Question about rods

[f350flatbed]

Hi all,

It's kind of hard to search for topics about rods because the search ignores the word rod. So I've been trying to get together a good set of rods but I've noticed that there are a lot of different ones based on the stamped markings. I'm just talking about the idi non-turbo rods here. I've also measured the length between inside the small end and the big end (new bushing in small end and no bushing in old end) and they are different lengths. What procedures can be performed to make these the same length? Also, some are very slightly bent, can they be straightened? Thanks.

Paul B

 

[icanfixall]

I feel you are getting some bad information somewhere. All the 6.9 and 7.3 IDI non turbo rods have the same number stamped into the I-beam. Its 1805615C1. Now the turbo rods for the IDI motor numbers are 1816619C1. These turbo rods have the exact same length as the non turbo rods. In fact the only real differance is the pin diameter. Otherwise they are the same length, width and the bearing size for the crank end is the same size. Where are you getting this diffreing information? You or I can't straighten rods without the correct tooling. They need to be sent out. Diesel Direct has both turbo and non turbo rods. Last I checked the turbo rods were $42.50 each without a core. They had 2 pallets full of them. Usually the core charge will be around $25.00 to $50.00 per rod anywhere so the $42.50 price is outstanding.

 

[f350flatbed]

Yes the "model" number is the same but on the other side there are other numbers that I guess are like serial numbers. They probably are a manufacturing sequence. But when I measure them they are all slightly different lengths, several thousanths. What I am concerned about is that these different rod lengths will produce different compression ratios in each piston.

 

[icanfixall]

There is no differant rod lengths for either the 6.9 or 7.3 idi motors. If you have a rod that measures differantly I would feel something is wrong with "that" rod or your measurement is suspect. I don't want to say you can't tale a mearurement but they really are all the same length. Otherwise some motors would be a differant stroke and displacement from the factory. What measureing tool are you using to get this length?

 

[f350flatbed]

I'm using a digital micrometer. I'm wondering if the difference is from the bushings in the top end or maybe some of the rods are stretched. How many thousanths will it take to make a difference in the compression ratio? Also, I noticed in your sig that you have the pistons all weigh the same. What about the rods? The lobes on these rods look to be different in size, can these be ground to bring the weights closer together?

 

[icanfixall]

The digital micrometer must have a ball anvil and post to get a proper measurement on the round surfaces in the top and bottom of the rod. Otherwise you will not be measureing the exact curve in the top and bottom of the rods. Yes, the top and bottom of each rod has a balancing pad that is ground to make them all weigh the same but... You will need two gram scales to find this weight of each end of the rod. You can't just weigh each rod and make them all the same weight. You need to know how much each end weighs. Then adjust so that everything is the same when your done. Its called moment balancing.

 

[f350flatbed]

So when they balance an engine is this the kind of thing that's involved? What about balancing the crankshaft?

 

[icanfixall]

The crank needs to "know" what weight the rods and pistons are. Then that amount of weight is atached to the rods journels and is called a bob weight. Then its spun up and the computer tells you where and how much weight is needed to be added or removed from several areas of the crank. I had my crank balanced twice. The first time it was done I ran the motor 5200 miles and burned it down when a freeze plug let go pulling a grade. The second time I went to another shop and they ADDED 42 grams of weight because it was too lite in the center plane. The first shop suxs big time. So it depends on who you trust doing this. When I worked in a large machine shop we would balance steam turbine rotors that weighed 50 to 60 tons and get it down to 2 ozs of perfect balance. Smaller diameter items turning high rpms are much more difficult to balance. Our turbos turn around 130,000 rpms so they are not something the everyday guy can balance at home. Two grams at 8000 rpm equals about 40 lbs at the end of the stroke in any motor. That will cause a shake for sure. Our motors can run with pistons being 15 grams differance from piston to piston and still run smoothly... So the Mahle piston engineers told me....... Thats what I think of their statement.