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| The small end of the rod is sent out to the machine shop to get milled down. We take a bit extra off for clearance between the piston and rod. The rods and crank we received were from preproduction units. The big end of the rod journal was on the tight side, while the crank OD was on the big side. When we went to measure out for the bearings, we found that the clearance was to the point that there was no bearing to select from. This was an easy fix, however, since we were at the end of the spectrum where the rod ID could be opened up to a bearing spec of our choice. We also found that the small end of the rod was too thick for our box-style piston. Since the wrist pin was shrunken down, the actual thickness of the rod wouldn't fit. The thickness of the rod measured out to 24.79mm while the gap from pin journal to pin journal was 20.3mm. Since our piston is already so narrow, we decided that it would be wiser to have Ultra Performance Machine mill down the small end than cut out anymore material from the piston. We had them cut the rod down to 19.3mm giving us a 0.5mm clearance on each side of the rod.
| Here you can see on the white marks where the block interferes with the crank. This can be easily grinded down to allow the proper amount of clearance. Another problem we ran into was the crank itself. Since the factory crank is only half counterweighted, the factory casts the block with this in mind. Unfortunately for us, our new fully countered crank hits against small sections of the block and griddle.
| Here the girdle is also clearanced for the now fully countered crankshaft. The bright side is that the area interfering with the crank has no real critical usage and can easily be grinded. Since we were already grinding away at the bottom of the block, we decided to clean up any casting imperfections that were leftover. As we discussed in the previous article, this will allow oil to flow freely back into the pan.
| Our digital scale reads down to the 1/10th of a gram. To ensure that the scale is calibrated correctly, we use a certified stainless steel precision weight. Once the parts were back, we were finally able to balance the pistons and rods. Here we are looking out for vibration caused by rotational imbalance. Since the engine is moving in a rotational motion, it experiences vibration much like a wheel. This vibration especially at 7,000 to 8,000 rpm can contribute to premature ring, piston skirt, and bearing wear. When revving up an engine with a base rpm of 6,000 to 8,000 rpm, the amount of centripetal force on the crank increases exponentially. This is a direct correlation with the amount of vibration experienced by the engine.
| When taking measurements of the block, the torque plate should be installed. From where we left off, we just received the block back from Ultra Performance Machine. We wanted to double-check that the final bore matched our specifications. To do this, we have to bolt the torque plate back on to simulate the distortion seen from a cylinder head. From here, we went on to work on the pistons and rods. This is where some of our problems started.
| When removing material from the crown of the piston, take care not to hit the skirt or the pin journal. Make sure to deburr and clean before you take your next measurement. We first start by measuring the base weight of each component and logging them down. Our goal is to get our mean weight within 0.5 grams. Remember, these are precision parts, use precision tools to do your measuring. We don't recommend a mail scale. Try to find a digital scale that will measure to, at the least, 1/10th of a gram. Starting with the pistons, we determined the lightest piston of our group and used that as the target. To take weight off, we choose the thickest areas that would least affect structural integrity. This is usually in the crown on the bottom side of the piston. NPD's box piston design limits how much can be taken out directly from the bottom of the piston, so care is taken when cutting away material.
| The big and small end of the rod is measured separately on the fixture. Each side is balanced independently but the overall weight of the rod is kept in mind. When balancing the rods, we use a balancing fixture to measure both the big and small end. Each side is measured and a base weight is determined much like the pistons; except that both sides have to be balanced. When removing weight from the rod, we cut away at the metal around the big and small ends of the rod. Weight can also be shaved off at the beam itself but be careful not to grind too close to the oil feed holes. Be sure to clean them before every measurement to get an accurate figure.
| NPD suggests measuring the ring at the position it sits at TDC. Ring fitment is done with the torque plate installed as well. Once everything is balanced, we also measured out the rings for the proper end gap. This is also done with the torque plate installed on the block. The generally accepted way to do this is by pushing the rings halfway down to ensure that they're as straight as possible, but NPD suggests measuring the rings at where they'll sit at TDC. This is where the most pressure is forced against the rings and where ring seal is the most critical. The ring end gap is opened up to the manufacturer's specifications. NPD pistons are made of 4032 aluminum instead of 2618, consisting of higher silicon content. Since the expansion rate is lower on a 4032 piston, we can close up the ring gap. This will decrease the overall amount of blow by we get and burn much less oil; commonly experienced with forged pistons.