Matching rod weights

[bbh03]

Anyone have a picture of the setup for weighing the rod ends separately? Is it as simple as supporting the rod horizontally with one end onthe balance and one on a solid support?

[paulweir]

The only way I have ever seen it done proplerly is on the balancing scale made just for the purpose in a well equiped automotive machine shop (like ours) This is expensive professional grade equipment and the only way to do this job.

[speedkillz]

From what I remember from my younger days... 26 or 27 grams to an ounce. so 1/2 gram is GOOD.

[bbh03]

I have very accurate balances available to me. I would hope I caN replicate the setup. The only two ways I can think of are the one already mentioned (supporting the end not being weighed such that the axis through both ends is horizontal) or having the end not being weighed pivoting on its bearing surface and the end being weighed on the scale pad.

[snowman]

THe rod is horizontal. The one end on a scale, the other on an ideal pivot, ie no friction so that only the end on the scale is weighed. Anyone with a good scale and any decent ball bearing that will fit into the other end is equiped to do the job. A level for the horizontally challenged could make a difference, but only minor.

Yeh and 1/2 OUNCE is good, 1/2 gram is, well, really really good.

[snowman]

The following photo shows how the big end of a rod is weighed on a commercial rod balancing setup.

First the big end is weighed, the lightest rod becomes the "standard" that all others must match.

All the big ends are machined to weigh the same, by removing weight from the big end. Usually by grinding from a pad specifically provided for this purpose or by even removal from the bottom of the rod.

Finally the small end is machined to match the overall weight of the rod to the lightest or standard rod. The reason the small end is not weighed like the big end is that it isn't necessary. Why? Because if all the big ends are the same and all the small ends are the same, the overall weight must also be the same. THerefore if the overall weight is the same, the small ends must also be the same. or in other words you can do it either way, but the overall is easiest.

For the math types:
Total rod weight = big end + small end





note that ball bearings are used to support both ends of the rod. The rod is perfactly horizontal and the supports are adjustable so that no side loading is possible. Any home made fixture could easily do the same thing. Even knife edges could be used instead of ball bearings. The big round adapter also seems to supurfulous, ie any bearing should work, just so no side load is present. No magic at all just a simple scale. like frys sells for $10 would do as good as this big buck contraption.

[bbh03]

Thanks for that picture. I can replicate that close enough.

[snowman]

An important PS

The small end is hung from a short vertical length that has a bearing on both ends. TO tell if there is a horizontal load one just lifts up the small end and sees if the vertical support wiggles or moves. If it does, then the horizontal support distance is adjusted until the small end can be lifted without any movement of the small end vertical support.

[sammyg2]

In my shop we balance rotating assemblies to a tolerance of 4W/N where W = the weight and N= the speed. That is the API tolerance that was developed from submarine technology where vibration = noise = boom. That is about twice as tight as the ANSI recommended tolerance. Balancing to a tighter tolerance than that doesn't make and difference, it just wastes time and money and gives the uninformed a warm and fuzzy.

One of my customers asked me if I would balance his stuff to 2W/N. He figured that tighter is better. It aint. I told him to just send me a check for an extra $1000 and I would pretend to do what he asked. My balancing machine (Schenk-Trebel CAB 802, worth about $80K with the stand) simply isn't accurrate at the tolerance requested so even if it said we hit the tolerance we would be fooling ourselves and the customer. I have about $10k worth of calibration equipment just ot make sure it is as accurate as possible.

Tolerances are there for a reason. People who suggest a tighter tolerance than engineered is better usually don't understand the process or theory.
I don't personally perform balancing anymore but when I did run the machines I was pretty good at it. Even had a bunch of them square things hung on the wall with my name on em
Even if the measuring equipment and procedures can repeatedly and consistantly provide an accurate measurement, it is prolly a waste of time and effort. After a point of diminishing returns the effort makes no measureable change in the vibration amplitude or severity.
Did I mention that I also have some square things on the wall for vibration analysis from CSI?

[afterburn 549]

Thanks for the input and insight....if you have the time, could you go into your balance theory a little bit more so some of us not so enlightened can get a grip on w= and n= (speed) I am sure I did not digest that all at once and need an other serving just to make sure I liked it.
A Big Thanks!!

[Turbo_pro]

Originally posted by Henry Schmidt
Why use a grinder? Just find a beaver to chew on them until they weigh the same, sand the old rod bearings, then take a rock and a chisel and reinstall the rod nuts.

At some point, you have to use some of the principles of proper engine assembly.
I repeat "A 20 gram difference is more than most balancers would care to work with."

This series of posts is one more reason why choosing Supertec was the right choice for me.

Quality repair and a sense of humor
You have to admit, that is funny.

[scottc]

In all this talk of grinding I have not read any mention of how Porsche themselves balances the rods when new. There are no telltale grinding marks that I have ever observed (unlike many other manufacturers) and contrary to Snowman's assertion 911 rods don't have the customary balancing boss on the bottom of the big end that would otherwise offer and easy place to grind excess weight from.

My understanding for the last 30 years has been that Porsche balances their rods on a lathe by turning a small annular step on the two outer surfaces of the big end. This is the surface that will be adjacent to the crank web when the rod is installed. All the 911 rods I have seen have this small step and in the absence of grinding evidence I took this as gospel.

Does anyone know if this is wrong? Or right?

It seems sufficiently over the top to be a Porsche only affectation. They go all the balancers I have heard of one BIG step better. Not only concerned with end to end balance they are concerned with rotating balance. Again an over the top solution that is one of the reasons we are so enthralled with these cars.

