A: First of all, I've felt your pain. We've built up engines that should have been bulletproof and run forever. I've built engines for my friends before, and one in particular ran for about 5,000 miles in his 1/2-ton work truck before breaking the crankshaft! The engine still ran, and he drove it to my house just fine but with a loud knock. Pieces can and do fail without warning. Can we push them too far and cause them to fail? Absolutely! Mixing the wrong components can cause parts to fatigue and break before their time. With your 496 it sounds like it was purpose-built for RV use and mild components that were properly utilized. As the mechanic said, sometimes valves do break.
The 502 H.O. should be an interesting engine for your motorhome. If you purchased the engine new it will be a Gen VI big-block with the hydraulic roller camshaft. These engines have been durability-tested back in Detroit, and the components will live a long and happy life.
Since the engine has a roller camshaft, you have some latitude on your oil choice. Joe Gibbs is a very good break-in oil with high-shear additives for break-in. I've been really happy with Delo 400 LE 15W-40 Chevron diesel engine oil for my engines for performance use. This new engine oil was released in 2007 with the advent of low-sulfur diesel fuel and the low-emissions diesel engines. Its applications have spread from diesel-only oil to high-speed four-stroke gas engines. It has an API (Automotive Petroleum Industry) rating of CJ-4. Even though the oil is low in emissions, it still carries a good amount of phosphorus and zinc, the high-shear additives that have been stripped from our standard gas motor oil. These levels aren't as high as previous Delo oils but help with flat-tappet camshaft wear and sliding contact components. It will take the high loads and temperatures of your motorhome use.
A couple of tips for your installation. First, you must us a melonized steel distributor gear on your HEI ignition. This gear must be used to mate with the billet-steel hydraulic roller camshaft in your 502 H.O. This gear has very close tolerances on its machining and has a secondary coating operation to prevent wear. These gears were used on all GM roller camshaft engines. You can pick one up from any GM dealer under PN 10456413. Second, keep the engine speed to a minimum until the engine oil has come up to temperature. There is a great amount of load on this gear during cold starts, when the oil pressure is highest. Third, keep that thing cool. The 502 blocks are a Siamese bore design, which means there is no coolant passing between the cylinder bores. These blocks have thick cylinder walls and will run cool as long as your system has the capacity. Finally, your motorhome should have had some type of engine oil cooling system. Either the engine oil was plumbed through one of the radiator tanks or had an external cooler. We would look to install an external cooler to keep the engine oil at a nice 200 degrees. Getting that cooling load off the radiator will free up some capacity for the engine coolant. Check with Fluidyne for very nice air-to-oil coolers that will pull the heat out of the oil; you can plumb the universal kit into your system.
Good luck with your traveling home. Hope the new 502 power will pull your 22,000 pounds down the road like you expect.
Roller Tappet Swap
Q: My '66 Chevy II SS bracket car is equipped with a moderate 383ci small-block featuring a forged Eagle rotating assembly with 6-inch rods, 10.3:1 forged flat-top pistons, fully massaged 186 casting heads, a port-matched Victor Jr. intake, and a 750-cfm Holley double-pumper. The cam is a solid-lifter Lunati that specs out at 0.530/0.550 inch max lift with 242/244 duration at 0.050 inch lift. I want to "detune" the car enough to be able to drive and enjoy it during the off-season.