Replace stock O235 engine with O320-E2G AirModsNW STC

Bob Steward suggested I create a page here in an attempt to describe my process of performing the engine upgrade from the O235-C2C that our AA1x models came with to the more appropriate O320 engine. I started with a core that I purchased from Gil Alexander.

O320 core ready to ship

I'm told Gil did me very well in selling me this core. It's an O320-E2G, which is the stock engine for the Cheetah. This worked out best for me as I wanted to use the STC available from Ken Blackman AKA AirModsNW. There are three STCs available to choose from to perform this modification. If you are considering the upgrade gather as much info as possible on all three before making your decision. Each has its positive and negative sides. Each owner needs to weigh the pro and con of each for them-self. My decision was made on recommendations of others who had used Ken's STC and by my A&P after describing the process involved and each of the STCs. Since my particular A&P/IA had never performed an O320 conversion on a Grumman AA1x before (C&P brand yes, just never an AA1x) I wanted to go with the one that had the most documentation provided with the STC.


For the engine I chose new ECI CermiNil® Process cylinders http://www.eci.aero/pages/services_advantage.aspx. I picked Aero Engines of Winchester 540-678-1661 (ask for Tom) for the overhaul of the engine. This STC limits the HP to the stock 150HP pistons and since the crank did not pass the AD inspection, it's my understanding I would not have been allowed to use it in my engine with the 160HP pistons. I was told by several reliable source gurus on the GG that I'd never miss the 10HP. They were right; going from 108HP to 150HP has been such a dramatic increase in power it's like I'm flying and entirely new airplane. I've started uploading photos of the project as I put this article together. So keep an eye out for updates.

O320 test run at Aero Engines of Winchester
O320 test run *man those pipes were loud*


Day 1: Removal of the old O235 engine. Removal was a fairly simple process. It is a REAL good idea to keep track of every nut bolt and screw as well as making GOOD documentations of what wire came from where. If not just be sure to label every wire indicating what terminal it goes to. A good digital camera helps here too showing routing locations etc.

O235 eng removed

Day 2: Firewall prep & engine mount repair. I ran into a snag with the lower right engine mount extrusion. Fortunately I was not in excess of the allowable limit on this and after a day of research and sweating was able to save a ton of $$$ and at least 3 days of work.

Eng mount repair
Eng mount repair before painting firewall
Bottom view of bushing
Top of bushing
Eng mount repair
Bushing in place


A separate article should be written on this repair and a good procedure for inspection for this. I stripped the firewall down of dirt and paint. Since I've never been real great with a spray can I elected to have Jeff paint my firewall. Before and after photos of this appear below.

Firewall striped
Ready to be primed
Firewall primed
Fresh paint on the firewall


Day 3: Hanging the engine (AKA how many washers & where do those spacers go?). Not having a manual that has a visual for the dynafocal mount I was unsure of the placement of the spacers used with this type mount. Fortunately Shearer Aviation had the AA5x manual on file. Numerous phone calls later I did acquire the info I needed along with comparing to the AA5 drawings. The spacers are placed the same as for the AA5 and AA5A. Finding the drawing for an AA5 at the shop proved beneficial. Basically all four spacers go between the engine & mount. There are also washers that are used to shim the engine up/down or left/right. Every application is going to be different. If using new Lord mounts (which I'd highly recommend) make the spinner slightly (I used 1/4") high in relation to the nose bowl. The mounts will settle in and at that time the alignment will be correct. That is what I used and now (almost two years later) it has settled in just right. I cannot recall which Grumman Guru that one came from. Thanks to who ever did!

