Last month, we tore down and inspected the tired 340 engine extracted from an original '70 Dart Swinger 340. Some of what we found was normal wear, though the engine had obviously seen some amateur wrench work in the past. It was clear from the cheap .030-inch over cast-replacement pistons and .010/.010-inch under crank that this engine had seen a complete rebuild before. This time around our goals were simple: build a reliable powerplant as close to factory-original specs as practicality would allow. We intended to build the engine to last, while keeping cost to a minimum.

Minimalistic Machining
We wanted to keep a budget approach, and stuck to the philosophy of, "If it ain't broke, don't fix it." Usually, shops take a comprehensive approach to machining in a complete rebuild, but we held the line on machining operations, saving money which can be applied to higher-quality parts elsewhere in the buildup. Why pay for decking the block if the decks are already flat? Ours checked out okay, so we left them alone. Same goes for line-honing the mains; often done as a matter of course, it is frequently unnecessary. Ours remained untouched. In fact, the only machine-shop operation done to the block was to hone the already .030-inch oversize bores to the next oversize, .040 inch. Unlike the other steps we passed on, good bores are vital to making the engine perform as new.

Our crank was already reground .010/.010 inch on the rods and mains, dating to the last rebuild over 20 years ago. The crank journals showed some mild wear, but mic'd right at the middle of specs, so we just had the journals polished rather go for a regrind. This saves dough and saves the next-undersize regrind for the future. The rods, however, needed the full treatment. The most likely part of the engine to blow is the rods, especially with tired old bolts. The rod bolts would get replaced, which meant that the big ends would need to be resized. Inspection revealed that the small ends, factory bushed for full-floating piston pins on this 340, were worn out. Though replacing the piston-pin bushings will more than double the cost of reconditioning the rods, ours really needed them.

Of course, the heads would need machining. Our heads were rough, with extensive valve-seat recession. The only practical way to fix it was to have 16 inserts installed. Some of the seats already had inserts, and they were gone too, making the job even more complicated. We took the easy way out and replaced the heads with another set. I've had a set of '70 340 "X" castings collecting dust on the shelf since 1986, so it was time to put them to use. These were treated to a performance valve job by JMS Machine, a new set of guides installed, a .006-inch clean-up surfacing cut made, and hardened inserts were added on the exhaust side for use with unleaded fuel. Basically, the heads got the works; there was nowhere to cut corners here without hurting performance, so we did what was required.

So that's it for machine-shop operations; just a hot tanking, fresh machined bores, resized and rebushed rods, a polished crank, and rebuilt heads. Some engines may need more while others can get away with even less, but we did the minimum needed to get factory-fresh performance from this 340.