After accumulating an hour or so of numerous shakedown runs, the engine abruptly stopped one day just after a full throttle test. The prop appeared to be sickeningly trapped between a pair of hard stops separated by what I measured to be some 390 crankshaft degrees. Right after this happened I had to go out of town for a week, and so I had something to think about while I was gone.
When I returned, the first thing that I checked for was a piston striking a valve being held open by a dislodged seat that had become wedged between the valve and the head. This possibility was high on my worry list because during construction one of the seats that I pressed into the head needed suspiciously less force than the others. Repairing this, assuming an irreplaceable casting hadn't been damaged, would require separating a head from its cylinder block, and this was something that I hoped I would never have to do.
After draining the fluids and returning the engine to its rotisserie assembly stand where it would be easier to work on, I removed the valve covers so I could manually exercise each valve. All seemed OK.
My next guess was that a rod bolt had come loose, worked its way out of its cap, and was hitting the oil manifold mounted to the tops of the bearing caps. I rotated the rotisserie 180 degrees so I could remove the lower crankcase (oil pan) and was surprised to see that everything beneath the oil pan looked OK. The symptom had changed, though, and now the crankshaft had much less free rotation. The cylinder bores, as much as I could see of them, had nice shiny mirror surfaces; and only one showed signs of minor piston skirt scuffing.
I turned the rotisserie back over so I could insert a borescope in the spark plug ports of the number two and five cylinders in both the port and starboard banks. The pistons in these cylinders were near the tops of their strokes, and now the crank was completely seized. Everything I could see inside the four combustion chambers, though, looked reasonable.
As an aside, I was really impressed with how clean the Viper spark plugs looked. They looked essentially new. I initially feared I'd have continual fouling problems with these tiny plugs which are mounted on the sides of the combustion chambers. Since they're located in front of the exhaust ports, though, the exhaust heat may be helping to keep them clean. There was a lot of coking, however, on the cooler walls of the huge exhaust ports cast into the heads outside the combustion chambers.
The center bearing cap, which was machined extra wide, establishes the crank's thrust clearance to something just under a thousandth. I then thought perhaps that some debris had found its way into this gap to create a bind. I removed the center bearing cap, which also required removing the oil manifold, but the crank was still stuck fast.
Although it didn't start out feeling like a starting system failure, the symptom now seemed to be pointing to an issue inside the wheel case. I removed the manual and electric starter shafts so I could look inside the wheel case with a borescope. The starter gears still looked good, but my visibility inside the wheel case was too limited to determine much else.
I was mentally preparing myself to begin disassembly of the wheel case when I decided to first remove the front drive cover. With the symptoms I had been seeing, I had no reason to suspect the problem was in there. Inspecting the front drive, though, would at least eliminate the last of the easy stuff. As soon as I pulled the cover, I saw two halves of a mangled screw laying in the bottom of the housing. One of the six button head cap screws that attaches the large driven prop gear to the prop shaft had backed out and fallen in between it and its drive gear. Three of the other screws were also on their way out.
The problem was created by a too small circular pattern for these six fasteners. The screw heads ended up too close to the fillet on the gear hub. When tightened, the heads dug into the fillet around the hub instead of bottoming on the flat surface of the flange. The vibration of the prop load likely caused them to back out over time. Two of the still-loose screws had too much thread damage to be safely removed without breaking them off. I was able to retighten them, but for good measure I first flooded them with a wicking grade of thread locker. Before replacing the other four fasteners, I turned down the diameters of their heads so they no longer touched the hub fillet. I also installed them with blue Loctite. The high spots on the damaged gear teeth cleaned up with a fine file.
I was able to take advantage of some of the unnecessary disassembly to better seal a few annoying areas around the lower crankcase that had been seeping oil. In fact, I discovered my two coolant leaks weren't created by the dreaded o-ring'd transfer tubes between the heads and cylinder blocks after all. They turned out to be associated with the coolant distribution manifolds on either side of the cylinder blocks. These, I was eventually able to fix.
To my relief, after reassembly, the engine fired right back up, and I seem to be back where I started. Actualły, considering that the drip pan no longer has coolant in it, I'm considerably better off.
So far, I don't believe the engine overheats, but I'm still working on determining that for sure. The cooling system seems to be sufficient for the three minute runs I've made so far, but I could sure use a temperature sensor inside the header tank.
When shutting the engine down, I've learned it's best to first kill the fuel pump and let the engine run itself out of gas before powering down. If not, fuel that was sucked up to the input of the supercharger will rain back down the long vertical intake and drip out through the carb's intake when the engine stops. In this engine that's actually quite a bit of gas. - Terry
