A65 versus A75

[Frank DeBartolo]

I am in the process of doing a major overhaul on my Continental A65-8 engine that is on my BC12-D. A friend suggested that I should upgrade the engine to A75. Because funds are definitely a limiting factor, I have several questions for the forum that I hope someone can answer.

The limited information I have regarding modifiyig the A65 to A75 is that larger cylinders are used and that the carbeurator has to be modified. If this is true how does this affect the other accessories. I have several questions for the forum.

Does this mean purchasing different--larger cylinder assemblies.

I have a McCauley 7443 metal Prop on the airplane now. Does the prop have to be changed.

Does the existing exhaust system have to be changed or modified.

What is Cruise and redline RPM on the tach.

What is TBO on the A75. I understand it is 1800 hours on the A65.

Does the extra horsepower increase max gross and useful load over the A65.

Is an STC required for the change in horsepower.

What paperwork is involved in this modification.

These are the questions I have at this time. All comments are deeply appreciated.

Thanks

Frank DeBartolo
N43684
BC12-D

[jgerard]

It's not an STC approved mod, but there are plenty out there just flying around with them installed. The only difference between the A65 and A75 was valves, rod caps, piston pins, different mag timing, and main jet in the carburetor. Most 65's have been rebuilt with the stellite valves which is good. You'll also find the later built 65's also have the drilled rod caps. Now it's down to piston and wrist pins. The A75 had a different wrist pin's because the engine spins faster. I don't remember if the 75 had the smaller ones for less weight at the end of the rod, or the beefier ones to cope with the increased load, All I remember at this time is they are different. Again you will find a few of the 65's have the 75 style wrist pins installed already. Do the above, change the timing, put the bigger jet in the carb, and fly!

The only reason the 75 makes more HP is because it spins faster, 2600 rpm redline. You need a smaller prop to get those higher rpm. most 75' s spin about a 70-41 sized prop.

Most people run the 75 without any paperwork and they don't change the redline on the tach. The only visible difference is the shorter prop, which most people don't recognize.

Make sure your IA will continue to annual the plane if you do the mod. Or get a field approval. It's worth the extra HP and costs way less than a C-85.

Jason
N43643

1800 hr TBO I think???

[Garry Crookham]

Frank:
Jason and Ron have covered most of the differences between the A-65 and A-75. You asked about cruise and redline for the A-75: Max rpm for the A-75 is 2600 rpm and normal cruise is 2350 rpm. Published TBO is the same as the A-65: 1800 hours. Your existing exhaust will work with the A-75.

To convert your engine to an A-75 you need to know how your connecting rods are set up. The A-75 requires that the connecting rod cap be drilled to help cylinder oiling. Late model rods used in the A-65 are already drilled and most mechanics have the rod caps drilled when overhauling an A-65, so it is possible that your rods are already drilled. The A-75 piston uses an .859" diameter piston pin. Depending on what piston is installed in your engine, you could have either an .859 or a .922 diameter pin. If you have the .922 pin your rods would have to be rebushed to be used in an A-75 engine.

The latest valves furnished by Continental were used in both the A-65 and A-75, so again you will need to know what valves are installed in your engine.

I don't think you could convince the FAA to allow an increase in gross weight by changing to an A-75. Increasing the gross weight during a conversion to a C-85 requires some airframe mods.

I am not aware of an STC to install an A-75 in a BC-12. There have been a bunch of field approvals of this install and I'm sure someone could furnish you a Form 337 to help the approval process. I would advise you to discuss this with your mechanic and have him check with the local FAA FSDO to make sure they will approve the change before you convert.

If it were me, I would not convert to an A-75. The late model A-65 piston, p/n 40731, is a much better piston than the one used in the A-75. The increase in horsepower is gained by turning the engine 300 rpm faster for takeoff and 200 rpm at cruise. If I want a little extra go I would run my A-65 at 2300 for cruise.... you won't hurt the engine and you will be only 50 rpm short of an A-75 at cruise.

Good luck

Garry Crookham
N5112M
Tulsa

[allen]

I have a one-time STC for the A-75 installation in my BC12-D. I worked with the Seattle Flight Certification Office a few years back; it wasn't too hard to do. I can send you copies of my paperwork and one-time STC if you would like to use this as reference material. Send me your email address and I will attach the paperwork.

