Page 17: Rod & Piston Work
October 2005

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Progress on the pistons and rods has been sporadic. After initial cleaning up I thought I detected a small crack near one of the bolt holes. I stopped work until I could have them all crack checked. I found a place that Magnafluxed them for $2 each. No cracks were found. I then sanded them all down to 287.5 grams (the weight of the lightest one after sanding). However, in the process I discovered a crack in one.

Sanding exposed this crack after Magnaflux determined the rod was good. Methods like Magnaflux only find flaws on the surface. This had to be replaced.

Fortunately GT Car Parts provided a used replacement in short order. All my rods were inscribed with "494." This was not the exact weight in grams of any of the rods assemblies (rods, caps, bolts) but it was close, so I suspect it was a grouping where all of the rods were "about" 494 grams. The replacement was marked 500, and so it took a bit more sanding to get it to match all the others.

Here they are, all sanded to 120 grain and weighing 287.5g. The next step will be to sand them to 400, shot peen them, and polish them, and open the oil hole (barely visible on the top of the rods) slightly with a 45 degree cut.

A few kind souls have written to point out the virtues of end-for-end balancing. I tried to get repeatable measurements by using various techniques to suspend one end of the rod: Threads, hooks; nothing worked. I finally wound up building my own jig from some scrap metal and a couple of bearings.

The bearings worked beautifully. I was able to find the exact size to fit each end of the rod. This enabled me to get fairly repeatable measurements. The other problem that I have not (yet) completely overcome is the effect of drag on the balance scale. When the free end of the rod moves up and down on the scale, it is actually moving in an arc and wants to drag horizontally over the surface of the scale. This friction has prevented repeatable measures to the accuracy I want (0.1g). Adjusting the hight of the jig helps by minimizing the arc. I may solve this yet, but the ends are close.

On to the Pistons

The first step was to clean them up and weigh them. I taped the sides so not to touch them, reduced the bead blast pressure to 40 psi, and gave them a careful cleaning with glass beads. The weights were surprisingly diverse. The spread is from 326.4g to 342.3g. (I found that the counter-weight I was using was 200g, not 300g, so the labels are all off by 100g too much.) This is a 15.9g spread, which seemed like a lot.

The heaviest piston (left) was different from all the rest. Notice that it has a flat surface machined into the bottom while all the other pistons were rounded (right). I suspect this piston is from a different lot.
Graphing the weights it's easy to see that the heaviest piston is over two standard deviations from the mean. Also, the next heaviest piston looks far from the rest too. If those two are removed from the sample, the remaining 10 are much better statistically behaved. The standard deviation goes from 4.6g to 2g.
I expected the engine to be MUCH better balanced than this. I thought the parts would weigh within a gram or so. Grinding 16g off a piston with a tiny sandpaper drum on a Dremel-like tool is taking a long time. I have spent hours and only removed 6g so far. I'm going to see if there is a better way to remove metal to balance the weights. I will get them all within 0.1g, like the rods. Then I'll balance the rod caps. Finally I'll weigh the pins, bolts, and nuts to make assemblies that are within 0.1g. It should be a very smooth V-12 when it's done.

SV Leaves Bobileff Motors

Remember that horrible SV back on page 14 in May 2004? Here's how it looked:

Well, here's how it looks now. This photo was taken the day it was shipped off to its fortunate owner. (This somehow makes me feel worse.) See Miura 5096 on the Lamborghini Registry site for more photos of this beautiful car.

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