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Connecting Rod Balancing

How I did mine

By Patrick Panzera, EAA 555743, Editor@ContactMagazine.com

 

I recently received a letter from an Experimenter reader seeking advice on “blueprinting” his VW engine conversion. Put in the simplest of terms, blueprinting is the same as “zero timing” an engine, paying close attention to the specific tolerances prescribed by the manufacturer. Where ever possible, any dimension or weight that is specified as a range, the builder will optimize this range.

In addition to dimensional “perfection,” balancing is paramount. Each rotating or reciprocating part should be balanced both as an individual part and as a system when applicable. For example, the pistons should all weigh the same, with rings installed (after being gapped) and with pins and their retaining system. The connecting rods should all weigh the same, end-for-end, with bearings and bolts (with nuts) installed, and the crankshaft should be dynamically balanced with the flywheel and any other attachments (such as a prop hub in our case, or a harmonic balancer, if applicable).

Some engine manufacturers will do their best to have pistons and rods assemblies be “close” in weight to its opposing mate, with little concern for the adjacent pairs. As long as each pair (in an opposed engine) is close, they are good with that. When blueprinting, all the assemblies are the same, or within very close tolerances.

As I reported in my introduction in the first issue of the Experimenter eNewsletter, I’m a Corvair engine junkie. I found a rusted, greasy hulk of a core engine and in the process of the “zero time” overhaul, I balanced my connecting rods. My cylinder bores, piston clearance and weight, ring end-gap, rod and crank bearing clearances were about as perfect as they could be, just the way they came from the supplier or rebuilder, so after careful measuring, there was nothing more for me to do. The only thing that I didn’t have done, but still may have done, is to have the crank rebalanced as an assembly, although each individual component (except the prop hub) was previously balanced separately.

Balancing my forged pistons was pretty straight forward. All I had to do was install the rings, the wrist pin, and place the pistons on my digital scale. Perfect. But balancing the rods, end-for-end, proved to be quite difficult. I could not manage to get the rod placed back on the scale in exactly the same spot every time. Sometimes I had as much as a 50g difference between one placement to another. Every jig I cobbled together was either not accurate enough, added friction resulting in uneven support, or added loads to the rods, which resulted in erroneous readings.
 
I posted an e-mail message to the CorvAIRCRAFT e-mail list, explaining my situation and seeking advice and I shortly had my answer.

Jig

Don McGehee sent me a private e-mail with the above drawing of a jig, which was drawn from memory, of one he saw in a hot rod engine shop.
 
 From his drawing, I made this jig.

Jig

Jig

I turned (on my lathe) two phenolic disks that slide freely but precisely over a pair of shafts I made from cutting off the heads of two one-quarter inch bolts. The phenolic disks in contact with the rod and the shaft lowers friction (eliminating side loads) and allows me to change the disks from one shaft to the other, for balancing each rod end.

jig

Although the centers of the shafts are spaced as preciously as I could manage, the chain all but eliminates any unwanted side-loads due to misalignment or being out of level. I was amazed at how little side-load it took to throw off the readings.

balancing the small end
Balancing the small end.

balancing the large end
Balancing the large end.

The base is made from three-quarter inch particleboard. The digital scale was purchased from a local office supply store. It measures in ounces or grams with a flick of a switch, grams being more accurate. I located it on the particleboard by attaching quarter-round pine strips against its peripheries, locking it in place but making it removable by simply lifting it up.

A piece of one-quarter inch thick pine was through-bolted to the platen so that it could be removed easily and the scale used for other projects. The off-the-shelf angle bracket was permanently bonded to the pine via five-minute epoxy. In retrospect, I should have used plywood instead (Baltic birch) as the pine has a propensity to cup from humidity. It certainly doesn’t affect accuracy, it just looks bad.

The upright was built from half-inch pine and has performed flawlessly. It’s held erect by way of a triangular support that’s screwed to the particleboard, in addition to a wood-to-wood bond from more five-minute epoxy.

Balancing

I’ve been able to remove and replace the connecting rod on this jig, and if it’s not exactly the same weight when I make the change, it’s only off by 1g.
 
One good thing I’ve found out from this exercise is that the stock connecting rods were very close as installed at the factory. From my heaviest to my lightest, there was only a seven gram difference, certainly “close enough” for the 3000-3400 rpm these engines will see in flight.

Using the balancer
Looking at both ends of the connecting rods, one can see the balance pads. This is the area on the rods that was forged into the part, specifically to be removed as needed to balance the rods one to another.

The image on the top is obviously the small end (piston end) of the connecting rod, and the bottom image is the crank end. The piston end has been ground at the factory. Since all the rods weighed about the same (within seven grams of one another) and only the piston end was machined, I assume that this was the factory practice; to simply weigh the rods and remove weight from only one end.

Following Richard Finch and Bob Sutcliffe’s advice, I balanced each end separately.


Here’s the completed stack.

before
Before

after
After

All in all, not much material had to be removed from the piston end of the connecting rod.

Rather than just grind more from the top of the pad as the factory did, I removed most of the mass from the corners, rounding the pad quite a bit. This was pretty straight forward. Depending how much material I had to remove, I either started with my four-inch angle grinder with an aluminum oxide disk and finished up with 120 grit sand paper in my vertical belt sander, or I just used the sander alone. Either way, I didn’t want to introduce deep scratches into the part as they might propagate cracking.

Notice how in the above photo, the rods second from the top and the very bottom don’t have any work done to the piston end balance pad? That’s because these two were the lightest, at 113 grams. The others were anywhere from 115g to 123g.

On several of the crank ends, I chose not to use just the balancing pad for weight removal. In fact, if I had, I would have not only run completely out of material to remove, but in my opinion, I would have weakened the cap. So I opted to remove excess material before starting to grind on the balancing pad. I did this to every rod, even if only a gram or two was to be removed.

balancing the small end

balancing the small end

 The top image shows a rod that has not been ground. The bottom image shows one with only a little bit of weight removed. The material removed is part of the excess that the “pros” remove when they lighten rods, so I felt it was safe to start here.

For cutting the concave shapes I used a grinding stone in my Dremel and finished up with a drum sander chucked in the moto-tool. 


This rod needed just a bit more weight removed than the rod in the previous photo…
 

...but this one needed a bit more. This is about as far as I would go before starting to grind the balancing pad on the cap.

Any and all grinding was followed up with some very fine sanding (polishing) to reduce the stress riser effect of the scratches left by the grinding wheel.



 

This one, however, has a good 4-5 grams removed. Notice that I dished out the pad, rather than just grind it off along with the ribs that make up the “C” section beam.


On the top is a stock rod bearing cap.  On the bottom is one that has been ground down by about 1.5 - 2 grams.

And this is the finished stack. The top one has just a little removed, the next two substantially more, the next two none, and the bottom one, a whole bunch.

I started out with the small ends weighing anywhere from 113g to 123 and the big end weighing 308g to 316g. Now the small ends weigh within 1g of 113g, and the big ends weigh within 1g of 305g.

All together I took 10g off each rod.

This may not be the best way to balance rods; I’m sure there are countless ways to do it. Some websites show people not bothering to weigh both ends; it makes sense that if you get all the small ends to weigh the same and then grind enough off the big ends to make all the rods weigh the same, the net result would be the same as weighing the big ends too, but I guess I’m too particular to not go through the full exercise.

Just remember, the intention of this article is not to tell you how to balance your rods as much as it’s a story on how I balanced my rods. I welcome comments, especially if you can elaborate on the specific details of “blueprinting” an engine in the form of an article that we can publish for our friend who posed the question in the first place.

 
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