I was examining a set of aluminum rods the other day and something occured to me. Wouldn't the "weak link" be the threads in the aluminum body of the rod where the cap screw is inserted? I don't know anything about the engineering aspects of threads, it seems that the threads would give before the bolts. Obviously, the rods work. So I am missing something and the threads must be stonger than I am imagining. If someone could explain it, I'd appreciate it.

Rich

 

I think you're going to get wildly varying opinions, just like the thread on Ti valves, but here's typically what I've heard (the work hardening part, at any rate) in relation to why aluminum rods aren't recommended for use on the street...

"In a blown application, it is highly recommended to use aluminum rods. The reason for using aluminum rods is not because they are lighter, (in fact by the time you add more material to the rods to make them strong enough they can be almost as heavy as a steel rod). Aluminum rods are used because they are softer than steel. That makes them act like a shock absorber every time the piston is forced down. This gives longer crankshaft and bearing life. There is one disadvantage, aluminum rods will only last so many runs. Every time the rod compresses and absorbs the shock of combustion, it work hardens the rod and the rod becomes more brittle. A rod will only take so many cycles and then it will break. How often to change them depends on how hard your running the engine (RPM, blower boost, etc.)."

No mention is made of the threads, but I've never seen a broken aluminum rod that had the main bearing end damaged. They're always twisted or broken in the center, on the "shaft".

 

My understanding of threads, back from my days of FEA stuff, is that... for a fastener to function at it's potential (tensile strength, etc, etc) it's length of engagement must be "X".
Of course, with aluminum being a softer material, that "X" dimension is gonna be larger than it would be in an steel alloy rod. So.... if you have enough thread engagement, then the determining factor will be the bolt strength.... providing class limits were held on the thread.

That said, I'm with jimlab. Every failed aluminum rod I've ever seen was broken in the middle. They are tougher than people will give them credit for and I think alot of that comes from recommendations like, "replace every 20 runs", which were more geared towards 2000+ hp engines. Seen em go a few hundred passes in your typical nitroused 9 second ride with bolt changes as needed.

-Mindgame

 

There was a thread on one of the racing forums a while ago about aluminum rods. Yes if you have a 1500+ hp engine then you're going to be replacing the rods aften just to be safe. One of the racers said his bracket car has had the same aluminum rods in it for 9 years now with no problems.

Rod bolt strength is to hold the rod to the crank when the piston reaches TDC. That's where all the energy is when the piston and rod are trying to go through the head. Lightweight pistons and a light aluminum rod reduce the amount of weight trying to fly off the crank so even if the amount of threads is the same as a steel rod, there's not as much force being put on the threads.

My Howards Rods have 7/16" rod bolts. At the end of this season, providing the engine stays together for a full season, I'm tearing it down again for new bearings and rings. I'm going to send the rods out to be measured and resized. It will give me an idea about how much they're going to stretch or compress over a season. Unless I abuse the engine somehow, I expect the rods to last a long time.

 

I thought I'd share my thoughts and some facts (EDIT: WRONG) on aluminum rods, especially for an endurance application.
Aluminum's thermal expansion can be the killer of aluminum rod engines. Oil temperature needs to be kept in check. A drag race engine doesn't typically see very high oil temperature. Certainly not as high as a street engine that has been running for a while. As the rod heats up the big end changes dimentions and can actually reduce oil clearance at the bearing. (edit: nope) As the rod expands it will do so in all directions. This can make the rod bearing bore actually smaller, (edit nope) scraping off the oil film with the bearing insert. Rod side clearance also reduces and I have seem some old used rods that were galling against each other from tight running "clearance". Many engine builders run an extra .001" oil clearance when using aluminum rods to ensure proper oil delivery and film on the crank. If the engine is built from the start with street use in mind then I think that they will run satisfactory. In fact short track engines are sometimes built with aluminum rods. I understand that some rod manufactuers can recut serrated caps found on some rods to enable them to be resized. The labor rate is probably outta sight though. My favorite machinist resizes rods for customers that have rods with non-serrated parting lines with success. My SuperRods have been with me for quite a while, but I confess that they don't have a ton of time on them. I had them on the street a few times and noticed no ill wear pattern on the bearings, but it wasn't out for very long. An oil cooler should be on the list of must haves as far as I'm concerned if a person wants to run them on the street. This should negate the extra oil clearance that is sometimes used. I think many of the rod failures seen are the result of the wrist pin galling in the rod. Most aluminum rods aren't bushed and rods that are supposed to be "pin fit" are very often too tight. If a thin wrist pin is used to save weight the pin may flatten out during use deforming the skirt and ring land area of the piston and causing galling in the pin bore of the piston as well. If the pin sticks for any reason the rod can break in the beam due to the leverage present.
I remember Mike Moran at memphis in '94 running the 20 mile cruise for prostreet. He had a new Sonny's motor and ran the engine very lean for the cruise because he had limited fuel capacity. The engine ended up getting so hot during the cruise that the connecting rods were squeaking as he drove by. I'm not kidding. We found out later what the sound was. He had a new shortblock airfreighted in to the airport to run the next day. This was the old Casper Camaro for those of you who followed that sort of thing.

EDIT: I could have deleted this post, but that would waste all of the good information presented after it. I feel its important to share as much information as possible for the benefit of everybody in the future. When I'm wrong I have no problem admitting it. My knowledge is a compilation of life experiences and the experience of others I trust. In this instance my conclusions were dead wrong regarding the effects of heat and the housing bore of an aluminum rod. While these conclusions were based in part by information shared to me by a beloved machinist friend, I belived it to be true.