Scott Johnston

[snowman]

How a rod is balanced is not important. As long as the big ends all weigh the same and all the small ends weigh the same, all is well. Usually all the big ends are matched, then the total rod weight is made the same, using only the small end, the overall rod is then balanced. The rods all weigh the same, the big ends all weigh the same and finally all the small ends weigh the same. ITS BALANCED!! It dosen't matter how the weight is removed, or added. Lathe, mill, drill, grinder, file, whatever. Just so you do not take metal off a critical area. The bottom end is generally not critical, the sides of the rods are generally not critical, the top of he rod is generally not critical.

[scottc]

Jack;

Thanks for the reply to my inquiry about how Porsche balanced their rods. What you say makes sense about a balanced rod being a balanced rod and that would be my gut response too. However perhaps we are missing something.

In my experience good engineers don't do things for no good reason.

While many engine designers do leave bosses on both big and small ends of the rods that are clearly for material removal and balancing, Porsche does not. Could we infer from this that Porsche engine designers have used another engineering solution to meet this need? If so why (other than Teutonic oneupmanship)?

Could it be that Porsche felt that, from a structural standpoint, those big end sides were the only place to remove the matrerial? To me that seems the more likely reason.


Scott

[snowman]

Porsche does leave an area on the big end that can be used to balance it. Its not a balance pad per se, but you can grind on the big round piece of metal on the bottom, without harming the rod. Of course you would do this as far around the entire bottom as possible, so as not to make a big NICK in the rod. However Porsche does it is not important, the rods are very close to begin with, so only minor ammounts need be taken from any part of the rod.

In my opinion all rods should be shot peened after all other work has been done on them. This critical step prevents cracks from developing or propagating. Polishing just dosen't cut it. Metal improvement company is the ONLY company that I know of that can competently shot peen rods. Carrillo and the factories may have solid shot peening facilities, but they are not generally available to the public.

If you have anything shot peened and the company does not give you a bunch of slightly bent metal strips back with your job, they were not competently shot peened and may not be as good as a non peened product. These thin metal strips are the only control the company has on their process, if they do not use them, do not use that company.

[Henry Schmidt]

Porsche doesn't really balance their rods. They actually just assemble weight groups. They manufacture the rods and weight them to determine weight variances. Then they put them into weight groups.
They(Porsche) believe that 4 grams is close enough. We have seen many rods in first time tear down that were as much as 9 grams out from end to end. (Same overall weight).
I know it sound strange but it is true.
We would never run an engine with rods more than 1 gram out and generally get closer (equipment limits) than that. I know it's not necessary but setting up the tools is almost as much work as balancing the rods. We feel that a job worth doing is worth doing right.

What is right? I guess we all have to figure that for ourselves.

[Superman]

I had my rod inspected by a woman recently who reported that it seemed heavy. I let her go ahead and work the rod for a bit, and it seemed like she was trying to take some material off. Eventually, the rod did get lighter. She didn't charge me anything, but she did use a special tool.

[afterburn 549]

woman are the best inspectors...they know what and where to look for...

[Walt Fricke]

Snowman says:

"Porsche does leave an area on the big end that can be used to balance it. Its not a balance pad per se, but you can grind on the big round piece of metal on the bottom, without harming the rod. Of course you would do this as far around the entire bottom as possible, so as not to make a big NICK in the rod. "

I don't think you would want to take any significant weight off of this area. That web around the rod cap sure looks structural to me, acting like a weight efficient gusset or flange or web to keep the cap round. Chevy rods and the like have a honking big squarish block on the cap tailor made for grinding off some weight as might be needed. But not the 911 rods I am familiar with. I once had some 911 rods balanced by a shop that does US stuff. They ground a whole lot off some of my rod caps, down to half height on some. The concensus was they ruined the rods. I got another set. It only takes having a rod cap get loose once to make one rather cautious about this part of the engine.

And the rod cap needs all the stiffness it can get, seems to me. I had a top end disaster (was it the piston that pulled apart at the wrist pin, or the valve whose head pulled off? Can't remember), and the rod cap bent so there was plenty of air between the inner mating faces (luckily the good aftermarket bolts, though both bent, held and I didn't lose the case and all the other stuff).

The balancer I usually use is one of those perfectionist crusty old machinist types: we do it my way or you can get someone else to do it. He might polish the cap web a little, but mostly he takes a little off the square corners on top and bottom of the sides by the rod bolts. Presumeably most of the stress is in the cylindrical area around the bolt axis, so these areas are structurally superfluous. A previous post had a picture showing a rod balanced this way.

And this lack of good meat to grind pretty much limits how much you can remove, buttressing the common sensical notion that you should start out with a pretty closely matched set before doing any balancing anyway.

Walt Fricke

[snowman]

The area on the bottom of the rod is not critical, only around the rod bolts is. Sounds like your crusty machinest is taking it off the wrong area. And as Porsche rods are generally balanced pretty darn close to begin with you only have to take off a very small ammount. Of course you must have the rods re shot peened after all machining has been done. All rods should be shot peened. as an aftermarket supplier I would only trust a company called Metal Improvement Company to do the shot peening. Is not that expensive. Metal Improvement Company is a division of Curtis Wright, you know the company that invented airplanes and airplane SI engines.

Rods are really tough. I have rods that have been twisted, by sticking a bar in the small end and holding the big end in a vice. How many twists? 4 complete twists and the rod looks like a pretzil, but it didn't break. Some rod bolts can stretch to several times their original length, without fracturing. But usually with cycling, like an engine does when the bolt is over stressed, will cause a break. If your lucky it will just hammer the rod and crank to junk. The bolt is always the weakest link in a rod so grinding a small ammount from the rod is no problem, as long as it isn't in a critical area like the rod bolt radius.