That's basically true; steel doesn't have the finite life cycle like aluminum so long as the stresses are below the yield tensile strength. Here's the catch with that- how do you know that no part of the rod is being stressed into minute deformation? The first the thread or so of the bolt hole comes to mind. Yes I'm sure these were run through an FEA program to attempt to predict where the stresses are highest, but that pretty picture is only as good as the programmer.

I've always wondered how much torque a rod sees at RPM.

I'd venture to say that you can make a sufficiently stout 'H' beam lighter than the 'I' profile but the 'I' will always be stiffer overall. It's all about where you put the mass. Just a guess from some old statics knowledge.

Compression and power stroke help push things together.

Reason why HP1 keeps mentioning TDC at overlap is because there is little to no pressure (sometimes a slight vacuum) as the piston just rose and is now being yanked back down. Ie: Just after TDC the crank is trying to pull the rod cap away from the rod and piston. This is why we need killer bolts, not just good rods.

At BDC the forces are trying to push the piston and rod beam into the rod's cap.

Imagine if you will - throw a 1lb rock out straight as possible and yank it back while it;s still in the air, as slow as you can. Now, shoot that 1lb rock out of a cannon and as the rope just becaomes taught, yank it back as soon as you can. Your shoulder will seperate from your body.

Look at the following animated pic for a bit, and then imagine it spinning many times fast. You'll then see how the piston and rod are still wanting to go upward with much force, while the rod's cap and bolts are trying to pull it down.

https://d2t1xqejof9utc.cloudfront.ne...9a5/medium.gif

Here's a better animation, Starts at 500rpm and goes up to 7k rpm by the end.

https://www.youtube.com/watch?v=s5n-2gb8yfM&feature=player_embedded

There was a discussion about rod bolts about this time last year. Basically what was discovered was at the rpm level we are spinning these motors a stock fm rod bolt would suffice.

I'll throw this one out there. Who has had a stock gm Rod fail?

Great demo, I guess the part I'm missing is the rod should be many times stronger than the bolt securing it due to shear mass, new bolts is a no brainer. I also see that if your wallet is over full then just go with new and be done with it, but that's not where the whole discussion began :flag:

Well, we know how many stock 502 shortblocks with GM 7/16" 6.135 rod has been hammerd and all has been well. 800+hp even. Everyone remember, 4.00 stroke. When these are ever torn down, many people will rebuild with a stronger rod bolt, just because.

Now, with the 4.25" stroke motors how many stock rods where kept ? Probably not many, therefore we really won't know. Most step up to a 6.385 rod (or longer in some cases) and thus aftmkt rods and bolts.

Longer strokes = more piston speed.

You can't use a stock Rod with a 4.25 the piston will not clear the crank. Unless you can find an external balance 4.25 crank...

Back in the day I used to play around with Pontiacs. Back then nobody was putting Chevy rods on a Pontiac crank and there was no aftermarket parts for them. We would use stock Chrysler Rod bolt on the Pontiac rods 1200hp+ and spin them to the moon and never had any issues unless the tune-up was off and then parts we go flying...

Scat makes a cheap cast steel 4.25" crank that can take a 6.135 rod. Only a few I've seen use this. I wouldn't bother.

Ive seen a GM rod come apart at 6000rpm in a buddy of mines cat. It wasnt pretty.

The other thing i think that plays a role. Take 2 engines, both identical 900hp for example.

First engine goes in a boat that runs up and down the river all day, encountering the occasional pontoon wake. He runs hard, lots of wot time.

Engine 2, goes in a boat, that runs the ocean, great lakes, etc. Hes out there every weekend working the throttle like bob saccenti in and out of rough waters. His engine is rapidly decelerating, and accelerating quite often.

Which ones gonna be harder on rods, or more likely to break one ?

Start talking long stroke, long rod, heavy piston fire breathers beating it up offshore, you def dont want some ebay special rods in there

That's exactly why I bought my rods off amazon.