Even though I hadn't planned another Offy post, I thought I'd share a few things I've learned working with it during these past several weeks. Since I usually enjoy making engines more than running them, mine often end up on a display shelf with only an hour or so running time. It seems that once they're running reliably, I'm on to the next project. So far, even after running a liter of gasoline through it, I still enjoying playing with the Offy.
Being obsessive about oil leaks, the Offy was almost my first leak-free engine. There's some seepage around the forward end of the crankshaft due to an o-ring I omitted, but a proper fix would require a new front cover machined for a lip seal. Since it's relatively hidden by the front bracket and doesn't drip, I've decided to live with it. Fortunately, some of this oil seems to be wicking into the starter clutch and lubing it.
I've had doubts about the grease-packed seal inside the water pump I designed, and it started leaking right after a long hard run. In addition to leaking onto the display stand, coolant was also finding its way into the oil. Adding an o-ring shaft seal just ahead of the pump's rear bearing turned out to be easier than expected and eliminated the problem, but several oil changes were required to completely flush out the coolant.
Many cold garage starts and short runs soon caused water (a by-product of combustion) to begin accumulating in the oil. Although the head heats up quickly while running, the crankcase doesn't get hot enough to drive out moisture. Over time, its chocolate color warned that the oil was becoming a corrosive mixture of water and crankcase gasses. I expected to see puke oil inside the clear PCV hoses that had been routed to the oil tank, but they never contained anything but clean condensed water. It didn't make sense to continue piping this into the oil tank, and so I eventually vented the PCV hoses to the atmosphere.
The most frustrating issue I run into was trying to control the oil smoke in the exhaust. The oil holes in the piston groove just below the scraper ring don't seem to be enough to keep oil out of the Offy's combustion chambers. This same oil control scheme has worked well in deep sump engines, but seemed to be overwhelmed by the oil whipped up by the connecting rods inside the Offy's confined crankcase compartments. Testing showed the problem to be worse with straight 30W than with 10W-30, and so 5W-20 might be a worth try. If I were re-making the pistons, I'd mill oil return slots rather than drill holes.
My 'easy fix' to the smoke problem was initially to limit the amount of oil in the crankcase drains by maintaining a minimum amount of oil in the tank. Since I'd originally placed the tank's outlet near the bottom of the tank, this allowed only 10 mL of reserve oil - an amount determined by removing oil from the tank (while maintaining a flow in the pressure pump hose) until the smoke went away. Even with this minimum level, the plugs still showed signs of oil. A new problem created by such a small reserve, though, would be its increased vulnerability to crankcase vapors. To get around this, a new outlet was installed higher up on the tank which allowed me to increase the reserve oil nearly 10x.
During a two minute 3000 rpm run, the upper half of the radiator becomes noticeably warm since about half the gets moved through the engine. A thermometer showed the coolant in the upper part of the radiator to be just over 100F.
The actual flow was measured by temporarily diverting the output of the upper radiator hose into an external container. Coolant flows through the engine at all running speeds, and the flow rates roughly agreed with the measurements made during the water pump development.
However, worrisome air bubbles show up in the coolant at high rpms. There's no oil leaking into the coolant, and the bubbles haven't gotten any worse since the engine's very first run. I've noticed these same bubbles, which are likely due to a head gasket leak, in the Youtube videos of two other Offy builders. And so, before tearing into the engine, I decided to run a compression test to see if I had a major cylinder problem.
Compression tests on model engines are complicated by the small volumes of their combustion chambers. While running a test, the volume of the tester is effectively added to the volume of the combustion chamber which reduces the pressure readings. An additional issue with the Offy is the limited access to its spark plug holes created by the water outlet pipe.
I converted an inexpensive mechanical tire pressure gage purchased from Amazon into a compression tester. The meter and valve body were reused, but the Schrader end was replaced with a custom adapter specially machined for the Offy's 1/4-32 spark plug holes. The tester's internal volume was also determined in order to come up with a correction factor for the Offy's pressure readings. The huge 41% correction factor is an indication of the measurement problem.
My pistons produce a theoretical static compression ratio of 7.85 which by design is less than Ron's high compression pistons. This reduced c.r. should theoretically produce readings of 115 psi. Although I was prepared for one or two dramatically lower readings, all four were within a generally accepted 10% variation. My corrected readings were: #1 = 99 psi, #2 = 107 psi, #3 = 113 psi, and #4 = 99 psi. Although the two outside cylinders are a bit low, I'm enough satisfied with the results that I have no plans for a teardown. To be honest, I'm not sure how I'd improve on the current head gasket.
The engine continues to start easily and after a brief warm-up idles reliably at 1100rpm. The air bleed adjustments had more effect on the idle performance than I expected. There are no hot-start problems and, even without a choke, starting in a 55F garage hasn't been a problem. The stainless steel exhaust gets incredibly hot, and its previously polished chrome-like finish has become a golden yellow. With 25 degrees advance the engine will rev to its top-end of just over 5k rpm. The #1 and #4 carbs seem to be well matched to each other and so are the #2 and #3. There seems to be only an eighth turn adjustment screw difference between the two pairs. The plug insulator colors aren't the ideal tan color that I've been able to achieve on my other engines using TruFuel 4 cycle gasoline, but that might be related to the still excessive oil.
The radiator fan turned out to be more effective than I initially thought. At the end of a run, the engine cools down twice as fast with the fan running, and so the air it's blowing over the head is having more effect on it than on the radiator.
Lastly, our pond fish and turtles survived our infamous week long Texas freeze and power blackout, but it will take months to replace all the landscaping we lost. - Terry