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Mystery rod failures

Raging Bull Engines

Raging Bull Engines

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I have had several rods smoke the big end lately. All rods had bolts lubed and re torqued before honing on my sunnen machine. All rods were checked for roundness, clearance, and verified that the pin bore and big end were parallel. Two of them stuck after 2-3 races. A couple of others fried on the dyno. It looks like they start pinching the crank at the part line. My process has not changed in several years but the issue is new. I ball honed a new properly honed rod a few strokes, something I don't normally do, to get a visual. I pulled the rod out after 5 minutes run time and one quick pull and can see the shiny spots in the bore on both sides near the bolt threads. It was torqued with good torque wrench. The rods get tight in this area only after it has been ran. These have been LY rods. The rod in the photo was ball honed after being ran, then I put it back in and spun the flywheel by hand a few times and pulled it back out and saw the two shiny spots. If it only happened on the dyno I wouldn't care too much. When it happens to a customer during the 3rd race on the motor it bothers me.
 

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there’s not enough meat between the bolt hole and big end so it distorts when torqued and when it stresses it puts pressure on the crank there. Was the crank good and round? Those things flex and get a bit out of round too. I ain’t gonna lie I make mine big time loose on the big end and run 0w20 and 5w20 oil. Dykem the rod up good on the flats and torque it and run it ...see what is getting cleaned off. A lot of them side load a bit just because the machining on the big end flats don’t have good clearance there once torqued.
 
Maybe needs seasoned like an old flathead .
run a bit then hone .
Yes new unusual problems need pinpointing .
 
Anybody remember the Chinese rods on the Raptor? :)
There's a solution, but I'm guessing it'll cost you fifty bucks or so per motor.
 
BigMoneyGrip said:
I think the rod threads are formed (rolled) and not cut. But I might be wrong!
Click to expand...
Rod bolt threads are rolled. (As are most bolt threads)
The holes are cut with a tap. Thread quality checked with qualified tooling. There are different quality of thread engagement.
Percentage of thread engagement affects thread strength.

If there is a mismatch, there will be distortion.
Cheap production line, low rpm engines.
All tooling wears.
Tap the rods, with the cap on, and opposite bolt tight. Then hone rod with rigid hone, with bolts torqued.

Rigid hone may be the answer.
 
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Since you have ruined several rods I would be looking at the crank. I don't see any reference to putting a mic on the crank to check roundness, have you done that? Have you used Plastigage on it? How much oil clearance did it show?
 
Interesting Captain Flathead, thanks for the heads up. I will keep an eye out for this, but I have not seen this yet myself.

Where are you getting your cranks and rods from? I'm using 110 in/lbs for rod bolt torque. What are you torqueing to?
 
Do you notice it following a crank? I'd be more inclined to chase a crank journal that is out of round. Any chance you've swapped cranks with the same rod in and monitored?

I've seen a lot of cranks lately (not really a lot as I'm no big time builder) but several that were out of round a few .0001's both new and used.
 
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