Horsepower to run an AC66 combine

Charlie M

Well-known Member
I'm wondering what size motor was used by AC if you bought an AC66 combine with a motor instead of a PTO. I've twisted off 2 PTO shafts on mine using my Farmall M (nothing out of the ordinary for horsepower on
my M) and I'm wondering if its a bit too big for the job. If its not too much horsepower am I doing something wrong to make it happen. The PTO shaft on that combine doesn't look to be built the strongest it
could be. My dad had an AC66 ran with a Farmall 350 which is a little bigger than my M and it worked without an issue.
Allis never had superbly hefty PTO shafts (at least, that's my experience with one of their square balers, two all-crops, and two rakes), but if you're twisting it off with an M something's awry. They have a slip clutch beneath the grain bin: Is yours adjusted too tight, or has it been bolted/welded/brazed together? The slip clutches on those all crops could really wear the cutch halves that engage together if they were slipped a lot. If this happened, they would slip too often, and someone might have got a little too creative with how to prevent it. A previous owner put a few stitches of weld on the clutch halves of my 90 to keep it from slipping. I cut the stitches free and found the halves were extremely worn and not meshing well at all, so I drilled it for a 3/8 shear bolt instead.

I've run our 66 and 90 quite a bit with a Ford 5000, 6600, and Deere 2120 (60-70 HP) and never had an issue, so if you're M's causing grief something's definitely awry.
I should add: The lack of live PTO on the M is certainly going to give the combine's driveline a bit of a jolt when you engage it, so I can see why your father might have run the same combine on a 350 without issue.

If your M's giving it that much of a load, I would think something might be binding up in the combine. Are the concaves adjusted to tight, or is a bearing seized somewhere? I'd check the concave clearance, then put a pipe wrench on the PTO and try to turn it by hand. It shouldn't take much effort to spin that combine.
Never dawned on me about a slip clutch being there. It has never slipped since I've owned it so maybe past owner tightened down too much or something rusted tight. The machine turns freely and cuts well so I don't think anything else is wrong. I've got almost 60 years of running tractors that don't have a live PTO including running them on a baler so don't think thats a source of the problem. Didn't really think I had too much horse power but didn't know how much AC was using on them. Someone mentioned the power of a WD or WD45 - is that the size engine they used if you bought one with its own engine?
On my AC66 it has a slip rachet device that is spring loaded. Not a true slip clutch with disk. May need to back off the nut on the one large spring.
I could be wrong, but no literature or info I've ever seen has indicated that Allis offered the All-Crops with a separate power unit. I'm sure many were converted by owners, probably with the same Wisconsins that New Holland and others used - about 25-35 HP depending on the model. The M definitely isn't out of line with what was intended.

I suspect your slip clutch being locked/seized is the main issue. Thinking about my 66 and both my Allis rake/tedder combos (built around the same time as the 60/66), they all slip just a little when you first engage the PTO. Allis might have intended them to slip a little at startup to take away some of the shock load. Those rake/tedder combinations were notorious for shredding gearboxes if you had the slip clutch too tight.

One more thing to consider: Do you have the original ball and socket hitch, or has it been converted to a standard drawbolt? If converted, is the pin location at the same location the original ball/socket would have been? If the bolt location is further ahead, you might not have enough overlap of the sliding sections of the PTO shaft and it would be more prone to twist off. It's not unheard of with converted Allis hitches - you saw it a lot with the roto-balers. They're especially weak going around a corner where the shaft extends more.
And Westga is right: They were more of a spring-loaded sprag clutch, not like a conventional slip clutch. I said 'slip clutch' for lack of a better term.
I've got the original ball and socket hitch. Interesting comment about the pto sections overlapping. Any rule of thumb how much over lap would be typical if it were behind an AC tractor? Only AC tractor I have run was a WD45 and was about 55 years ago.
I would guess you want at least 16'' of overlap for that long pto length, more being better. Lots of other equipment runs with less, but because that shaft is so long you need it both to transmit enough torque and also to stabilize it. The distance from the ball (or pinhole) to the end of the pto shaft should be 14'', which is the standard measurement for all 540 pto tractors, regardless of make. Some tractors have adjustable drawbars that can be unpinned and re pinned underneath at a longer length, but you want to make sure it's set back to about 14'' for pto work. An inch or so longer or shorter usually isn' to enough to cause concern.
The original Allis Chalmers 60 and 66 harvesters were PTO driven (often with an Allis Chalmers WC or WF tractors) or they would be fitted with the same engine as the Allis Chalmers B and C tractors. i.e. not that powerful.
Never saw any with anything except the factory AC engine, Our 60 had that. And when live power came out many were removed and up on canival rides to power them. If you rode a feris wheel it likely was powered with a AC combine engine. Just about every ride at the fair was powered by the AC combine repurposed engine. Later they were replaced with electric motors. The lit for a 66 combine said a 35 horse engine so I think it at that time was the CA engine
Was he raising rhy? Neighbor did and used a Wards tractor because of the slow speed. That was his hog feed. So much straw it had to go slow.
We had several around here. Last year they were made. These had WD engines on them. Several were scrapped and the engine went into their tractor. A few mod's were done to the governor's.
I would bet the majority of those engines powering the carnival rides were actually not combine engines. Allis sold a ton of them engines as power units with clutches just for such purposes.
Your spring loaded slip clutch is stuck or over tightened. My dad owned 2 AC 60 combines and he only used a IH H tractor with no live pto never had a problem with them.

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