Took to machine shop with a $3.12 bushing to have repaired.I orderd a new oil pump,pan gasket and clean out gasket,and head gasket.He should get it back together next week.I need one of the round clips that hold the piston pin in if anybody has a spare.Thanks for the help and replies. Russell
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