The only machining operation...
The only machining operation to the block was having Jerry Goodale at Specialized Motor Service hone it to the next oversize, +.040-inch.
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.
The engine had obviously been...
The engine had obviously been custom balanced as part of the last rebuild. We weighed our new TRW pistons and compared them to the old cast-replacement pistons and found them within 2.5 grams. We subtracted the weight of the carbon and varnish on the old ones and decided they were probably bang-on. The weights were much closer than production tolerances, so we opted not to rebalance the engine.
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.
JMS Racing Engines in Monrovia,...
JMS Racing Engines in Monrovia, California, installed new cast-iron guides and cut a precision high-performance valve job on the stock 340 "X" heads. The heads were finished at JMS with a light surface mill of .006 inch to ensure a flat gasket surface.
While Ishmael at M&R Machine...
While Ishmael at M&R Machine in Glendale, California, completely rebuilt our stock rods (including glass beading, fitting with ARP bolts, and resizing the big end, then rebushing and sizing the small end), the already .010/.010-inch undersized crank checked out okay. Rather than regrind to the next undersize, M&R simply polished the crank.
The haul from PAW, everything...
The haul from PAW, everything we'd need to build a blueprinted stock 340, but with the enhanced reliability offered by tougher-than-stock parts such as the forged pistons, moly rings, and stainless-steel valves.
We reassembled the heads using...
We reassembled the heads using this $20 lever-type spring compressor. The DynaGear valves were stock size, and the same tulip form as the originals. The PAW SSI-brand spring and retainer kit were matched to our 340 HP cam. Locks were also SSI in hardened steel.
On to the block, first it...
On to the block, first it was washed-out and air-dried, and then all of the plugs were knocked or screwed in. We used brass freeze plugs, which will not corrode or rust out.
A new intermediate shaft bushing...
A new intermediate shaft bushing was knocked in to replace the old one. These usually need to be sized for the shaft by burnishing, reaming, or honing, but it fit well with the seasoned old shaft we were going to reuse.
We knocked in some new cam...
We knocked in some new cam bearings using a tool set from Powerhouse. It was the first time we'd done this job, but with the right tools it was cake.
The crank went in with our...
The crank went in with our new bearings from PAW, using STP as the assembly lube. Bearing caps always go in notch-to-notch on the bearing tang.
Crank endplay checked out...
Crank endplay checked out at .007 inch, well within specs.