broken piston rod
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Junior User
Joined: May 2005
Posts: 92
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From: Grand Coulee
broken piston rod
got a freebie some time ago. 77 f250 w/ a siezed 400. took it apart and found that a piston rod had broke and the lower piece got wedged between the block and the crank (motor seized at this point). Doesn't appear too much damaged occurred. Just wondering if i would be able to put in replacement rod. would i need a new crank. crank doesnt look damaged. now i think of it. what caused the piston to get stuck in the first place? maybe need rebored? Any info would be appreciated. the 400 is just sitting in my garage now. if it didn't cost too much, it wouldn't be bad to have a spare motor if i needed one.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
I doubt the piston stuck to break the rod. More likey it was spun too fast (too many rpms) or the rod bearing seized, which is pretty common. I'll bet it was one of the rear cylinders? Like number's 4 or 8 ? Although the block may not look damaged, it still could be cracked. You'll need to completely take it apart, have it cleaned and magnafluxed to make sure there's no damage.
Posting Legend
Joined: Sep 2001
Posts: 30,255
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Sometimes the rod bolts are not installed correctly and they contact the radius at the corner of the flat spot where they go thru on the big end. This causes a stress concentration at that point that will break the rod. The rod bolt heads must never contact the radius area. It does not take too many RPM to make them break with this problem. Few 400's will rev far enuf to cause a bearing to seize outright. Usually the bearings seize because of lack of lubrication in a worn out 351M/400. Bearing lube failure is usually accompanied by discolored rods or crank journals. Metal transfer and galling can sometimes be seen due to lube failure. High wear rates are evident and the journal may be out of round and undersize which means it must be ground. Lack of lubrication can also egg the connecting rod itself. Rods should always be replaced in sets in an engine build. Mismatched rods are a leading cause for poor performance and unbalance in parts store type bulk remanufactured engines.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
Originally Posted by Torque1st
Mismatched rods are a leading cause for poor performance and unbalance in parts store type bulk remanufactured engines.
I swapped two rods in my 68 Merc's 390, 3 years ago when I tore it down for an overhaul. It needed two pistons and I just happened to have a spare set of rods and pistons ( cast std bore,10+ to 1 flattops & C7AE rods) that were given to me 6 months before ( what luck 
) So I swapped those two rods and pistons. Couldn't have worked out better. And I also did a buildup using a parts house roller 5.0 with hyper flattop pistons topped with Canfield heads, B303 cam, and a Vic Jr. Swapped the stock bolts, in place, in the assembled engine. One by one, replacing them with ARP bolts. That engine lasted well over a year, at first with ported E7 heads to 6500 rpms, then for a few months with the Canfields to 7500 rpm, daily "freeway" blasts. In the end it wasn't the rods that failed it, it was the block. It cracked a cylinder wall in the middle of a 7500 rpm shot.
Freshman User
Joined: Jun 2006
Posts: 38
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From: Texas
Two motors I had that broke factory connecting rods on nitrous passes had different amounts of internal damage.
The first rod broke when I lifted hard at the finish line & got off the throttle & nitrous at the same time. The slicks got loose just before the finish line (don't know why), I was stupid & lifted hard. The rod pulled apart in the center, and a section of the rod got lodged between the crank & the block and seized up the motor. To my suprise the crank checked out OK, (not bent & no cracks), but the block was junk & had a bunch of cracks where the crank & rod section jammed up. The cam was in a few pieces, & there was a bunch of bent pushrods. The other seven rods & pistons escaped damage.
The other motor broke a rod about 60ft off the line at a lower RPM, but did much more damage. Nothing could be salvaged from the bottom end.
You might get lucky and the damage might not be too bad, but it would be smart to pull it down and get every thing checked out.
The first rod broke when I lifted hard at the finish line & got off the throttle & nitrous at the same time. The slicks got loose just before the finish line (don't know why), I was stupid & lifted hard. The rod pulled apart in the center, and a section of the rod got lodged between the crank & the block and seized up the motor. To my suprise the crank checked out OK, (not bent & no cracks), but the block was junk & had a bunch of cracks where the crank & rod section jammed up. The cam was in a few pieces, & there was a bunch of bent pushrods. The other seven rods & pistons escaped damage.
The other motor broke a rod about 60ft off the line at a lower RPM, but did much more damage. Nothing could be salvaged from the bottom end.
You might get lucky and the damage might not be too bad, but it would be smart to pull it down and get every thing checked out.
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