Notice on NPD's piston the cutout section for the box-skirt design. This also dramatically reduces the size of the wrist pin.
With nearly no aftermarket support for this engine, Naoto offered his KA24 wisdom and advice on what steps to take for the bottom-end build, starting with a set of his very own NPD-prototype KA24DE pistons. Their pistons are 90mm, 9.5:1 compression and made out of billet aluminum. NPD utilized a box-skirt design to cut back on the weight of the piston. Because of how long the stock stroke is (96mm) and our target rpm of 8,000, the overall piston speed would be high. NPD pistons were designed to reduce the overall rotating mass of the assembly, but keep the strength of the piston. Since the skirt is cut down, the pin is also shortened to save weight. NPD kept the piston at a 90mm bore in order to have ample piston ring selection.
The stock rods were tossed and replaced with a set of off-the-shelf BC Pro Series rods. In place of the half-counter stock crank, NPD has developed a fully countered billet crankshaft. The main and head bolts will be replaced with ARP studs from AMS. AMS developed an upgraded head stud, increasing the size to 11mm that requires machine work to the block and head. Any rotating parts that touch any metal surface will be WPC treated to reduce friction and improve overall strength. With this combination of parts, the engine should hold up to the extreme amount of power we'retrying to obtain.
NPD's crank in conjunction with BC rods ensures the reliability needed for making six times what stock outputs.
Now that you have an overall idea of what the build is about, let's get to work. The first thing to do is tear down the engine to inspect, because the majority of KA blocks have seen high mileage. Since the majority of machine shops hae trouble line-honing main journals, you should take the time to measure them for wear and out of roundness. It's better to make sure you have a good block than to spend the money to get a cylinder hone only to find out you need a line hone. With blocks like the KA, it's better to scrap the engine and try another block than risk having a machinist improperly line hone the main journals.
Naoto performs the procedure by first reapplying the proper torque to the main girdle, then checking each journal one by one with a bore gauge. Make sure you work in the correct degree of measurement.
The difference in bearing sizes is in the thousandths of a millimeter. (0.001mm is equal to 0.000039 of an inch) This means that if your machinist only works in thousandths on an inch, he will never be able to get the exact clearance you want; every step he takes he's moving is too large of an increment. Remember, you're paying them to do work for you. Do not accept anything less than what youasked for.
Make sure to properly torque the main girdle, as small variances will show when working with such tight tolerances.
To get a good sample, Naoto takes a measurement at three different axis points: X, Y and Z. He then goes back and measures the crank. With both measurements in hand, he looks up the factory clearance and finds out if there is a suitable bearing that will fit. If you can't find a bearing that will fit what you have, then you can always take that chance and gamble at the machine shop or roll the dice at the junkyard with another block.
After determining that your block is good, you need to get an accurate measurement of your piston. One important tip that Naoto offers is to never leave the measurement responsibility up to the machinist. He takes accurate measurements of the pistons so that he can tell them what bore and finish he needs. We look up the piston clearance and add that into the size of the piston. Again, stress to your machinist that you want it to be accurate to the hundred thousandths of an inch or thousandths of a millimeter. Do not accept anything less.
Naoto recommends taking a good sample group and going back to recheck it to assure a good measurement. The gauges are checked for calibration after every journal to assure accuracy.
NPD also machined an aluminum torque plate made for the KA24. The torque plate simulates the stress of the cylinder head keeping the block assembly nice and tight while it is being machined. Without it, the center cylinders tend to come out more out of round thanthe outers.
Now that we know we have a good block and the exact bore size we need, we can send the block out to the machine shop. With the block, we sent the head studs as the block needs to be resized to run an 11mm stud, a headgasket and the torque plate for machining. We'll also be sending out many of the parts to WPC while the block is at the machine shop.
Stay tuned. In part two, we'll discuss the valvetrain and porting of the cylinder head.
Most new cranks are pretty exact since they're freshly machined from the factory. Never the less, checking for irregularities will save you headaches and add to a better performing motor in the end. | 
NPD uses Mitutoyo gauges to get a measurement of the pistons down to 0.0001mm. | 
The torque plate keeps the assembly true as it is being machined. This is especially critical with open-deck motors, since they tend to slightly give in the machining process. |