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Building a "Super 71". Looking for Input.

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Paul Thieme

Sep 26, 2022

I've been using a Cub 71 in regular mowing service for about two years now. I have a 1-acre yard. It was the first engine I ever rebuilt, supervised by my expert mechanic cousin when I was a kid, probably 15 years ago now.

It's getting to the point where it just needs to be gone through again. The engine is getting tired. The PTO clutch needs rebuilt, it will need a drive clutch within the next 100 hours,
etc, etc. So my plan is to just rebuild it form the ground-up as a worker, NOT a puller, into a "Super" 71. I want your advice on three things:

1. What is a good cutting deck that I should use? I'm dissatisfied with the OEM deck. It cuts fine, but not great. It's getting to where it needs spindle bearings anyhow. I plan on custom fabricating mounts for a deck, so I can sling any deck under this thing.

2. The steering has some slop in it at the linkage on the left side of the front axle. I was just planning on brazing this closed and re-drilling the hole to remove the slop. Any suggestions on a sturdier fix?

3. Should I replace the clutch with OEM equivalent parts, or should I upgrade it somehow?

The details about the rest of the build:

- Keep it as OEM-looking as possible
- Regular yard mowing
- Snow pushing (Indiana winters) (add tire chains maybe)
- Pulling rollers, aerators, carts, people at tractor shows, etc
- Stationary electrical generator (PTO) operation. For HAM radio stuff and remote power.
- Stationary air compressor (PTO) operation.
- Snow blower if I can find one

- Kawasaki FH680D 25HP V-Twin (Yes, I know I will need to modify the frame. Yes, the deck lift will clear. No, I will not be cutting the hood.)
- Dual stainless exhaust out the back, under the rear axle, chevy-style with twin mufflers.
- Electric front PTO clutch actuated by OEM deck engagement lever (add: spring / switch)
- Oil pressure, charging current, and system voltage gauges from a Farmall Cub or similar / to appear OEM
- 2 gallon fuel tank under the seat with 12V booster pump

- New clutch (upgrade?)
- Ford 9" rear-end limited-slip differential in OEM cub housing
- Creeper if I can find one
- 540RPM PTO. There are a few ways I have thought to do this... TBD on final implementation
- Duals on rear
- Fabricate 3-point lift system if I can't find an OEM one

- Gauges as mentioned above
- Add automatic oil pressure engine kill switch (rewire OEM sensor) (add "T" for gauge pressure sender)
- LED headlights in the grille, NOT in the "headlight" position, since my mower didn't come with headlights
- Add fenders and rear light / have fenders, need to referb them
- Add 4-pin trailer plug on rear with brake sensor switch
- Add "cigarette lighter" somewhere that looks OEM-ish
- Maybe add tachometer? If I can find one the same size as the other 3 gauges / that matches

Hydraulics (TBD... may not add this system):
- Live hydraulics via pump on front output shaft
- Reservior location: TBD
- Add 1x rear SCV
- Potentially convert deck to hydraulic power instead of belt drive??

I think that's everything... looking forward to hearing your thoughts.

KISS, just rebuild the 7hp engine or put a K301 in it and save your money. Most of those mods will not let you put a deck under it. Right now a 38 ,42, or 48" deck will fit with the current setup on the tractor. Otherwise have fun and keep us posted on your progress.
Cheaper and simpler to get most of the way there would be to sell the 71 to someone who will appreciate it for what it is, and get a 982/1872/2072/etc. and put your choice of engine in it. That will get you provision for a better deck, hydraulics, live rear PTO (not 540 RPM though) and check most of your other boxes without having to reinvent the wheel. It will work perfectly for what you intend to use it for.
Hey all,

While I appreciate your input, and don't mean to sound ungrateful for your trying to help me with my project, the answers I received were not to the questions I asked. I am going forward with this project, and just wanted to see if anybody had pointers before I start digging in. I'm going to sling a 54" deck from an 82 series under it, and by all my measurements it will fit just fine, with some light modification to the raising gear.

What I didn't tell you is that I got the FH680D engine for free, freshly rebuilt with just break-in hours on it. That and I have time, iron, a shop, and a welder.

Also, my brother has a Cub 100 and could use the brand new starter / generator, new voltage regulator, and refurbished deck off of my mower as his are basically worn-out. So I'm not the only one benefiting from this project.

Anyhow, I got the needed frame and engine modifications done over the weekend. Pictures below. I did some of the welding outside, and we just got the remnants of Hurricane Ian while I was working outside (a LOT of wind), so some of my MIG gas got blown away. Welds on the outside of the frame suffered from this... but ground down flush nicely.

Installed new clutch while I was in there. It's a good thing I pulled the engine and inspected the frame, because there were cracks where the front axle mount is welded to the frame, and where the front grille bolts on. Patched the cracks before continuing.

