KR builders, With the events of late there seems to be a sudden search for a suitable aircraft engine for the KRs. I am not going to tell people what engine they should use, but I will walk through a comparison of the series of Continental and Lycoming engines that *COULD* be suitable. This is not an endorsement or recommendation, but only a comparison based on my experience with these engines. FWIW, I have overhauled and flown behind every engine listed here with the exception of the Franklin 125.
For you Corvair and VW owners, this isn't meant to suggest that they are not viable engines. FWIW, the VWs have proved themselves to be reasonably reliable with proper care and feeding of the valve train. It is my personal opinion that the Corvairs may still be a viable engine if operated conservatively to reduce crank stresses. While they lack some of the toughness found in "certificated" engines, it really comes down to ones individual choices for risk management and acceptable risk. "A" series Continentals. If your KR is light, why not use an A series engine? The complaint I hear is that they aren't equipped for starter or alternator. We know the Porkopolis Pig was/is a fine flying plane that appeared all over the country and I believe it had an A-65 mounted to it. Build your plane light, and It will fly just fine behind an A-65 Continental. Others have used a Lycoming style ring gear on the front of the A series engines and mounted an automotove starter and alternator. While I haven't done so, I know it can be done. The SAE-1 ring gear off the O-235-L2C Lycoming will fit the Continental flange. If you have an A-65, it can always be upgraded to an A-75. The differences lie in the RPMs, valves & seats, pistons, rod bushings, wrist pins, and oil ports in the rods. The A-75 engine was discontinued at the end of WWII due to the introduction of the C-75, while it's little brother, the A-65 was updated with some more modern parts (like cam ground pistons and larger wrist pins). However, the A-75 at it's essence is an early model A-65 that is turned up from 2300 to 2600 rpms. Any A-65 that has been overhauled in the last 30 years probably has the modern valves and drilled rods. The A-65 uses a modern cam ground piston while the A-75 uses a round piston. Additionally, the A-75 has a waffle pattern cast into the bottom side of the piston crown and uses oil squirted from the opposing drilled rod cap to cool the piston crown. A-80. An A-65 can also be upgraded to an A-80. Like the A-75, the A-80 also turns 2600 rpm, but also has 7.0:1 compression vs the 6.3:1 compression of the A-65 and A-75. However, the A-80 uses a 5 ring piston, which creates a lot of cylinder drag and the pistons can be difficult to find. I don't see any real advantage to building an A-80 vs an A-75. C-85. The C-85 engines are a fine engine. I flew my KR behind a C-85 for the first 350 hrs to finish running out the engine. The down side is that the C-85 cranks are hard to find, although there is an STC to use an O-200 crank as a replacement. However, with the boom in O-200 crank sales, the price has taken a big jump. $3300 last time I checked the price for a crank. Additionally, the A series and C-85 cranks were not nitrided when they were new, so they are almost always worn out when it's time for overhaul. They can be turned -.010 and -.020, then they are done. If you send the crank in to have it ground, it will likely come back nitrided, which will really help with wear resistance and longevity of the crank. Same for the A series cranks (which are dimensionally identical to the C-85 crank). C-90 & O-200. The C-90 is essentially a derated O-200, but there are some minor variances. Both the C-90 and O-200 use a 1/4" longer stroke than the C-85 with a 1/8" shorter piston. Bore is the same as a C-85. I often hear how you can easily put C-85 pistons in an O-200 to make a high compression screamer. I tried it. The pistons hit the spark plug bosses inside the cylinders. I clearanced them to fit and flew with the C-85 pistons for a few years. I had a valve guide issue I wanted to address, so pulled the cylinders off my O-200 to replace the valve guides. I found that the top ring on 2 of the 4 cylinders was overrunning the top of the cylinder bore and was actually hitting the aluminum head. It had caused some really bizzare ring wear and clearly wasn't a good situation. When I honed the cylinders, I allowed the carbide cutter to slightly overrun the end of the cylinder to keep the rings from simply smacking into the head. When I put the cylinders back on, first time the piston went to TDC, the ring expanded out into the head and locked the piston into the top of the cylinder. That was the end of the C-85 pistons for me. I went back to stock O-200 pistons. I actually got a net gain in performance when I reduced the compression by improving the exhaust at the same time. Lycoming O-145. Rated at 65 hp, but less torque than an A-65 Continental. No electric. Yes, these engines are out there. Many of the Mooney Mites were flying behind them. I'm sure a light KR would as well. But, parts are difficult