Converting A65 to A75

[waltermrich]

Does anyone know of the Continental service bulletin that tells how to drill the connecting rod of a A65 to an A75, from what im reading most of the later rods were all drilled on the small Continentals, also from the Continental manual the A65 rods had a larger piston pin than the A75 so using the A65 rods for A75 pistons might be a mute point.Any one might have a set ?

[Ragwing nut]

why? by the time you shorten the prop to turn it up you dont gain anything.

[waltermrich]

Not what I have heard , Had a friend that put the A75 pistons and rods and the appropriate venturi in his Carb and has a marked improvement in performance changed his prop too.

[Garry Crookham]

The A75 connecting rod is the same as the A65 rod with the exception that the rod cap has a 1/16 inch diameter hole drilled in the cap and a different bushing for the .859 inch diameter piston pin. Also the A75 valves have stellite faces. If I remember correctly ( all my manuals and service bulletins are at the airport, I'll look tomorrow) both the A65 and A75 have a 1 1/4 inch carb venturi.
All of this info is in the Continental A65, A75 overhaul manual and parts catalog.

[waltermrich]

On Harry Fentons web site on Continetal engines he claims the A75 carb venturi is 1-5/16 inch the main jet is #46 and the airbleed hole is #60 all of this in the C75 manual.same carb as A75

[Edwin Otha]

Did an A75 for another aircraft. Like Mike said, after you get to the RPM required to make the 75HP fuel burn is too high and the prop isn't enough to pull the airplane around. It was more of a marketing gimmick than an actual 75HP engine.

[waltermrich]

Im hearing conflicting reports, I rode in a 39 Tcraft with an A75 and was impressed.

[3Dreaming]

On Harry Fentons web site on Continetal engines he claims the A75 carb venturi is 1-5/16 inch the main jet is #46 and the airbleed hole is #60 all of this in the C75 manual.same carb as A75

Also on his website for parts interchangeability the list says A series is 1 1/4, and the C-75 is 1 5/16. So contradiction even on his site.

[3Dreaming]

A simple light pre war airplane will perform much better than a later 1200 pound gross airplane. It could be as much as 150 pounds lighter empty weight. My 1941 BL that I restored was sub 700 pounds, and I have seen BC12-D's that were over 850 pounds. That 150 pounds does way more than the supposed extra 10HP.

[waltermrich]

On another note the rod caps are also drilled with a 1/16 inch hole on the A75 which as said before many 65s all ready have as mine does the piston pins are smaller though. So you cannot put 75hp pistons on 65hp rods unless you change the pin bushing. Harry Fenton thought the 75 hp grade was worth it.

[Ragwing nut]

there is no difference in the engine that gives you any more performance. It is merely gained by shortening the prop to wind up to higher rpm. The venturi change is only so you can properly feed it. Take it from someone who has built and flown all versions, all on a t-craft.

[Ragwing nut]

A simple light pre war airplane will perform much better than a later 1200 pound gross airplane. It could be as much as 150 pounds lighter empty weight. My 1941 BL that I restored was sub 700 pounds, and I have seen BC12-D's that were over 850 pounds. That 150 pounds does way more than the supposed extra 10HP.

We have 2 champs on the field. One is a stock light 7ac with a65 and the other is a 7ac with o-235 and full interior and fat. the a65 will out climb the o-235 all day long.

[Garry Crookham]

Harry Fenton knows a lot about small Continental engines but I think he is wrong about the venturi. I have 13 overhaul/parts manuals for the A65 A75 dating from 1941 to present and they all show the A65 and A75 use the same carb. Also continental service bulletin M47-16 gives instructions on converting an A65-8 to an A75-8 and no carb work is included. I'll attach that to this post. Also the Stromberg NA-S3A1 overhaul manual indicates that the A65 and A75 use the same size venturi. I'll also attach the spec page from that manual.

[Garry Crookham]

A couple of other A65, A75 points: The latest A65 piston p/n 40731 uses the large .9215 dia piston pin so if your engine has those pistons installed the rods will need to be rebushed to use the .859 dia pin for the A75 piston. If your engine has the earlier A65 pistons, p/n 4648 or 40312 these pistons use the .859 dia pin so there would be no need to install new bushings. Also there are two .859 dia piston pins, p/n A-21422 and A-25127. A21422 is a thin wall pin with an id of .626 and is subject to failure and AD 46-36-01 requires replacement with p/n A-25127 (.594 id). I've never run across an engine with the thin wall piston pin installed in the 60 years I've be around these engines but have been told there have been a few found to be installed after all these years.

[waltermrich]

Thanks for presenting that