rod mix-up? question

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mopar410

mopar410

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I have a set of supposedly 340 rods,I got with my 340 block and x heads unassembled.So asI'm cleaning the rods up I notice the 7 are 645 bushed and 1 is a 496 bushed rod.So I assumed I had a set thrown together but then I noticed the stamp is the same as on all the rods and the rods themselves look almost exactly the same other than the 496 has a rounded belly in the web and the 645 is more squared off,more like an I beam.
So my question is,could or would the factory let one slip by or is there really no difference in the rods.Also it apears they have been balanced so could the PO put this together and it all balanced out?I'm in the process of hunting down a scale.
This is a side by side to show the similarities
 

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here are some
Showing the bellies of the web
 

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Here is one of the stamp
 

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If it was me and I did not personally know this engine had been run that way (like you didn't know the seller prior to the sale) I'd pay to have the rods checked to make sure they weigh the same. Just in case.

And by that I mean using the right balancing fixture - not total weight, but large and small ends respectively.
 
Yeah thats the thing,not knowing.But if they measure out the same could the factory have mixed and matched these or is it probably just the PO throwing parts together.By look and feel there seems to be no difference between them as far as weight goes.
 
This was posted by 2DART in one of the threads Oklacarcollecto posted.

Yr, engine, pin type, part number, forging number
64-71, 273/318, Floating, 2406785, 1518699
72-73, 318, Floating, 2899495, 2899496
74-86, 318, Pressed, 3751015, 3418645
68-73, 340, Floating, 2899495, 2899496
71-86, 360, Pressed, 3751015, 3418645

The first part number is the light duty rod. The others are HD. Hope this helps.
Click to expand...

So that means both of my rods are HD rods and not a light one as I thought.
 
First, you couldn't feel by lifting if they truly "weighed the same". Identical factory rods don't. Factory weight matching consisted of grabbing 8 rods and stamping them 1-8. Performance balancing requires the individual ends of the rod be IDENTICAL. We're talking fractions of grams here, not something you can notice, not something most typical (even postal) scales can notice. Plus you ned to weigh each end individually.
Or, just slam it together and hope it's good enough to work.
 
Just slam it together and rev it to 7500. If somethin's wrong, you'll see it on the ground.
 
mopar410 said:
This was posted by 2DART in one of the threads Oklacarcollecto posted.



So that means both of my rods are HD rods and not a light one as I thought.
Click to expand...
3418645 are the rods in my 68 340 stock and my 71 has them to floating pins interesting.... thanks
 
moper said:
First, you couldn't feel by lifting if they truly "weighed the same". Identical factory rods don't. Factory weight matching consisted of grabbing 8 rods and stamping them 1-8. Performance balancing requires the individual ends of the rod be IDENTICAL. We're talking fractions of grams here, not something you can notice, not something most typical (even postal) scales can notice. Plus you ned to weigh each end individually.
Or, just slam it together and hope it's good enough to work.
Click to expand...
What I meant by no noticeable difference was in referrance to the rods being "light" and "HD" mixed together.As I mentioned I originally thought these numbers were 645 being HD and the solo 496 being a light rod but the light bushed rod would be 495 that you would be able to see and feel a difference beside a hd rod.My point is that the rods are mix-matched but seem to be all the same in a geometeric manner.My question was more in line with if anyone had experiance with factory rods being thrown in together if they were all HD or light rods.I know the process of balancing rods and if I get a scale soon I'll check them out but I do have a set of press fit rods that I may use as is or have them bushed as I already bought the bushings before I cleaned the rods up and found the odd ball.
 
update....after buying a cheap 4"x4" gram scale,which seems to be working pretty good right now,all the rods weigh in the same range from each other.All around 753 grams +/- 5 grams with the small ends within 5 grams of each other.Seems the rods are all within spec of each other which isn't totally suprising since there are all the HD rods.
 
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