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Old 04-22-2009, 09:33 AM
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Wrist pin walking?

What causes wrist pins to do this? Just pulled a sbc 350 apart my buddy gave me. Cylinders 1 and 7 both did this after 3 hours on a fresh rebuild. They are press fit style. Block is already bored to .040 so probably not saveable.
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Old 04-22-2009, 09:56 AM
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In a press fit rod/piston, it usually is from not having enough interference fit between the rod and wrist pin. You typically need a minimum of .001 interference to hold the pin securely in the rod, more is better. Its possible the rod could have already been pin fit for an oversize wrist pin in the past,and a standard size wrist pin was put back in it, and the installer didn't check or pin fit the rods before he put them together. If it was a floating wrist pin set-up, then obviously a wrist pin lock came out. Bummer....
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Old 04-22-2009, 10:44 AM
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It could also be that the wrist pin is too long. Did the pins come with the new pistons?
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Old 04-22-2009, 11:13 AM
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Not sure if wrist pins came with pistons or not. The pistons are H345NP. Which is just a hyp speed pro piston. Not sure if rods are stock or not either, they have an x on the big end. I bought this motor for $100 and it has nice cam intake, heads, and accesories so i already got my moneys worth out of it so not too worried about the block, just thought i would get some input from some people with more experience than me. Also almost every bearing was down to the copper. It was only run for about three hours did this person just not have a clue about what they were doing and clearances? Thanks
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Old 04-22-2009, 11:29 AM
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Quote:
Originally Posted by zildjian4life218
Not sure if wrist pins came with pistons or not. The pistons are H345NP. Which is just a hyp speed pro piston. Not sure if rods are stock or not either, they have an x on the big end. I bought this motor for $100 and it has nice cam intake, heads, and accesories so i already got my moneys worth out of it so not too worried about the block, just thought i would get some input from some people with more experience than me. Also almost every bearing was down to the copper. It was only run for about three hours did this person just not have a clue about what they were doing and clearances? Thanks
Well, with only 3 hours of run time I would say that he did not check anything while assembling. If you are keeping the crank, the rod & main journals need to be checked to see if it was turned or not and that the journals are round. It may be that he did not use any assembly lube other than oil when he assembled it. The clearances are either too tight on the bearings that are worn or the oil system was not working properly OR he did not prime the oiling system before startup.
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Old 04-22-2009, 11:32 AM
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Lol oh man, this kid went to a college by me for motor sports. Good reflection on the college i guess. The counter weights on the crank also have gouges in the side of them. It looks like they were coming into contact with the rods. Could it be he installed these backwards?
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Old 04-22-2009, 11:34 AM
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It`s also possible whoever installed the pistons on the rods didn`t know what they were doing and left the pins too far off center. The cast iron grit from the bores is what killed the bearings after the oil pump picked it up. If the piston was hitting the crank it gives the impression the rods are bent.
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Old 04-22-2009, 11:41 AM
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so should i throw out the rods and pistons? or could i save the pistons? Just hang them on a new set of rods.
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Old 04-22-2009, 12:25 PM
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It looks like who ever pressed the wrist pin in the rods wasn't fast enough getting the pin in the rod all the way and the rod got cold and it shrank before they could get it centered properly on the piston. It probably didn't damage the piston but the block naturally needs to be rebored. Time for some bigger pistons or a different block.
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Old 04-22-2009, 01:06 PM
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the block is already at .040 will taking it to .060 clear up those gouges? They look deep.
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Old 04-22-2009, 01:15 PM
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Take it to a machinist to get it mic`d or have someone you know that has the instruments needed to measure how much it`ll need to clean up.
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Old 04-22-2009, 05:24 PM
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Quote:
Originally Posted by zildjian4life218
What causes wrist pins to do this? Just pulled a sbc 350 apart my buddy gave me. Cylinders 1 and 7 both did this after 3 hours on a fresh rebuild. They are press fit style. Block is already bored to .040 so probably not saveable.
The pin wasn't being retained by the press fit, one would think the installer would have seen this. But!

- Things you need to check is pin outside diameter against the rods inside diameter. Some pistons come with pins made to be press fitted and others come with floating pins

- Were the rods accidentally sized for floating pins?

- Are either of these rods cracked across the pin boss allowing the bore to open up when the pin is pressed?

- I'm assuming the pins were positioned properly when installed as any simple observation would have made a pin not centered pretty apparent, but if there's one thing I learned, is you can't assume anybody did anything correctly.

- A rod where the wrist pin bore is not parallel with the crank pin journal bore will try to push the wrist pin out. The piston would be tipped sideways to the bore. This may not be so much measurable upon installation as the rod will move on the journal to take up the misalignment in its side and bearing clearances. But the rod bearing will not be footed flat upon the journal. When the engine is running, the rod will try to set the bearing flat to the journal, the resultant twist force will want to drive the pin out of its bore.

- The H345NP is a "claimer" piston, these aren't anything I'd recommend using. When I see the word "claimer" associated with any part I shudder at the thought of it coming apart and walk on by.

- A 350 block will take .060 pretty commonly so you've got .020 to play with to clean up the damage so all may not be lost. But don't bore more than what it takes to get a clean wall, if going to .050 does it stop there.

Bogie
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Old 04-22-2009, 05:52 PM
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thanks everyone for all the info
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