New engine mounted showing four vane oil cooler on firewall as was mounted with the O235
O320 mounted, right side view. Don't forget to paint the nose strut when you paint the firewall
A split nosebowl is a no brainer if not allready done at this time. I don't think it was necessary to use all of the screws though


Begining of Day 3 Somehow we missed painting the longer nose strut when we painted the firewall and baffling. Note the difference between the old (white) and the new (primed) nose strut. Here is how you get the extra prop clearance. It also can be gained by swapping out the nose fork depending on which one you have already. Also make sure your alternator brackets are in good shape. Needless to say mine were not. A call to Ken Blackman at AirMods NW had them shipped to me next day in short order. Ken is also the guru who gave me the info on how to make the engine mount repair. I guess using his STC he had a vested interest in me making it right.

New and old nose strut
Close comparison of the longer nose strut to the stock one
worn out alt mount
Another view of alt mount



Day 4: Baffling. Using the STC from AirModsNW proved the most beneficial here. When Cliff Hansen said he spent 40 hours I thought, no way (he wasn't kidding). Having engineering drawings as well as full scale templates for the baffles proved invaluable. I photo-copied the drawings to keep the originals in good condition (thanks for that idea Paul). Using the photo-copies of the templates I was able to create baffling using the aluminum from the RV6-O320 kit (great tip from Cliff) that fit my engine perfectly (well almost, I had to modify the forward baffles some after I thought I was done; see below). Fitting to the cowling took some hours. The rear and sides went together well. Keep in mind do not apply the flex baffle to the top until you have verified the height of the upper cowl.

One of the guys helping me got the cart before the horse on this and we had to cut the baffles down after he spent quite a few hours applying the flex baffles. The two front baffles took more time than all of the other baffles combined. I'm still not pleased with how they turned out and plan on modifying or changing them out at some time in the future. My temps on 1&3 cylinders are great but I'd like to get #2&4 down about 50 degrees. It was suggested to check to make sure I don't have any flashing issues before tearing my baffles apart *again*.

Baffling showing 2" scat opening for oil cooler
The baffling took a long time. When Cliff told me fourty hours, I thought no way.... WAY



Several months later: Replaced the oil cooler with a seven vane. The STC does require an oil cooler be installed. The four vane unit I had with the O235 did meet the minimum requirements of the STC. In the cooler spring weather when the engine was installed the oil temps were fine. However once summer came around I could not get my oil temps down to an acceptable level to me. Once again Gil pulled through for me. He had a brand new seven vane cooler just laying around the hanger that he was not using. It was actually the one he had planned on using with this engine in his RV. Since he was going with a different engine he advised he needed a different cooler and let me have it for what he paid for it years prior. I got this seven vane for less than a six vane would have cost me from Spruce. I figured more was better and have been happy with it.

Old & new oil coolers I think this should solve my cooling problem eh?
Both coolers and boxes to compare
Baffling showing 3" scat opening for new oil cooler


I do have to put a blocking plate over the scat tube hole in the winter months or my oil never gets anywhere near minimum temps. I was able to only find 3" adapters for the scat tube at the cooler box and rear baffle. I picked them up at a boneyard in slower DE (Jenkins Airport 15N). Ken said I only would need the 2" but he also said I'd be fine with the 4 vane cooler. It must get a lot hotter in DE than WA. Old man Jenkins also had a replacement VS for the repairs when I was doing my paint. That too will be another article. I built the airbox for the cooler and did the cooler upgrade while she was down for the paint in the summer of of '07.


Then a while after that: I relocated the batery to behind the baggage compartment. When the W&B was calculated we determined 64L had gained a bit of wieght in her years. With the engine swap she had her CG fwd a good bit and to keep her inside the envelope I had you fly with a 50# weight in the baggage compartment. We had to place a placard on the panel stating this also. This never caused a problem flying alone but with a passenger it easily could have put me over GW. Ken Blackman supplied me with most of the hardware and some photos of the job being done by another AA1x. Copies of some 337s were included. The job wasn't as hard as I thought it was going to be. I would like to make an easy access panel in the rear of the baggage compartment for servicing the battery. Although using the Concord RG cell reduces the requirement of adding water so less service required. Pics of that for another day.