I don't ask too many questions here but i do have one that i do not know the answer to nor have i been able to find anything.

I decided to do a aluminum rod engine as a new setup this season. We had a failure.. This was basically one of the first pulls under more load after we did roughly 11 miles of break-in on the dyno. Upon teardown, we found a rod cap to have a crack going from one end to the other width wise. Well i guess it depends on how you are looking at the rod. It is not the part that goes from one rod bolt to the other side. The smaller width. One side actually has three cracks. We believe this crack is what cause the tolerances to get off enough during load and eventually spinning that bearing. I know for 100% that all clearances and torque was dead on.

Has anyone seen a aluminum rod cap crack? I thought aluminum would bend, not crack? For what its worth, the largest crack, is perfectly down the center where the dowel hole/ pin is pressed into the rod cap. Could it have been pressed into a hole too small and causing it to weaken that spot?

I know there are alot of what if's, but am hoping to get some opinions and hopefully some first hand experience from some of the bigger names out there. Thanks in advance for any info.

Aaron

 

Sounds like a premature failure. When people speak of the small ends of aluminum rods failing, it is due to stretching over time, not in 11 miles though.

 

What brand of rods?

 

/\ We also think it was a failure of the rod cap and was already cracked from the get go and just wasn't able to be seen until it saw some "stress".

I would like to keep the name brand of the rod out of it until i get some expert opinions by my local machinist etc.

Aaron

 

post pics...ive done a few aluminum rod(groden and bme rod brands) engines and i havent seen anything like that before...what were the rod bearing clearances?..what did ya tq them to?...sounds like you got a defective rod

 

I sent pics to the owner of the company who makes the rod as requested. He also says he has rarely ever seen a rod cap crack in the middle area like mine. It is cracked pretty much half way down the cap. Says he has seen rods crack near the rod bolt area more often.

The one thing he did ask me was how old were the rods. Obviously i told him they are brand new. I special ordered them from a very reputable company, and then i put them in my engine and ran them within a month or so of receiving them. He wanted to know how long has the company who specs them out has had them. So i sent him pictures of the box with the order number etc on it. Apparently from the box etc, these rods were 6-7 years old. So basically they have been sitting for that long on a shelf or whatever, fully torqued, which is not good. If aluminum rods are going to not be used and sitting, your supposed to thoroughly clean them, oil them, and leave the rod cap un-torqued until you use them? I don't know how that affects the rods though.

Rod bearing clearances were set at ~.003", and torqued to 55ft lbs with good quality engine oil. The torque number is what's required with the type of forging the rod is. Needless to say, i am F'ing pissed.

Aaron

 

Thats the biggest load of crap ever.

Metal fails from fatigue.. not sitting.

 

I didn't say they would fail from sitting. I was just saying that they recommend un-torquing them if they are going to sit for long periods at a time, like say over winter.

Aaron

 

That's a lot of extra work to untorque your aluminium rods every winter and then retorque them in the spring...

If they were sitting on the shelf, new in the box from the mfgr, and they knew that could cause a problem, whey wouldn't the mfgr put a "sell by" date on the rods, or just not torque them? That does not make sense...

 

Sounds fishy

 

Its a connecting rod --- Not a gallon of milk.

 

hahahaha omg