Allen N27599

[taylorcraftbc65]

A-75 performance

Frank,
I have a modified A-65 in my BC-65. The first time I took it up, I had the guy I bought the plane from with me, a generously proportioned fellow. I weigh more than I look, as I power lift, and work out quite often. Anyway, the plane was five pounds under gross weight, and it was summer in El Paso, Tx.. The dencity altitude was 7000 feet, and we took off from the 3000 foot dirt overrun on the east end of the strip. We had a short takeoff roll, Rotated at 45 IAS, climed out at 60, and the plane just jumped off the ground concidering the conditions. I cruised it at 95, as I like good fuel economy, we stayed up an hour, and burned about 5 gal of 100LL. (The engine only had 8 hours on it since a total rebuild). I hope this helps, Sabrina

[Frank DeBartolo]

What do you mean by a modified A-65? Is it still a 65 with some A75 components? Or is it a full A75 with the shorter prop etc as explained by Ron Coleman and Gary Crookham in the previous posts in this thread?

Thanks,

Frank

[taylorcraftbc65]

A-65 to 75 HP

It's an A-65-8 that has had the conrods, pistons,and carb upgraded, and in the case of the engine in my plane, an intake spyder with a larger inside diameter orifice. Hope this helps. Sabrina

[Frank DeBartolo]

Does this change TBO?

[barnstmr]

Increasing the gross weight during a conversion to a C-85 requires some airframe mods.

Garry,
Please tell us which airframe mods are required.. Does this apply for both the BC-65 and the BC-12? I am not that familiar with the airframe differences.

[Garry Crookham]

Sorry for the delay in answering the questions...just got in from a week at Oshkosh...and yes the factory was there with a display airplane.
Terry: my airplane was converted to an 85 in 1955 and according to the 337 was "modified per manufacturer's recommendations:
1. larger fuel lines
2. front spars redrilled to take fiber bushings which were changed to 5/8" OD
3. strut and butt fittings were changed to .095" material and a shear plate added to strut fittings.

It is my understanding that the Gilberti/Harer STC requires the same modifications.

Ron, I'll try to take a picture of the late 65 hp piston and post it. Something to be aware of: there are a couple of different pistons that have been produced for the A-65 and at least two for the 75 hp engines. Some have 5 rings, some 3 rings...some use the .859 piston pin, some use the .922 pin.

I would be very careful about mixing A-65 and A-75 parts...almost all of the parts are the same, and most mechanics wouldn't know,but during an annual a savvy IA could refuse to sign off the annual if he found something that didn't conform.....(guess this is where I should insert my give your IA a break speech..don't try to put something by him, or ask him so sign off something thats not proper, etc, etc, but I won't).

Garry Crookham
N5112M
Tulsa

[jgerard]

http://www.taylorcraft.org/resources.html

This website has a pdf. file regarding the small continental parts interchangeability. Its a great reference tool to have on hand.

Jason
N43643

[Garry Crookham]

Ron.
The 75 hp exhaust valve is legal in the 65 hp engine.....the only real differences between the A-75 and A-65 are the pistons, carb jetting, and mag timing (assuming that the rod caps have been drilled-----the rods will have to be rebushed for the small piston pin to use the A-75 piston).
The latest revision of the A65/A75 overhaul manual and parts catalog (dated August 1977) shows the current piston and valve part numbers. The intake manifold is the same for A-65 and A-75 engines.

One item to note: if you are thinking of installing Superior Millennium cylinders on your A-75 be sure to get approval from your local FSDO before you buy the cylinders...Superior says they are for an A-65 engine and the local FAA folks here wouldn't give a field approval to use them on an A-75. Also, if installing these cylinders on an A-65, the pistons that comes with the Millennium cylinders (they come as a complete kit with pistons, rings, etc) use the .922 diameter piston pin, so make sure your rods are bushed for the proper pin.

Garry Crookham
Tulsa

[alwaysoar]

Are there any drawings or specs available to show how to beef up the strut fittings for the 85hp STC?

Ed @BTV VT
TF 527

[alwaysoar]

Are there any drawings or specs available to show how to beef up the strut fittings for the 85hp STC?

Ed @BTV VT
TF 527

[alwaysoar]

Are there any drawings or specs available to show how to beef up the strut fittings for the 85hp STC?

Ed @BTV VT
TF 527