1968 Plymouth Valiant Project 7.0 Small Block Engine Build

Rotating Assembly
The small-block Mopar engine was blessed with a tall deck height, which makes it well equipped to handle an extra long-stroke crankshaft. While the factory engineers didn't go larger than the 3.58-inch stroke used in the 360 engines, the aftermarket has been making 4.00-inch stroker kits for many years. The 4.00-inch stroke crankshaft provides 408 cubic inches when used in a .030-inch over 360 block, and up to 416 cubic inches in a 340 block. While the four-inch stroke setup has become a very common small-block combination, we felt we needed even more displacement than that for our project.

A little bit of research on the internet found several options, including a Callies crankshaft with a 4.250-inch stroke, a Hughes Engines crankshaft with a 4.180-inch stroke, and a K1 Technologies crankshaft with a 4.125-inch stroke. While any of these would've worked for us, the K1 crank was available with the 340 mains and smaller, early-Chevrolet-sized rod journals. We wanted to use the smaller rod journals in order to use less expensive connecting rods, as well as to tap into a wider selection of rod bearings.

Because the 4.125-inch stroke setup is a fairly new combination, we had to order custom pistons. Of course, custom pistons aren't really a difficult task for the industry these days with all of the CNC equipment and CAD files on hand. Our combination with the rods we chose was a little unique, but the folks at JE Pistons were able to provide us with a set of pistons that matched our requirements. The pistons needed a 20cc dish to keep the compression at a pump gas friendly 10.5:1, and we asked JE for their special Tuff Skirt coating. The finished pistons weighed 450 grams, while the piston pins weighed 130 grams.

The bob-weight for this rotating assembly with the small-journal rods and custom pistons was 1,780 grams. The K1 crankshaft had a natural bob-weight of 1,950 grams, so we needed to remove weight at each end of the crankshaft to balance properly. On a crankshaft like this, reducing the counterweight radius by 0.10-inch will remove about 100 grams of weight. A large lathe was used to take material off of each end counterweight, leaving a few grams for the engine shop to remove during their final balancing process.

Our block came with ARP main bolts, but we wanted to use studs rather than bolts. Unfortunately, ARP does not offer a main stud kit for these Mopar race blocks, so we had to piece our own kit together. You can duplicate what we did by ordering a standard ARP main stud kit, then buy six individual studs and six washers and nuts.

Roller Lifters
Our original concept for this engine was to use a hydraulic-roller camshaft. We felt that with the long stroke and small heads that our horsepower would peak before 6,000 rpm and that, therefore, we wouldn't need the higher rpm capability of a solid roller camshaft. Our other thought was that a hydraulic roller setup would be fairly quiet and reliable, which sounded like a good fit since the car does see its share of street miles. With those thoughts in mind, we went ahead and ordered a hydraulic-roller camshaft and matching lifters from Comp Cams. We did understand that Mopar Performance doesn't recommend the use of roller lifters with these race blocks, but we had already machined away the extra head bolt lugs in the valley, so we assumed the lifters would fit. As we found out, even with the head bolt lugs machined away, the block walls are so much thicker on the 340 resto block that the lifters just could not be installed. We tried to grind additional clearance in the valley with a 30-grit sanding roll, but the amount of material that needed to be removed really had us worried. Even the constant use of a sonic checker didn't provide us with enough reassurance, so we eventually stopped grinding clearance and started to look for an alternative.

Lucky for us, we learned that Comp Cams was in the process of introducing a brand-new solid lifter that is designed to work with the Mopar race blocks. The design change with these lifters was to move the link bar from the outside of the lifters to the inside in order to provide more clearance with the valley wall. The 59-degree lifter angle in the small-block Mopar allows the inboard link design to work because the pushrods exit the lifter at such a steep angle. The new Comp solid lifters dropped right in place in our block without any interference, and the pushrods easily cleared the in-board link bar. Comp Cams engineering told us we could run the solid roller lifters on our hydraulic roller camshaft by setting the hot lash at 0.006 inch.