Monty the Answer Man Archive

Continental C85-C90 Info


Subject: Re: GC-1A
I'll take the info under consideration when the time comes. Can a C85 hp be converted into a C90? -- Roy

Yes, and no... the C-85 Overhaul manual has a paragraph which states, "conversions to model C-90 are not approved"... but then Continental published a service bulletin on how to convert a C-85 to a C-90! In the year 2000 it is really not necessary to convert the engine, an O-200 crankshaft can be installed in a C-85 by STC (along with the O-200 rods and pistons) so, you can effectively get 100 hp out of it. (a C-90 has the same crankshaft as an O-200) You need a knowledgeable mechanic to do this, there are several little "tricks" to do which are not published. -- Jim

Subject: Re: GC-1A props w/C-90
What prop do you like on a GC-1A with a 90 ?

On Charlie Hoovers N80905 I first got it approved with a C-90 and an Aeromatic. He decided he wanted a fixed pitch prop, so I applied for and got a McCauley SCM approved. (I think it was a 71x54) I looked it up, it was a McCauley 1A105/SCM. Then he bought a Sensenich 72CKS6-54 (72 dia.) which he installed with a 1 1/2" spacer. I got that one approved too. He later went back to the McCauley, which he likes best. I rode with him a month ago or so and it performs very well. He has beat every 125 he has run against and also several 145's. -- Jim

Subj: GC-1A C90/alternator
From: Ron Best <>
Hi Jim, I have a few engine questions regarding the GC-1A. I am restoring S/N 205 and am wondering if anyone that has done the C-90 conversion has installed an alternator like the Cessna 150 with the O-200 engine? As far as the C-90 goes what dash number (-12F, -14F or -16F) is the most common used in the Swift? The -14F that I have has the O-200 style lord mounts, will that work with the original C-85 mount? The original C-85-12F engine had two engine driven fuel pumps, I want to install the electric fuel pump like the stc'd type for the GC-1B and would like to know if that has been done to the other C-90 conversions? Thanks for all the great information that you provide to the news letter. Ron Best, Calgary, Alberta

An alternator would work fine on a C-90 in a GC-1A. The only thing is, you can't get rid of too much weight forward of the firewall because a GC-1A tends to be tail heavy. The good news is, the GC-1A aft CG limits are further aft. (+35.5) The C-90-14 will not work very good, it has lord mounts like an O-200. The best engine is a C-90-16. The later C-90-12 had thru studs and they are fine too. DO NOT use an early C-90-12 without thru studs. I'm sorry to tell you the C-90-14 is not the engine you want, you'll have to either get another engine or change the case to a C-90-16 case or a C-90-12 case with thru studs. The C-90-14 will bolt up to the mount but the engineering is not right and it will sag on the lord mounts and the engine will be too far forward by about an inch and not fit the cowl and baffleing perfectly. The electric fuel pump will work fine, don't even think about that extra engine driven fuel pump. I have done several of these conversions and I can give more detail if needed. I even have copies of the 337 forms if that will do you any good. -- Jim

MORE ADVICE FOR RON RE: O-200 ON C-85 MOUNT...(010201)
Subject: Re: GC-1A C90
From: Ron Best <>
In a message dated 1/6/01 6:02:19 PM Central Standard Time, wrote:
<< The -14F that I have has the O-200 style lord mounts, will that work with the original C-85 mount >>Ron Best

Mark Holliday told me that I should have mentioned they make some cone inserts for the O-200 style case to fit it up to a C-85 style mount. These are for homebuilders, I don't know about approval on certified aircraft. If you want to go ahead and use your C-90-14 you might at least check in to it. -- Jim

MORE INFO RE: O-200 ON A C-85 MOUNT...(010401)
From: Terry Straker <>
Subject: Re: January #3 GTS Internet Update
Denis- You may pass along that The Luscombe people use those cone inserts to install O-200's on C-85 mounts, so they may have them made up and some sort of paperwork available. I know when I was flying a Luscombe they published detailed plans on how to make the cones in the newsletter and that several guys were flying with O-200's installed. Now that The Luscombe Foundation is trying to go back into production and are rebuilding Luscombes, perhaps they would be a source for those cones. I could probably dig up that old newsletter as well if necessary to see a 1-A fly again. Terry Straker

(Editor says... Mark Holliday email'd Jim Montague who email'd me to say that the mount adapters are $294. Call 650-493-6916 and ask for Ralph.)

