The mechanical parts were basically complete at this point. I decided to make a leak down tester rig to sanity check the cylinders under pressure prior to running. The basic principle is you have two identical pressure gauges with a small diameter orifice restriction between them. Regulated pressure is applied to the input upstream gauge. The downstream gauge output is connected to the cylinder head by a fitting screwed into the glow/spark plug hole. With the piston at TDC & both valves closed mimicking combustion, the gauge pressures should read the same. If the downstream gauge pressure is lower and/or diminishes over time, it indicates a leak in the system. But it can’t distinguish leak between ring seal, ring gap or valves... because it’s a collective system. This is why the tiny orifice restriction between gauges is important, we want to see any pressure change without the supply replenishing just as fast so to speak. I don’t have a feel for how much pressure over how much time is acceptable. More of a Houston, we have a problem thing. My valve/seats were vacuum tested to the extent of negative pressure previously, so I was hoping this would show more ring fit results at elevated positive pressure.
The parts were relatively inexpensive Amazon components including the hose & quick connect fittings. I machined an extended length spark plug adapter fitting to match the thread & incorporated an O-ring to seal. Apparently on full size engine testers the orifice is of defined size. But because our model engine volumes are smaller, the orifice must be reduced. The smaller the hole, the more sensitive are the readings. I’m not sure if it should be reduced proportional to displacement, but I had a few data points from other model testers I came across. My friend used a #80 drill (0.0135"). I asked him how he came up with that & he said it was the smallest drill he had at the time & it works, ha-ha. Also, Don Grimm on HMEM published his tester plans which I subsequently stumbled on & I believe he used 0.026”. I found a tiny restrictor we used on RC pneumatic retract gear systems, but the hose barb diameter is very small ~1/16” so would have required extra machining to integrate. So, I made a restrictor by plugging a standard pneumatic brass coupler fitting with a #80 pre-drilled orifice segment, glued with Loctite. It seemed to work. Then I realized what I thought was a regular on/off pneumatic valve I purchased was actually a pretty fine adjustable needle valve. It already has threaded fittings so I installed it in between gauges to compare. If I just crack the valve, it flows quite slow to the extent the downstream gage takes a while to equalize the upstream gage with the tubing blocked off. So for simplicity & some flow variability, I would recommend going that route if you build your own.
I went ahead & tested the 5 cylinders thinking it should at least be indicative of apples-to-apples comparison among all cylinders. I used about 45 psi initially & then 60 psi when nothing bad happened. The pistons had a light coating of oil normal for assembly. The good news is, no problematic leak off was seen. The gauges equal one another & will sit that way for several minutes at least. I left the crankcase open so I could see the underside of the pistons. Over time a tiny bubble could be seen which I attribute to ring gap & brand-new rings/liners. If I push on a valve, it blows down, re-seats & gauges re-equalize again. I put the same test rig on a known used RC engine & replicated the result.
It is advisable to somehow mechanically lock the crankshaft at TDC before applying pressure. Otherwise, the engine will faithfully reproduce a combustion stroke with an unexpected & jump on the workbench. Ask me how I know.
