New rod bolts vs resizing

[Cranky]

Is it assumed that whenever you replace rod bolts, the rods have to be resized? Why is this? If the rod was right before, why does it 'go out' after the new bolts go in?

 

[Meep-Meep]

I've always wondered this myself. The best thing I can think of is the bolt shank that aligns the cap to the rod decides where those two components live and it's possible that different bolts will not repeat. If the bolt shank was ground like a shoulder bolt and it was a light press fit then it probably wouldn't matter, but they are just rolled or formed to shape so minor irregularities probably exist. I've had bolts come out during disassembly but just put them back in exactly from where they came from and nothing bad happened. I suppose if you have a way of measuring the profile of the big end down to tenths you can check it but by the time you set up to do all that you might as well just resize them. I'm not an automotive machinist, but seems to me that automotive machining is somewhat a lesson in economics.

Another rule of thumb I've always used is you have a limited amount of torque cycles on rod bolts. When the rods are new they are torqued to size them then again during assembly. A rering job will be the third time to torque, assuming you don't size them again. So I'd say 4 times, including a second resize, for rolling stock low stress low RPM stuff. For hot rod builds replacement should be mandatory.

More general info that I'm sure you know is if replacing with ARP or other high grade bolts it's often found that the torque is higher on those - from 45 lbs/ft to 50-55 lbs/ft on 3/8" bolts. The rods should be resized with the bolts torqued to the higher recommended torque and assembled with the same torque.

Just my $.02 (or I should say $20.00 accounting for inflation).

 

[condor74]

I recently asked this same question to a local high performance machine shop. He is is very familiar with building high horsepower Mopars also. Here is what he told me. If you go with ARP wave lock bolts you have to re size most of the time. If you go with ARP standard HP bolts. (Which are very good in anything up to 550 horsepower) then you don't always have to re size them. He said you should always check them. The wave lock bolts are pressed in much tighter than the OEM bolts and this can cause distortion in the big end of the rod. The HP ARP bolts are close in design to stock bolts and cause a lot less distortion than the wave lock bolts. He said about 70 percent of the rods that were good before the HP bolts were installed were good afterwords.

Basically you can get away with just installing the bolts but I would measure them to make sure they are not distorted. A fail safe method is to just go with the rule of thumb and have them re sized when ever bolts are installed.

 

[Cranky]

Another rule of thumb I've always used is you have a limited amount of torque cycles on rod bolts. When the rods are new they are torqued to size them then again during assembly. A rering job will be the third time to torque, assuming you don't size them again. So I'd say 4 times, including a second resize, for rolling stock low stress low RPM stuff. For hot rod builds replacement should be mandatory.

Funny you mention that. Some time back, we had a fastener company come out with this torque measure tool (forgot the name of it) for nuts and cap screws and it showed just how much a fasterner gave up after each torque down. The cap screw gave up a very little bit but the nut lost a bunch!! From then on, I replaced all the nuts on the final torque down on every build and ARP used to set just the nuts. Don't know if they still do or not. I think they now only sell to other resellers like Summit or Jegs etc.

I recently asked this same question to a local high performance machine shop. He is is very familiar with building high horsepower Mopars also. Here is what he told me. If you go with ARP wave lock bolts you have to re size most of the time. If you go with ARP standard HP bolts. (Which are very good in anything up to 550 horsepower) then you don't always have to re size them. He said you should always check them. The wave lock bolts are pressed in much tighter than the OEM bolts and this can cause distortion in the big end of the rod. The HP ARP bolts are close in design to stock bolts and cause a lot less distortion than the wave lock bolts. He said about 70 percent of the rods that were good before the HP bolts were installed were good afterwords.

Basically you can get away with just installing the bolts but I would measure them to make sure they are not distorted. A fail safe method is to just go with the rule of thumb and have them re sized when ever bolts are installed.

I forgot about the wave locks. I started this thread because a friend has argued with me about not resizing a set of rods I have when I swapped out the bolts but the rods checked out good before and they were the same afterwards....to the .0001! So, no need to spend the money on them...and they were good and straight too. Just had to get this out where he could read it from another horse lol

 

[Revhendo]

Funny you mention that. Some time back, we had a fastener company come out with this torque measure tool (forgot the name of it) for nuts and cap screws and it showed just how much a fasterner gave up after each torque down. The cap screw gave up a very little bit but the nut lost a bunch!! From then on, I replaced all the nuts on the final torque down on every build and ARP used to set just the nuts. Don't know if they still do or not. I think they now only sell to other resellers like Summit or Jegs etc.

I forgot about the wave locks. I started this thread because a friend has argued with me about not resizing a set of rods I have when I swapped out the bolts but the rods checked out good before and they were the same afterwards....to the .0001! So, no need to spend the money on them...and they were good and straight too. Just had to get this out where he could read it from another horse lol

Well gosh Wilbur, that's actually some nice to know info.

 

[polkat]

I have no great insight to add here except my own experience. A number of years ago I changed out rod bolts on a 383 and a 440 to the MP bolts without resizing. I tightened the bolts to torque and then used an inside micrometer to measure them in about 5 places around the hole. Looked good so I ran them and had no problems. When I pulled the rods later, all the bearing shells looked good. That was my experience but I am NOT saying it will be yours. I don't know about the wave locks, but with the MP's the shoulder is a bit longer, and tends to pull the caps into proper alignment, unless a bolt is actually bent, or the bolt holes are worn.

I'm probably an idiot for saying this, but I've come to believe that in some cases this concern is a bit over rated.

 

[Meep-Meep]

I have no great insight to add here except my own experience. A number of years ago I changed out rod bolts on a 383 and a 440 to the MP bolts without resizing. I tightened the bolts to torque and then used an inside micrometer to measure them in about 5 places around the hole. Looked good so I ran them and had no problems. When I pulled the rods later, all the bearing shells looked good. That was my experience but I am NOT saying it will be yours. I don't know about the wave locks, but with the MP's the shoulder is a bit longer, and tends to pull the caps into proper alignment, unless a bolt is actually bent, or the bolt holes are worn.

I'm probably an idiot for saying this, but I've come to believe that in some cases this concern is a bit over rated.

It's like I said earlier. Even if 50% of the rods work out without resizing, that means 50% will not work and what shop wants that headache? The 99.9% solution is to resize every time.

 

[Cranky]

It's the same with align boring and square decking. It's not hard to check a block for square but most shops won't spend the time to do it but will instead just throw it up on the machine and do it. After all, if it doesn't need it, then they won't make any money either so being a machinist anyways, I've developed my ways to check my junk before I waste money. And these days, the pawn shops are full of machinist tools so it's not that hard to pick up a few things. For example, a 0-12 wide base Starrett depth mic in perfect shape for 40 bucks. A 0-12 Starrett inside mics. And it's easy to check a set of rods. Besides, how do you check the machinist's work when you get it all back?? You just trust everything they do? The last race engine I had align bored came back with the main bores .003" out of round!!!! That would have lasted for a good while. And this came back from a shop that built Comp Eliminator engines!!