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I hate asking questions I'm pretty sure I know the answer and it's bad news, but maybe I'm wrong. I have a '68 302 that I bought last fall. Just got it down off frame jacks after doing the floor. While it was up there, changed the oil pan and, naturally had to change the oil. Used same oil that previous owner used. Anyway, all of sudden, I have a knock that seems to come from the bottom of the engine and goes away once it's warmed up. My guess is a main bearing or a rod.Motor supposedly has 45K original miles with only valve seals and a timing belt replaced. The guy I bought the car from is a mechanic. Suspect there was an "additive" that kept the motor quiet until I changed the oil. Am I correct on diagnosis and if so, anyway I can buy a couple months until fall comes and I have more time to pull it, get it fixed and detail the engine compartment while it's out? Mark
 

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Does The oil pan does not have a dent in it somewhere? I have never found a oil additive that would quiet,n a rod or main bearing knock. How about the oil pan pick up is it laying on the bottom of the pan?You need about a quarter of an inch clearance there. Robert
 

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Just curious about all the answers that you got. I would think that a knock in the lower part of an engine would get worse when it heated up. The oil would be thinner and the metal would be expanded.
Also my dads 302 went 100,000 miles with a lower end sound. Always thought it was a wrist pin.
 

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Neil Home got it right.The problem is piston related,be it a loose wristpin,cracked skirt,or excessive wall clearance.I've seen engines run thousands of miles like this without a problem.If it continues to knock when warm,you have a problem.A bearing will only get louder as the oil warms up...
 

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Check your oil pressure with a good gauge after market gauge. If it is low then you would get a rod knock my old truck knocks at start up but the oil pressure is really low. like 10psi. It's not a daily driver. Have seen rods knock at High Rpm and at start up. Have had a bad wrist pin on an engine but it always knocked. Have had lifters sound like rods before that could be a cheap way to go. Pull the valve covers and look for really loose rocker arm. Sometimes lifters bleed down after shut off for awhile when a piece yucky stuff get stuck inside.
Good luck. It'll drive you nuts!
 

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My 289 used to knock until it warmed up just like you are describing. I believe what was causing the noise in my car was a stuck lifter. I don't know how changing the oil would cause this to start, but I believe that was the problem in my car as when I took it apart, there were 2 lifters that i had to "coax" out.

Dave
 

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It does sound like a piston. My previous engine did that. When I rebuilt it, I found several skirts with cracks in them causing piston slap.

Why it would show up after an oil change is curious.
 

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If you are sure the noise was not there before you changed the oil, I would change the filter just to be safe. Stranger things have happened. Make sure you use a good brand of filter ( not a Fram!)

If that doesn't help, I agree with the other posts that its probably piston slap. Small block Fords are notorious for it. It's annoying, but it usually won't cause any problems. If oil consumption is within reason, just drive and enjoy it.
 
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