351 interchange question
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New User
Joined: Oct 2003
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From: Boise, ID
351 interchange question
I have a 1990 f250 with a 351. The cam bearings are knocking like a PowerStroke at about 2500 rpm. I have a chance to pick up a 1987 351W long block and was wondering about the interchangeability of the block and heads. I was planning on using all the same intake, exhaust and engine management systems. Any input would be greatly appreciated.
Hotshot
Joined: Jul 2002
Posts: 14,285
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From: Bend,OR
the blocks are the same. cam bearings wont knock. sounds like you may have detonation. does it go away when you ease up on the throttle? what is your timing set at should be 10* btdc with spout connector removed.
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New User
Joined: Oct 2003
Posts: 10
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From: Boise, ID
Thanks for the input on the blocks. It doesn't seem to be detonation. It occurs whether you 'sneek up' on it or power hard to 2300 to 2500 RPM. If you go past it's not there either. I've experienced the racket before with a chevy 6 cyl. that did a lot of highway travel at a specific RPM range.
Lead Driver
Joined: Feb 2003
Posts: 8,140
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From: Oak Harbor, OH
It could be worn main bearings. Easiest way to tell is check the oil pressure at idle with it hot. Use a real gauge not the idiot light on the dash. If the oil pressure is less than ~35psi at idle it's time to think about replacing at least the bearings. Get one new standard size main, and one standard rod main bearing. Use them and some plastigauge to check the clearances. If it's more than .003 inch record how much and look at the crank. If it's all scarred up that means they've been hammering pretty good. If it looks like it's in good shape get some ultra fine emry cloth about 3/4 inch wide and polish the crank journals. Order the right size bearings, take the amount of clearence you have and subtract .003, then clean up the block and install the new bearings. If it's not burning oil and the performance is still good the engine will last quite a while longer.
It could be carbon... I forget what year you said you had, but if it's carbed get a small glass of water, open the throttle up and pour in small amounts of water. You don't want to stall it just steam the valves to clean them up. If it's EFI then get a spray bottle and use the same method. Just spray it into the intake.
It could be carbon... I forget what year you said you had, but if it's carbed get a small glass of water, open the throttle up and pour in small amounts of water. You don't want to stall it just steam the valves to clean them up. If it's EFI then get a spray bottle and use the same method. Just spray it into the intake.
Last edited by pfogle; Jul 6, 2006 at 12:54 PM.