 

Thats a good point that people often overlook. People who don't understand when to utilize aluminum often only think "we'll make it out of Al, it'll be lighter" and sometimes people get in the mindset that if its aluminum it must automatically be better. BTW, when i say 'people', I'm not talking about anyone in this thread, this is a VERY general observation applying to rocker arms, bicycles, ect.

All this babble leads to the question, would an AL rod require more block clearancing making them impractical for large stroker applications? I'd assume most of the motors built using Al rodss are based off of aftermarket blocks so clearancing is less of a concern. I guess what i'm asking really is just how much thicker are typical Al rods?

-brent

 

Yep it's always a issue.... like we talked about on the phone today... There are gonna be problems with the cam and the block with a Al rod in a 3.75" stroke SBC.


Good story!

Bret

 

I think the most interesting thing to worry about is the thermal expansion and what that does to a bolt over time. As the rod heats up it has to be stretching the bolts over and over. I was nervous the first time I torqued the rods to 70 # with oil. The threads aren't a problem.

 

Ive been a machinist for 20+ years and I have never seen a metal bore get smaller with heat, they get bigger because there is no room for the expanding molecules to go.

Dean

 

Bingo. The hole won't get smaller with heat.

 

I'll be honest, I don't have a background based arguement to the statement that the hole won't get smaller, but get larger, and I have installed crank sprokets after heating them up in a pan of oil before, but. The aluminum will expand in every direction as it heats up. That makes the material bigger in general, everywhere. (making the bore smaller at some point in time) Now the bore may end up bigger after its sorted itself out, but the material is still growing, unless there is something cosmic or obvious going on that I need drilled into my head, and high oil temp kills bearings on aluminum rods and its not due to high oil flow from expanding clearance.

 

WRT aluminum rod weight.... they're all over the map. The standard weight aluminum rods are in the same ballpark as the lightest forgings. The "lightweights" are usually ~60 grams lighter and the really beefy rods are almost as much as some H beams. Like I said, all over the map.

So you could realize a nice weight savings if your goal is to run a lightweight piston/pin assembly NA.

And an aluminum rod's big/small end will expand with heat. If you were to measure the inside dia, the outside dia and the width between them at room temperature, then compare those readings to a 200ºF reading, you'd see an increase in all dimensions. Apply a linear thermal expansion equation to all three and you'll see what I'm talking about.

-Mindgame

 

It's not immediately intuitive but it can be shown mathematically (No, I don't want to do it here...). Think about it this way: If you visualize a block of your favorite metal with a circle of known diameter scribed on the surface and you heat the block up, the circle will measure larger hot than cold. Now, if you cut out the material inside that scribed circle and repeat the process, the circle (hole) will exhibit the same behavior. In short, it doesn't matter whether it's a hole or just a circle, the behavior of the material can't change.

Now think about a cylinder of the same material with the same diameter as the hole. It too will get larger with application of heat. Place the plug inside the hole so it just fits. If the hole were to get smaller with heat application, the plug, getting larger, would create an interference fit with the hole. That doesn't happen. Again, it's not intuitive, but the hole in metals gets larger with heat. That includes cylinder bores too. It's predictable and measureable.

 

I just found this thread. It's very interesting.

Rich's original question is a good one.

Nuts, the female part of a bolt/nut connection (ironic, huh?) are much stronger than the bolt (male part). If you want to test this, you can usually fail a grade 5 bolt using a grade 2 (hardware) nut. You might even be able to fail a steel bolt with a plastic nut. I think we did this in school (LOTS of years ago).

Did you ever break a wheel stud by overtightening the lug nuts? The body of the stud will fail in tension before you strip the threads in the nut.

Usually the internal (female) threads in the rod are long enough to mate with (I love how nature and mechanics are so similar ) all of the bolt threads, so maximum strength is available. But the mind wanders....

As for changes in size with increasing temperature, holes ALWAYS get bigger. All of the material expands so the circumference of the hole gets longer and the diameter increases by the circumference /PI. It's not necessarily intuitive to many, but that's how it works. The molecules around the edge of the hole are farther apart due to their faster motion (the basic definition of temperatere), but to fill in the hole they would have to get cozier with each other (closer together) which is the opposite of what they are doing. They especially aren't packing closer together which would increase the density. As they separate due to increasing temp, the circumference and therefore the diameter has to grow.

This can be easily demonstrated by heating a ring of metal and measuring it's ID and OD before and after heating. A classic demonstration uses a ring and a ball each with a welded on thin rod as a handle. At room temp, or if they are at the same temp, the ball won't pass thru the ring. If you heat the ring in a Bunsen burner flame, it expands until the ball will slip thru it. Let the ring cool and the ball won't pass back thru.This is the basis for "shrink fits" where you heat the female part, slip it over the shaft and let it cool so that it now has an interference fit with the shaft. This is a good way to assemble delicate parts tht might deform if you tried to press them together. Of course a good shrink fit really can't be disassembled because if you heat both male and female parts they grow. If the coefficients of expansion were different enough you could disassemble them in some cases.

Assume aluminum has a coefficient of expansion of 10 millionths of an inch per inch of length per degree F change. A 2 inch hole with a circumference of 6.283 inches (PI x dia). Increasing temp 150 F (say 70 F to 220 F) would cause an increase in circumference of 10 x 10^-6 x 6.283 x 150 or about .0094 inches. Dividing by PI gives a diameter increase of about .003 inches. Of course if we just used the 2 inch dia. we get the same answer.

.

Believe it....or don't.

My $.02

Edit: nateh beat me to it! Great minds think alike...

 

Boy that hurt.....

You guys all piled in here at the same time.

OldSS.... thanks for the example of linear thermal expansion at work, coefficients and all.

-Mindgame