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While we're looking at fancy new pistons, and since I've been hanging out with piston nerds a bit, let's look at some cool details you've probably never noticed before. Like this groove in the side of the wrist pin bore. Under extreme loads, the wrist pin is getting pushed on so hard it actually kinda flattens out on top, going slightly D-shaped. The deformation is very slight, but the pin bore clearances are also really tight, so this relief cut is made on each side of the bore so the fat parts of that D have a place to go without actually binding in the bore.
Here's another cool thing. Pistons aren't round, and their skirts aren't straight, because pistons, like everything else in the world if you pay close enough attention, aren't rigid. The skirts are actually machined into a barrel shape that's carefully engineered to initially contact the cylinder bore in just a tiny spot. This initial small contact area reduces friction, but more importantly, as side load increases, the piston skirt deforms, bringing more and more of the skirt in contact with the wall. This gradual, cushioned contact reduces piston slap noises and reduces skirt and cylinder wall wear.
Similarly, when viewed from the top, the skirts are machined in an oval shape. This promotes the same gradual contact.
Not only are these amazingly subtle and precise shapes machined into every piston, but JE is able to customize these shapes for each piston order. Their custom piston machining process is so streamlined that if an engine builder is having problems with scuffing or piston slap, JE can re-profile the skirts with a few clicks of a mouse.
Another fine tuning tool is the shape and position of the skirt supports. The entire support structure of that piston skirt can be fine tuned to ensure they're stiff enough to protect the piston, but not stiff enough to damage the cylinder wall.
And, while we're talking about nerds, and things you thought were rigid but really aren't, let's talk about machining the block. Over the course of countless SR20 builds, Clark Steppler has realized that the cylinder bores get distorted by a small but meaningful amount by not only the head bolts, but also the bellhousing bolts. If you think about it, this is obvious. Head bolts are torqued hard enough to stretch significantly. If they're pulling hard enough to stretch the bolt, they're also pulling hard enough to pull the aluminum block slightly out of shape. An open-deck block won't have this issue, since top of the bores aren't attached the part of the block being distorted by the head bolts, but some distortion is inevitable on a closed-deck block like the SR20.
So any time 5523 or Jim Wolf have a block machined they first bolt on this deck plate and cut-down bellhousing and torque all the bolts to their final assembly specs to pre-distort the block just as it will be when it's finally assembled. This way, when the engine is assembled, the bores will get distorted back to their as-machined condition.
The normal process when building an engine is to get the pistons first, then give the pistons and a target piston-to-wall clearance spec to the machine shop when they bore and hone the block. The machinist can then measure your pistons exactly, and hone the bores to exactly the clearance needed for your actual pistons. This eliminates any tolerance stack-up as tiny variations between the piston specs and their as-delivered sizes, and tiny variations in final bore diameter potentially take you outside your target piston to wall clearance
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| This setting fixture is used to calibrate the bore gauge to a target bore dimension before every use by measuring the distance to that hardened steel bar on the right. Some nerds worry that a few years of wear on that calibration bar could lead to small errors in final bore size. |
That's a nice theory anyway, but Chuck the piston nerd doesn't trust the measurements most machine shops make. His distrust is not of the machinists themselves, but their generally sub-aerospace control of the test conditions. In JE's test shop, measurements are taken in a temperature controlled room with equipment that is calibrated every few months. When measuring to the ten-thousandth of an inch, the difference in a morning and a mid-day measurement of an aluminum part can actually make a difference, as can a few years of wear and tear on the micrometer used to measure the pistons and the potentially different wear on the bore gauge used on the block.
So, at Chuck's insistence, we took the opposite approach, asking the machine shop to bore the block to 87mm exactly and JE would build pistons to match.
This gave us a good opportunity to double check that machine shop quality control by measuring the block in JE's clean room. After measuring each bore in 6 places (parallel with and perpendicular to the crank at the top, middle and bottom of the bore), every measurement was essentially within the measuring accuracy of our equipment (a window of about 0.0003″). In other words, dead on. So we can trust our machine shop.
That machine shop, by the way, is GRC PERFORMANCE and get this: they don't have a website. Consider this a good sign, as the kind of guys who do this kind of work are usually too old school to be playing around with computers. There is a GRC Performance on the interwebs, with lots of pictures of Mustangs and trucks and stuff. They might be good too, no idea, but the guys who got our bores right just have a phone and a mailbox, which you can get to thusly: (619) 443-3300, 8455 WINTER GARDENS BLVD, LAKESIDE, CA 92040
Ok, back to work…