Subj: GC-1A 85 H.P. Swifts
From: Art Nicholas <>
Hello, my name is Art Nicholas, I have a 85 H.P. Swift that has been sitting unused in my hangar for the past 20 years. After looking in a mirror the other day, I figured I had better do something with it, or forget it. It has a Beech Roby prop on it and I can't find anyone who can do any work on it. A friend of mine showed me how to get ahold of you guys on the internet , I found you and the GC-1A you owned and was wondering who has the STC to put a 90 H.P. on my Swift ? I would also appreciate it if you would give me your opinion on what your plane was like and what I should expect with that set-up. Thank You; Art Nicholas ( p.s. it is s.n.161)

My N80539 (now renumbered N28AG) had a one-time STC in the name of Leonard Lewis of Des Moines, IA for the installation of the C-90. He at one time told me I had permission to use his data, but when Mark Holliday and I converted N80760 the FAA inspector simply gave us a field approval. Later, we converted Charlie Hoovers N80905 to a C-90 and used N80760 as a basis for another field approval. Still later, the owner of N80523 installed a C-90 and used N80760 as a basis for his field approval. I eventually ended up owning N80523. The aircraft file can be obtained from the FAA on any of these airplanes. I do have a copy of the C-90 installation 337 in N80760.We usually used a fixed pitch prop with the C-90, from memory it was a McCauley SCM, 72" dia. and from a 51" to a 54" pitch. We used a spacer about 1 1/2" thick which helped the performance considerably. I don't know of any shops that will work on a Beech-Roby prop these days. I am cc'ing this to Pete King who may be able to help. I wish I had one of my GC-1A's back! With a good C-90-16, a fixed pitch prop, a prop spacer, and an empty weight under 1100 pounds a GC1A will outperform a 125 hp Swift. -- Jim

C85 REBUILD TRICKS...(090202)
Subj: c-85 rebuild
From: (Thurmon Kent)
jim, I read something you wrote on the answer man about rebuild tricks on c-85.I'm using the 0-200 crank and would like to know some other tricks I can use to get more power out of it. thanks thurmon kent

The "tricks" are not necessarily to get more power. The case needs to be modified to accept the late 0-200 crankshaft and thrust washers. Any good FAA approved Engine shop can handle this. The two center studs in the 85 case are not thru-studs. If the case halves are drilled thru, the lower hole hits an oil passage. Some guys just drill the upper hole and just use one 0-200 thru-stud. Other guys have "0' ringed the case to prevent internal oil leakage. I don't think either procedure is approved and would suggest you reject any crankshaft with any runout at the center main and not turn the engine over 2550 rpm. That will keep the case from cracking. TCM's manual allows .005 runout at the center main which is too much. The C-90 cam or the 0-200 cam will work in an 85. I do not recommend the 0-200 cam because very few of them have a fuel pump lobe which you may need and also it requires high rpm (2800) for rated power. The C-90 cam requires the C-90 lifters also. You can't run a cast iron cam (C-85) with cast iron lifters, or a steel cam with steel lifters. You need cast lifters on a steel cam and vice-versa. I would be more worried about the engine hanging together than trying to get every last horsepower out of it. With new cylinders and the 0-200 crankshaft you will get plenty of power. Use new Slick mags of course. -- Jim

85 to 100HP... (AUG 03)
Subj: stc
From: Dan Bevington <>
Hi Jim,
Do you know where I can find the STC for converting the C-85 to 100HP? Thanks, Dan

I believe you are referring to the STC which allows the O-200 crankshaft to be installed in a C-85 engine. The engine is still rated at 85 hp. In the real world of course the engine does put out a little more power. The STC is held by: Aircraft Specialties Services 1-800-826-9252 When I first read your mail I thought you were asking about an STC to install a 100 hp engine in an 85 Swift, there is no such STC. -- Jim

From: "Harry Fenton" <>
FYI, I have an archive of engine material for the small four cylinder Continentals that is also useful for the C125/C145/0300 series that you find interesting. Most of the information there is pretty good, but sometimes kind of rough in that it is kind of stream of thought answers (not unlike the stuff that has been archived from Jim's comments). I will eventually have a dedicated website for the four and six cylinder Continentals, time permitting. You can find my archived engine page at:
Anyway, feel free to contact me any time and maybe we'll meet at one of the fly-ins.
Best regards,
Harry Fenton
Gippsland Aeronautics