My K20 rebuild thread
I know there's not a whole lot of k20 action in the Atlanta area, especially naturally aspirated, but I assure you, I own the dubious distinction of having blown up more k20 motors than anyone else in the state. Along with that comes rebuilding experience, and I'm going to share a bit of my latest rebuild in this thread.
After 20,000+ miles, my 230ish hp 9000+rpm k20 bit the dust. More specifically, the cylinder #3 piston bit the dust.
What cylinder #3 looked like
What the rest looked like. You may be thinking to yourself "my, what a clean burning engine he has!" Why thank you, I like to think I know a thing or two about tuning. :p
Rods & pistons laid out
In case you wondered why they're called "H" beam rods.
Long story short, these were CP 12.5: pistons with a thermal barrier coating on the top and anti friction coating on the skirt. I was planning to run a healthy shot of nitrous, but I never got around to it. The pistons were installed with a piston to wall of 0.0035" or 0.0036" which is okay for an n/a motor. However, what happened in the 20+K miles that she lived is that the piston to wall clearances never tightened up to what they should have been when the engine was at operating temperature. As a result, the pistons were loose in the bore and the bottom edge of the skirts all showed irregular wear. Cylinder #3 was just the first to go. The skirt fell apart and then the piston turned sideways in it's bore. :p
So based on that experience, I decided that this time around I'd go for pistons made of a lower expansion alloy. The only off the shelf pistons for a k-series made of 4032 are the supertech.
Here's some comparison pics of the CP pistons vs. the supertech pistons.
The CP has a much more nicely sculpted dome with a wider peak. However, the Supertech has visibly deeper valve reliefs and yields the same 12.5:1 compression. Just from the way the top of the CP piston is machined versus the Supertech piston, it's not difficult to see why there is a significant price difference between the two brands.
The Supertech has a wider skirt that is also a little longer. The crown of the piston is also a little thicker. The supertech piston also has less taper from the crown to the bottom of the skirt. This makes sense as it is constructed from an alloy that expands less as it heats up.
On the underside of the piston, it's again clear why CP costs more. It's got much nicer machining on the underside of the piston. Dual wrist pin oiling holes are present on the CP piston. The supertech piston does have a thicker skirt. The wrist pin is also shorter on the supertech piston. The CP piston will accommodate a much wider rod, although that doesn't matter much if you are using stock width crank journals.
I got the block back on Friday and measured it out today.
All cleaned up.
The plateau hone looks really good.
My inexpensive bore gauge that gets the job done. I have to do some interpolation to read between 0.0005" increments. Also, the reading shown here is 0.00225" over since the gauge zeroed out on my piston measurement halfway between zero and one mark to the right.
So, the other thing I wanted to do tonight was get a better idea of if the piston to wall clearance I specified to the machine shop was suitable or not. Last night I made up a worksheet in MathCAD to calculate the expansion of the pistons based on coefficients of thermal expansion I looked up for 4032 and 2618 alloys. Those numbers indicated I'd end up with a hot clearance of 0.0011", which seemed reasonable. However, I grew up watching mr. wizzard and bill nye the science guy, so I had to take things a step further and determine the CTEs experimentally.
MMMMMMM, pistons, they're what's for dinner. Note that I marked the measuring points with a marker to ensure measuring precision. Supertech pistons on left, one CP piston and one OEM piston on the right.
They're cooking now!
After crunching the numbers, results were actually reasonably close to what the values I looked up were. Well, for the 4032 and 2618 alloys. Obviously there was no table with specific information on CTEs for the OEM piston material as CTEs for cast material vary widely depending on the specific way in which the piston was cast. But I'm getting sidetracked.
- CP CTE: 25.7 versus 24
- Supertech CTE: 23.2 versus 21
- Stock piston CTE: 16.9
As you can see, the 2618 alloy does in fact have a higher CTE than the 4032 alloy. As expected, the CTE of the stock piston was much lower than with either of the forged pistons. The mathematical model says that the OEM piston to wall in a hot motor would be 0.0002" to 0.0010". With the piston to wall I specified on this motor, my hot clearance should be right in the middle of the OEM range, 0.0006". In short, I chose wisely. Piston to wall is tight enough to help maximize longevity, but not so tight that I'd have to worry about seizing the motor if I run her hard on a hot day.
For kicks, here's a screenshot of my MathCAD worksheet.
I also measured the expansion at the piston crown. The aftermarket pistons generally expanded a good bit more along their thrust faces at the crown than along the wrist pin axis. The CP pistons were within 0.001" along each axis. the Supertech pistons had an average difference 0.006" between the two axes, with the longer axis being the thrust face of the piston. The stock pistons were slightly oval as well having a difference of 0.002" between the two axes, with the longer axis being the thrust face of the piston.
What I find interesting is that although the CP pistons were more round at the crown than the stock pistons, the stock pistons had much more even skirt wear. This highlights two factors that set apart truly great pistons from the rest.
- having a truly round shape at temperature
- having a skirt profile that matches the bore at temperature
Now, I'm not sure how the Supertech pistons will stack up, but it would seem that the CP pistons left a little to be desired in the piston skirt profile. Makes you wonder why some companies charge $1600+ for a set of four pistons and what kind of intrinsic value is engineered into the product.
That's it for now. There's a lot of work left to do before I get this motor together. Hopefully, she'll be stronger and longer lasting than the last.
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Originally Posted by Dr.G35
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I am in for numbers! Glad you got everything together!
