Converting an AA1 to O320 power

Although the AA1 series has a lot of wonderful attributes, take off and climb performance is not among them. I don't believe there is an AA1 pilot that did not wish for more power. There are a few ways to go. If you are looking to make an "investment" in your airplane, it probably would make sense to sell the one you have and buy one that is already converted. I don't think that the cost of conversion (about $15,000 in my case) is in proportion to the increase in resale value of the plane. However,  I liked my AA1C, it had relatively low total time and I had already made an  investment in avionics that wouldn't be recovered either.

 The easiest, and least expensive power increase would be to upgrade the O235 to 125 hp, ( This is only applicable to the AA1C, and I have been told that this mod rarely makes TBO with all of the original cylinders). I did get to fly a 125 hp AA1C and noticed little performance improvement.  If you have an AA1, -A or -B with a "straight" engine mount, you might consider installing a 125 or 135 hp O290 ( not a huge increase in power, and the O290 has been discontinued for quite a while), or an O320 of 150 or 160hp.  You could also have you engine upgraded to use the Cylinders from the -L2C engine by Bill Scott of Precision Engine. If you have an AA1C with a dynafocal mount, it would make sense to stick to the O320 series. I chose the 150 hp O320-E2G, which is the engine that is standard on a Traveler or Cheetah. Once that  decision was made, I now had to find an engine, which is complicated by the fact the overhaul shops and the factory only want to except a like core. In other words, they would not exchange an O235 core for an overhauled O320. I finally found a core from a wreck which I had rebuilt, using new millennium cylinders, new cam and cam followers, new oil pump, new mags with 2 impulse couplings, new carburetor, new vacuum pump and lightweight starter.

There is also a choice of STC's to use, the "Collier" sold by Fletcher for about $350 or the "Blackman" sold by Ken Blackman at Airmods NW for $700. I chose the Blackman because it was far more comprehensive, including patterns and drawings for the baffles, a gross weight increase, included the options of landing gear cleanup and the AA5 dorsal fin as well. It also allows the use of the engine at its full rated power. 

The AA1C is the best choice for the upgrade in my opinion, because it has a dynafocal mount, an oil cooler, the tallest of the nose gears installed on the AA1's, a higher (1600lb increased  with the STC to 1684lb gross weight) and the larger horizontal tail surfaces from the AA5. If you have a Yankee or -A or -B there is an approval to use the AA1C mount or use a "straight" mount engine, and drawings to fabricate the oil cooler mount as well.  Other considerations favoring the AA1C are the higher gross weight, no mixture cable or bungee AD either, and more elevator authority from the larger AA5 elevator.

Because I had an AA1C, with a dynafocal mount and oil cooler already the O320-E2G would be very close to a bolt on operation, with the throttle, carb heat, and mixture cable connecting right up as well. The exhaust system from the O235-L2C bolted right on as did the airbox, although Ken Blackman says that an AA5 airbox would offer slightly improved performance. All of the fuel and oil lines as well as the various pieces of scat tubing were the right size and length, I reused mine because I replaced them all two years ago when I had the engine off to do the oil pump AD, otherwise I would suggest replacing all of them as well as the Lord mounts. Those were all things that went right on........ However, 

The AA1C uses a Prestolite alternator which required a different lower mounting bracket than comes on the O320-E2G, the breather is in the rear of the O320 rather than the front of the O235, requiring a slight rework of the breather line. The AA1C climb prop (Sensenich 72CK-52)  was replaced by a Sensenich 74DM6-58 from a Cherokee 140 that I had repitched to 62". I was lucky to find the prop which was overhauled in 1983 and never used because the overhaul left it to short (72 1/2" ) for the Cherokee's type certificate requirement. However I was allowed to use it down to the prop's minimum length of 72"  so I only lost 1/4" of ground clearance and got a great deal on it as well.  The new prop also required new prop bolts ($150+) and changing the prop drive bushings in the crankshaft to 3/8" from the original 7/16" . The original spinner would not fit the bigger prop without being cut out and reworked. Given the already fragile nature of the stock spinner, I chose to go to a composite spinner and cut that out to fit my new prop. (That worked out quite well for the 125 hp AA1C newly based at my home field with a cracked spinner! ). Now for the big one. I had to fabricate all new baffles. The Blackman STC allowed for either modifying the originals or constructing new ones. Considering all of the cooling problems that the Grummans have, I decided to make all new ones and fit them nice and tight. The STC came with excellent drawings and full size patterns to cut the parts out, but it was still at least three times as much work as I anticipated. The plans called for either .032 or .040 2024 or 6061 aluminum, I chose to use .040 2024. I used the 3/32 neoprene impregnated fiberglass cloth for the seal material rather than the heavier red silicone at Bill Scott's suggestion. I was quite happy with the results, but it took me a long time.

While this was all in progress I  added the DMA fuel monitor system, EGT and Vernier mixture control and did an annual inspection, including replacing several control surface bushings and rigging with the factory tools. So while the plane was out of service for 5 weeks, a lot was accomplished.

Click on any picture for a larger version

25U no baffles.jpg (575816 bytes)  25U front baffles.jpg (486877 bytes)  25U O320 front.jpg (445434 bytes)    25U O320 left.jpg (513844 bytes)   25U O320 lf.jpg (553815 bytes)

The Results

After all the work was done it was finally to time to fly it and enjoy the fruit of all that work. It started up on the second blade and made oil pressure right up to the top of the green. A quick run up to minimize ground running before it was broken in, and then shut down for a leak check and another once over before flight. Everything was just right, so we cowled it up and ordered lunch. I was going to take it for a 15 minute shakedown flight and come back for my sandwich. I did not realize I would be walking back to the airport. Taxiing out, another run up and a static RPM check everything OK engine gauges in the green, line up and go. The take off was gratifying, climbing at about 1000fpm at 100kts, showing 12.5 gpm at full throttle. All the gauges were in the green when BANG! A violent vibration shook me awake. At first I thought I lost a piece of the prop, but when the prop stopped with another bang I could see that it was all there. This all took place between take off and before turning crosswind. There was no hope of making it back to the airport and the best choice of fields was on the other side of a housing development and some wires. I was able to land in that field with no damage to the plane, and upon removal of the cowl in that field, discovered what the problem was. There was a hole in the crankcase with a connecting rod, minus the cap sticking out of it.

25U hole 1.jpg (620293 bytes)   25U Piston 2.jpg (385273 bytes)

Here it is with my son, on its way out of the field on the way back to the airport. 25U field 1.jpg (103043 bytes)

It is now sitting in the hangar with the engine off again, waiting for another overhauled engine. I expect that it will be flying again in a week or so. About the only parts to be reused are the mags, carb and vacuum pump. Stay tuned for the next update.....

Second Flight