In the last issue, we covered the disassembly of the 402 big-block out of our AMD 1967 Chevelle. To recap, the motor had been pulled out of a salvage yard, given a quick and dirty refresh so it would run, then repainted and dropped between the fenders of our SS396. We knew it had a cracked piston in at least one cylinder, and overall needed a full rebuild so we could drive the car without fear of it grenading.
With the proliferation of crate engines today, ranging from budget minded to out of sight, some might be wondering why we're even bothering rebuilding this "small" big-block. There are multiple reasons. First, this is the engine we had. There are a lot of readers out there that don't have the budget for a crate engine, or just want the satisfaction of building what they've got. Then there are the class racers who have to run a stock displacement engine.
Second, just because a big-block doesn't have gobs of cubic inches doesn't mean it can't make great power. Along with that, the shorter stroke of the 396/402 gives this engine an easier ability to rev up fast, much like the difference between a 327 and 350 small-block. Third, we just recently did a build of a 496 big-block, so why do the same thing over again? And lastly, we just have a soft spot for the underdog.
After disassembly, we hauled the block, heads, and crank along with all our new parts to Tommy Eufemia at Bad Attitude Engines in Morriston, Florida, to be cleaned up, machined, and prepped for reassembly by Jason Rollins at Rollins Automotive in Gainesville, Florida. On arrival Tommy looked over everything, then set about the process of getting our Rat back into fighting trim.
1. The first order of business was getting our block and heads cleaned up. All three pieces were dipped in chemical solution and shot cleaned so all bits of grease, grime, and carbon were removed, along with any engine paint.
2. As noted in part I, the journals on our cranked had been scored by debris in the oil, and both main and rod journals needed to be turned to clean them up. Good big-block cranks can typically be ground .060” under, but Tommy doesn't like doing that unless there's no choice. Thankfully, ours only need an .020” turn to clean up.
3. After the crank was cleaned, it was time for balancing. Even though cranks are balanced at the factory, there's still a decent factory tolerance for this, and after years of heat cycles and revolutions, the balance can change. Tommy uses this spin fixture along with weights bolted to the rod journals, determining how much weight is needed on which throw of the crankshaft. Then he'll add Mallory metal (dense, heavy metal) to balance it out.
4. On the block, first thing to be done is torque down the main caps, then align hone them so the journals are all parallel and in line.
5. After align honing the mains, next up is decking the block. This will true up the deck surface, assuring us of good sealing between the heads and block.
6. Something we forgot to mention on our crank is checking for straightness. Over time, a crank can flex from torsional stress, heat cycling, and other factors. During a rebuild, it's good to check the crank for striaghtness. If it's out of spec, steps can be taken to true it up.
7. Next up is boring out the cylinders. Our block was virgin, but had a decent ridge at the top of the cylinders, and a lot of wear. A good 396/402 block can typically take an overbore of 0.090- to 0.125-inch without fear of the cylinder walls being too thin and not sealing properly. We went with a standard 0.030-inch overbore though, to give us as much strength as possible, and also leave room in case we need to bore the engine out again in the future.
8. Whenever a block is bored today, a torque plate is used. When a head is bolted to a block, the cylinder bores will distort from the pressure. The torque plate simulates a head being bolted to the block, so it can be bored to a true round shape just like when the head is attached.
9. After the heads had been hot tanked, Tommy went to work replacing the factory exhaust seats with our new, hardened ones from Summit Racing. At the same time, the seats for both intake and exhaust were machined out for bigger valves so our heads would be able to flow more.
10. Here are the stock combustion chambers fully cleaned and ready for larger valves. In our next story, we'll do some bowl work and cleaning up of the ports to get the most flow possible out of our factory heads.
11. Finding a set of low compression, forged pistons for a 396/402 is nigh impossible these days. There are plenty of cast options, but forgings are tough to find. Wiseco came to our rescue though, taking one of their on-the-shelf pistons and shaving 20ccs off the dome to help us achieve the near 10:1 net compression we want this engine to have.
12. Each piston and rod was weighed by Tommy to verify weight and work into his setup of making sure the engine was balanced as possible. Because manufacturing of these parts has gotten so much better thanks to modern metallurgy and machining technology, most piston and rod sets you get today have negligible weight differences between the individual pieces.
13. And there you have it. All our individual components are ready to go for reassembly. In Part III, we'll be putting everything together, and expand further on more of the parts going inside our real world big-block.
Powerhouse Machine Shop