Bent Rod, or what?

U

ursus

Well-known member
Just finished assembly of a short block with reground crank, new rods, and pistons. In checking over the piston heights I found that the #1 piston is lower in the forward end by 0.015 inches as compared the the rear. Is this evidence of a bent rod? The rod upper end is positioned in the middle of the piston and I don't see evidence of any obvious binding under rotation.
 
Switch the rods and see if the difference moves.
If the front is still .015 lower start measuring the crankshaft center to block deck, perhaps the deck surface got machined crooked.
Most likely the rod was machined with the center to center distance wrong.
The stroke could be wrong on that journal possibly
Start by moving the rod to other cylinders and measuring the piston projection
 
What I believe the O.P. is saying is that within the diameter of the #1 cylinder the piston top is angled/tilted .015. Lower at the front, higher at the back. I doubt the piston is mismatched but it is a possibility. Another possibility is the wrist pin end of the rod may be bent/angled. Having checked hundreds of A rods on a sunnen rod device and straightened many this would be my guess.
 
The other pistons are dead level so errors in machining the deck are probably not an issue. If the rod is bent at the wrist pin end maybe it can be tweaked back into alignment but not sure how that gets done.
 
0.015 inch is a lot. I am surprised that the piston could be cocked that much in the bore. That said, the clearance for a cam ground piston is not as tight in the fore and aft direction. I would take the rod to a machine shop that has the capability of straightening rods. Take the piston pin with the rod.
 
I guess I don't understand the suggestion about checking for a bent wrist pin. Ursus states these are all new parts. I would remove the piston and inspect the location of the piston wrist bore relative to the center and top of the piston. My guess is that if this dimension is out of spec the piston would tilt when in the fully extended position.
 
If these are rebabbited rods, the 1st question I would ask is how were the wrist pin bushings fitted to your new pistons. There are numerous ways to fit the (hopefully) new pin bushings and several of these do nothing to maintain the alignment with the big end of the rod. Sounds like you need to find a machine shop that has the equipment to check your rod alignment. Even new rods need the pin bushings fit to pins. If the piston is tipped enough to visually see a difference or if one piston is level and just lower in the cylinder at TDC than the others you still have an issue that needs to be investigated and corrected. Even if the deck was not machined correctly, it would be impossible to be so far out to have a .015 difference across the cylinder bore, and it would affect all 4 cylinders, not just a single cylinder.
 
I agree that the first thing to be done is check that #1 connecting rod. A person would need a crank pin mandrel and the piston pin with some good measuring equipment. For lack of that, I'd take it to a local machine shop. The piston pin bores would be next if the rod checks OK.
 
As an O.P. follow-up, I pulled the rod and piston and noted some scuff marks on the piston. Off to the machine shop where better measuring tools and a skilled machinest found that the bend was more that I thought - 0.025 inches off! A few iterations of tweaking and measuring and the proper alignment was restored.

I have found it a good practice to test and measure things to ensure that the moving parts are getting friendly with each other in the right way before buttoning it all up. In this case, a problem was found that might have had disastrous consequences for a rebuild.
 
Glad to see you did the right thing. We checked all rebabbitted rods on the sunnen rod checker. I will get a picture of one if anyone is interested. Also the bar used to bend, twist, tweak the rod. Now what does not make a lot of sense is that every rod bearing machine I know about and have used mounts the rod on the wrist pin and then grabs the big end on the sides to machine the bore which just about guarantees the big and little ends are parallel, square, and not twisted. So I have to wonder how the big ends where bored. .025 is a huge number. A twist or non parallel pin of .010 or less might be expected. We would use a .0015 feeler gauge with the testing device to ensure the rods where correct and square. Using the wrist pin for the set up also guarantees the rod is the correct length and all the same.
 
Dan McEachern said:
the 1st question I would ask is how were the wrist pin bushings fitted to your new pistons.
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Sounds like the rod was not level when the bushing was reamed. Either end of the rod being higher (due to chips/crud on locating surface) during machining would cause the bore in the bushing to be "tilted".
 
Terry Burtz said:
If all 4 connecting rods came from the same source, I'm willing to bet that the other 3 connecting rods have similar problems.
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Actually, the other three rods came from a small shop in Washington state and were straight. The bent rod was an older Federal Mogul product that appeared to be in pristine new-in-the-box condition. One never knows!
 
ursus said:
Actually, the other three rods came from a small shop in Washington state and were straight. The bent rod was an older Federal Mogul product that appeared to be in pristine new-in-the-box condition. One never knows!
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Thanks for your reply about different sources for your connecting rods. Do you know if they all have the same weight?
 
Terry, these are 0.030 undersize Model B rods and I had already weight matched them to a +\- 0.5 grams variance per unit hence the preference for fixing the problem versus sourcing a new rod of unknown weight.
 
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