Aluminum Rods cracking

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Aluminum Rods cracking

Post  LETHAL_DOSE on July 5th 2012, 9:26 am

I was getting ready to put this thing back together after it chewed up the push rod tips, and as I was cleaning the rods, I noticed a little crack from the bearing tang up the aluminum rod. These rods have 46 passes on them. So I looked a little closer and seen that #5 was cracked as well. The rods are GRP, and this is the second set that has been in this engine. The first set had 80-90 passes and all 8 of them did this same thing. Any ideas on what would cause this? The clearances are good; .0028-.0033 on rods. The bearings that came out of it looked excellent. Any Ideas?

Thanks
Mike
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Re: Aluminum Rods cracking

Post  hookin78 on July 6th 2012, 7:16 am

After a conversation with Bill Miller , he had mentioned to make sure that over winter or long periods of time one should back off the torque on the rod bolts. He spoke about a type of pressure corrosion that happened. This was the result, Has yur engine set for a whil all torqued up and not ran for months at a time.....? Just a thought

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Re: Aluminum Rods cracking

Post  LETHAL_DOSE on July 6th 2012, 8:18 am

This time it set probably longer than it has any other time. I made my last pass in October, then took the heads off in November and had them drilled for down nozzles. I got the heads back in late February, sometime in January the I took it apart checked everything and put it back together and was waiting on the heads. I put it on the Dyno at the end of March. It set from the end of March until late May when it was ready to fire up in the chassis. I am going to call GRP to talk them, but thought I would see if anyone else had any ideas.
It is probably going to sit until winter, because at this point in time I have no desire to spend any more time on it. I have tractor pulled for 20 years now, and it used to be fun. I always told myself that when it quit being fun it was time to walk away, and I am at that point. Give me a few months and I may be ready to go at it again, time will tell.

Mike
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Re: Aluminum Rods cracking

Post  ifucan on July 7th 2012, 10:36 am

LETHAL_DOSE wrote: I always told myself that when it quit being fun it was time to walk away, and I am at that point. Mike

I'm sorry to hear this. Take a step back for a year, toys aren't going anywhere...go jet skiiing, go to Montenegro, go do whatever it is you want to because right now it is too much work but don't sell out because with a 20 year track record of pulling, that is who you are and anything else will bring remorse and regret

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Re: Aluminum Rods cracking

Post  LETHAL_DOSE on July 7th 2012, 12:58 pm

I spoke to GRP yesterday afternoon, and found out that they make rods in 2 different materials. 1 is their pro material, which is the rods I have. This material has a shelf life is what they told me. If it is stressed(rod bolts remain torqued for long periods at a time) and remain in oil or salt water conditions or alcohol or corrosive conditions, etc the rod continues to fatigue. When they reach the end of their life, they normally crack at the bearing tang. He told me I need to go back to their 5100 series standard material rod. It can remain torqued, for longer periods of time and will not crack. I just wish they would have told me this 3 years ago when I bought this set, and I would have switched back then. They tell me this happens all the time. They get engine builders that call them up telling them they are going to build this killer engine, and need the best aluminum rod money can buy, so they put them in the Pro series. When actually all they really need is just a plain jane aluminum connecting rod. I believe that is what happened in my case as well, and when I replaced them 45 passes ago we went back with what the engine builder had stuck in it originally.


I'm sorry to hear this. Take a step back for a year, toys aren't going anywhere...go jet skiiing, go to Montenegro, go do whatever it is you want to because right now it is too much work but don't sell out because with a 20 year track record of pulling, that is who you are and anything else will bring remorse and regret

See I am already thinking about it again. You are right, I may have to get my brother's super semi out dust it off and go semi pulling a few times this year, just for something different.

Mike
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Fatigue

Post  mkopmani on July 10th 2012, 12:33 pm

I have heard BME's talk of backign off the rod bolts, and it's interesting to read GRP's response.

That said, with 46 passes on the rods, they seem about worn out to me. Not sure how much RPM you're running, but for my AR Boss, Miller told me to run them 40-50 passes then throw them away. Fatigue occurs as a result of cyclical loading which is near the yield point. Aluminum work hardens quicker than steel which is why they don't last as long.

When the bolts are torqued, the aluminum should not be yielding. If it is, there's bigger problems. Assuming that there's not a decaying factor (corrosion or the like), from a materials perspective they should be fine. Oil doesn't typically erode or oxidize aluminum that I've ever heard unless it carries something that is corrosive (nitro, methanol).

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