Loose & over-protruded piston

TrailTamer

TrailTamer

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I just rebuilt a 12v Cummins and everything went well until I test fire it. There was a bad noise within 5 minutes of run time. Borescoped & found valve impressions on top of piston #5. I had to pull the head to see why it no longer turn over 360* by hand.

I’ll pull the piston/rod soon. But, until then, what the hell have happened? How can the piston be loose, move by roughly 1/2” up/down by hand and is over-protruded, even after pushed it back down.

12v Cummins piston looses overextending - YouTube
 

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I think it’s wrist pin judging the feel when pull/push the piston up/down. The picture shown is with the piston pushed down. Not sure how it can be the rod bearing or cap since it’s extended above deck.

I will update this when I get the rod out.
 
Rod cap/big end problems could definitely cause that. I'd assume it to be on the big end before I guessed the small end, with that much play.
 
bateman said:
Rod cap/big end problems could definitely cause that. I'd assume it to be on the big end before I guessed the small end, with that much play.
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Agreed. I'm sure anything in the pin area would have been noticed during assembly. It's either a broken or stretched rod bolt, or missing bearing.

Please do update us on this.
 
TrailTamer said:
I think it’s wrist pin judging the feel when pull/push the piston up/down. The picture shown is with the piston pushed down. Not sure how it can be the rod bearing or cap since it’s extended above deck.

I will update this when I get the rod out.
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Rod bearing is doubled up or piggybacked on the big end of the rod. No bearing on the cap.
 
DISTURBED said:
Rod bearing is doubled up or piggybacked on the big end of the rod. No bearing on the cap.
Click to expand...

If the bearings were doubled up on the big end it would have been out of the bore on assembly, it also would have been the same overall diameter on the top to bottom and never fit side to side... Plus it would never bar over if you were able to wedge that bearing around the crank doubled up. I think he forgot to torque a rod bolt or forgot the lower bearing...1/2" is a ton of movement....How was your oil pressure?
 
T-MAN said:
If the bearings were doubled up on the big end it would have been out of the bore on assembly, it also would have been the same overall diameter on the top to bottom and never fit side to side... Plus it would never bar over if you were able to wedge that bearing around the crank doubled up. I think he forgot to torque a rod bolt or forgot the lower bearing...1/2" is a ton of movement....How was your oil pressure?
Click to expand...

I had mechanical gauge hooked up for the first test fire. It never dipped the whole time it ran. It was running at just under 100 psi. And yes the engine barred over just fine, no seizing to speak of and I didn’t forgot the bearings. Lol
 
From what your describing with only 5 minutes of run time is the cap either has loose bolts and it hitting the side of the block locking it up or 1 bolt broke and your very lucky. But until you see what damage in the lower end we don't know how lucky you are. The wristpin moving a 1/2" is near impossible and if the circlip fell out you would kill the cylinder wall before it came apart with only idle time... There's not enough room for the pin to slide out in the block.. This sucks on a new build... How much can you twist the piston in the bore?
 
Have to agree, if bearing doubled on big end it would have been eye poppingly obvious on rotaion of short block.
 
T-MAN said:
If the bearings were doubled up on the big end it would have been out of the bore on assembly, it also would have been the same overall diameter on the top to bottom and never fit side to side... Plus it would never bar over if you were able to wedge that bearing around the crank doubled up. I think he forgot to torque a rod bolt or forgot the lower bearing...1/2" is a ton of movement....How was your oil pressure?
Click to expand...

TrailTamer said:
I had mechanical gauge hooked up for the first test fire. It never dipped the whole time it ran. It was running at just under 100 psi. And yes the engine barred over just fine, no seizing to speak of and I didn’t forgot the bearings. Lol
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RacinDuallie said:
Have to agree, if bearing doubled on big end it would have been eye poppingly obvious on rotaion of short block.
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The bearing wasn't doubled on assembly. The bearing doubled up after it fired and ran because the the cap came loose. Hence why it's sticking out of the block the same distance as 2 bearing halves put together
 
Last edited:
Cap came loose and "stacked" the 2 bearing halves on the top of the crank journal.
They can also do it without the bolts loosening, all it takes is one half sticking to the crank journal and the other half riding up over it.

Mark.
 
DISTURBED said:
The bearing wasn't doubled on assembly. The bearing doubled up after it fired and ran because the the cap came loose. Hence why it's sticking out of the block the same distance as 2 bearing halves put together
Click to expand...

Soooooooo what was cause of this action? Phone ring while torquing rod caps?
 
RacinDuallie said:
Soooooooo what was cause of this action? Phone ring while torquing rod caps?
Click to expand...

Very possible. Thinking you torqued them all and forgetting a rod. Just a habit for me is I install all rods first and finger tighten the bolts. Then torque in sets of 2 as the rods rotate up so you can get to the bolts. Them I go back and put the wrench on them to make sure I don't forget any. Also a common practice for me is there is no one in the engine room when I'm assembling a bottom end. Anyone can distract you for just a second and that's when mistakes happen. I implemented that practice after someone stopped by to chat one time and I mixed rod caps as I was assembling. Couldn't bar it over and was pretty quick to discover my mistake after I made them leave.
 
We always paint lines across the fasteners so that they should all have to be indexed roughly the same direction once fully torqued, especially on TTY fasteners. I've also used two different colors to designate what sequence I'm on.

Sent from my motorola one vision using Tapatalk
 
DISTURBED said:
Very possible. Thinking you torqued them all and forgetting a rod. Just a habit for me is I install all rods first and finger tighten the bolts. Then torque in sets of 2 as the rods rotate up so you can get to the bolts. Them I go back and put the wrench on them to make sure I don't forget any. Also a common practice for me is there is no one in the engine room when I'm assembling a bottom end. Anyone can distract you for just a second and that's when mistakes happen. I implemented that practice after someone stopped by to chat one time and I mixed rod caps as I was assembling. Couldn't bar it over and was pretty quick to discover my mistake after I made them leave.
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Installing all rods finger tight first, then barring the engine over to torque them is a No No in my book
 
JWhite said:
Installing all rods finger tight first, then barring the engine over to torque them is a No No in my book
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I guess I kinda miss spoke. I use a small Snap-On impact that does about 60in pounds. Not really finger tight.
 
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