351 Knock
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New User
Joined: Mar 2006
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351 Knock
I just recently purchased a 79 F-150 with a 351m. It has a knock at warm idle but not when cold. I am thinking that it will need rod bearings. Please point me in the right direction. Thanks!
Laughing Gas
Joined: Jul 2002
Posts: 843
Likes: 1
From: USA
Because the crank is iron and the rods are iron they would heat at approx. the same rate so if a rod bearing is bad it will probably knock both cold and warm. On the other hand, pistons are alum. and heat at a different speed and rate. Good possibility it's a wrist pin. It's a rebuild either way. Get a 400 crank and pistons and you will have a much torqueier(is that a word?) engine.
Posting Guru
Joined: Aug 2000
Posts: 2,416
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From: PacNW
Best way I've found to pinpoint a knock in a specific cylinder is to "short out" each cylinder 1 at a time by pulling the plug wire while it's running. When you pull the wire on the cylinder that's giving you the problem the knocking should either quit or change significantly.
If it doesn't quit/change after you've tried all of the cylinders then it could be a main bearing or it's not in the bottom end at all.
If it doesn't quit/change after you've tried all of the cylinders then it could be a main bearing or it's not in the bottom end at all.
Senior User
Joined: Dec 2005
Posts: 229
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From: Fargo, ND
My old 351M did that as well, when it warmed up it really rattled and finely through 2 connecting rods right through the oil pan and thrashed the block. So if you want to save the motor try not to run it too long like this. I think it would be a rod bearing myself.
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New User
Joined: Mar 2006
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Well I dropped the oil pan and 7 rod bearings were worn out and one was spun. I replaced with a 400.
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From: Eastern Pa
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New User
Joined: Mar 2006
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I got a good deal on a 400, so after I dropped the oil pan and found the carnage, I just pulled the motor and it is sitting in the corner of the shop.
P.s. Where in Eastern PA are you, I just recently moved to Lancaster county
P.s. Where in Eastern PA are you, I just recently moved to Lancaster county