to find and the O-145 has a reputation for dropping valve seats. From here on down, you are likely building a hot rodded single seat plane, or you and your passengers are relavitely small people. Lycoming O-235. There are a couple of basic models of this engine with some significant differences. The early O-235 C series engines are rated anywhere from 100 - 115 HP. There are lots of them around, but once you get into one for overhaul, you'll find that parts for this engine are quite expensive, even by Lycoming standards. Then there is also the O-235-L2C model. This is the modern version of the O-235. It has a few minor differences like larger studs for the higher compression and is rated anywhere from 112 to 118 hp or 125 hp with different pistons. This is the engine that came on the Grumman T-cat, Cessna 152, Beech Skipper, and Piper Tomahawk. It is significantly less expensive to overhaul than the C series O-235, but costs as much or more than an O-320 because the parts count is the same but the production numbers are lower. The down side is that this is a relatively high compression engine. It is supposed to require 100LL only, but I would guess one could likely run it on premium mogas. It requires different pistons (9.7:1 compression) to make it 125 hp, which then would likely require 100LL or whatever fuel replaces 100LL. The whole O-235 series has solid valve lifters, so requires setting the valves every 100 hrs. Additionally, almost every O-235 will need to have the heads welded when they are sent in for overhaul. I don't know why, but they are almost always cracked. But they can also be easily repaired and made airworthy again. The welding usually costs ~$100 per cylinder. Lycoming O-290, G, D and D2. The O-290 series engines are orphaned. Some parts are difficult to find and extraordinarily expensive. The G models are converted ground power units rated at 125 hp. I flew behind one for several years in my Starduster I and thought it was a pretty decent engine. The D version and the G version are practically identical *IF* the G was converted correctly. I used a reinforcing plate on the prop flange of my G series engine as the crankshaft is somewhat lighter. The D2 model was slightly higher compression and rated at 135 HP. The D2 pistons are the only O-290 pistons that can be found reliably, although they are quite expensive. The G and D versions also had solid lifters while the D2 model used hydraulic lifters. All the O-290-D2 and larger Lycomings use hydraulic lifters. Lycoming O-320 - narrow deck series. Dimensionally, the narrow deck O-320 is exactly the same as an O-290, although it is a slightly larger bore. This is a pretty common engine, so parts are readily available at "reasonable" prices. An O-320 is much less expensive to build than an O-290 or O-235-C. Engine can be readily built to make 160 HP with Lycoming O-320 - wide deck. The cylinder bolt pattern on this engine is bigger than the narrow deck series. They are also a bit heavier. The engine can readily be built to 160 HP different cylinders, different wrist pins, pistons and rings. Dimensionally, the cranks, bearings, and wrist pins are the same in all the Lycomings from the O-235 through O-320 series. However, the part numbers are different and the differences are usually found in things like wall thicknesses and length of the wrist pins, or the thickness of the prop flange on the crank or the thickness of the wall of the crank. I have torn down an O-320 and found parts belonging to an O-235 inside. Franklin engines. Probably the only one that might be suitable is the 125 HP 4 cylinder Franklin. (I've never seen this model in person!) Generally speaking, I steer clear of the Franklins. Parts availability has been sketchy at best for decades, so reputable shops won't touch them. Some of the Franklin "Overhauls" I have seen I did as post mortems due to low time engine failures (including the one in my own Stinson 20+ years ago). In every case, I have found obsolete parts and/or improper engine assembly as the cause of the failures. There are too many shops working on them that don't care whether the engine is airworthy or not. In some cases, I found things like brand new worn out parts, welded cranks and overhaul logs signed by long ago dead mechanics where owners were clearly defrauded. I don't know if this information is useful to anyone, but is based on my 35+ years working on this stuff. Hopefully it will help some of you to keep from falling into some of the pitfalls I have found over the years. My recommendation for the KR is to build it light (which I didn't) and don't over power it unless you are dead set on building a real fire breathing hot rod and really understand the troubles (unreliability) you are letting yourself in for. Remember, Ken designed these planes to be light and fly behind a VW. Some of us (myself included) will never qualify as light, so the low powered VW may not be always be a good option. But, it is something you should keep in mind when you start dreaming about making yoru KR into a fire breathing Bonanza slayer. Jeff Scott Los Alamos, NM