Frame had to be cut and lowered 1 1/2" for the engine output shaft to line up, and a notch in the right side had to be made for the air cowling to clear. Engine fits now though, and looks like it will clear the hood once I relocate the air filter assembly. Reinforced the frame at the cut with some 1 1/2" OD square tubing, which should provide plenty of rigidity.

Here's the front output shaft of the 680D, cut off and ground parallel with a surface grinder. Drilled and tapped center for an alignment bolt, then bolted on custom-made engagement plate with friction disc engagement pins. Bolt just keeps it aligned during welding so everything is square.


Then held everything in place (don't forget the debris screen!) with a clamp during welding.


Then dropped the motor in after the aforementioned frame modifications. Everything lines up.


Here's the structural square tube I added for frame rigidity before welding.


And here's a picture of that frame cracking I went through and repaired.


So as of now - engine is in, frame modifications are 95% done, and new clutch is installed. Next I'll re-mount the front grille with a 1 1/2" spacer I'll make out of some more of that square tube and get all the hood clearances figured out.

Then its on to wiring, locating the fuel tank, and welding up an exhaust manifold that clears the hood. Might need to put the fuel tank under the seat.

After that she should run and drive. Then on to deck mounting, and wiring up the electric PTO clutch to the original clutch engagement handle with a momentary push-button switch and a spring.

Will keep you all posted with progress.
Might want to rethink not having a pilot bushing for the driveshaft. It is there for a reason. When that 50 year old clutch disc lets go because you are putting 25 hp through it, things might get interesting.

The frame will need more reinforcement than that to not have issues later on. Those square corners will crack, and that square tubing isn't really doing much, to be honest. Some flat stock the thickness of the frame or a bit thicker extending both forward and behind that entire spliced area, welded all the way around the perimeter would give it a chance.
I would have welded a little thicker plate onto the stub, so that I could turn it and true it up in the lathe, keeping things completely true while welding is mission impossible. At the speed it turns, a few thousandths becomes a vibration. Holding to about 5 thousandths is all you can hope for once the welding rod is applied.

I'd also put some metal along side the motor flaired at about a 45 degree angle to replace what was cut out, would help make it look more streamlined and add some strength.
I would have welded a little thicker plate onto the stub, so that I could turn it and true it up in the lathe, keeping things completely true while welding is mission impossible. At the speed it turns, a few thousandths becomes a vibration. Holding to about 5 thousandths is all you can hope for once the welding rod is applied.

I'd also put some metal along side the motor flaired at about a 45 degree angle to replace what was cut out, would help make it look more streamlined and add some strength.
I agree completely. Good idea about the 45 degree piece - I'll do that.

I did balance the assembly by the way, up to 4,000 RPM. I just didn't show it. The vibration was negligible, and all the engagement pins remained straight.

I'm about to post an update to this project as there has been considerable progress. Give me a bit here to write it all up.
Continuing on with part 2:

In this stage I focused on installing the engine, relocating the front grille, ensuring the hood cleared everything, and doing all the electrical wiring to make sure everything will fit. I will take it apart and paint the completed frame and hood later, after it is determined that this will all actually fit and play nicely together.

When designing grille standoffs, I noticed that the front axle hinge pin was not making contact with the front bushing in the frame, and that the pin was frozen in the axle bad. I didn't even bother trying to beat it out with a sledge, I just cut the pin with an M18 sawzall and took it to the press.

You can see below that it took ~22 tons of pressure to press that pin out. Yikes.


After greasing the snot out of the axle mounting holes in the axle and the frame, I replaced that pin with a 3/4" x 6" grade 5 bolt. I added a lock washer and some flat washers, tightened enough to give a touch of resistance on travel without any slop, then drilled the nut and threads and put a cotter pin through them to prevent it from backing off.

Next, on to grille spacers.

I used some more of the aforementioned square tube to drill four spacers, to bring the grille back up to proper alignment with the tower, and thus provide a level hood when closed. I clamped these in place above each frame mounting hole before drilling, and center-punched the location of the mounting hole. After these were made, I welded them to the frame. This should add a bit of rigidity as well. I stamped them "A" "B" "C" and "D" to prevent mix-ups during welding.


After spacing... looks like the hood sits level.


Now unfortunately here is the part where I didn't take a lot of pictures. There was quite a bit of "massaging" needed to those grille spacers to allow the engine to slide back far enough to disengage the clutch disc. This required me lifting the engine out of the frame about half a dozen times to ensure proper fitment. The results were worth it though, because this engine fits like it was made for the tractor. I cannot stress how little room there is under the hood.


Obviously the air filter needed to be relocated, and a fuel tank needed to be added. I thought about mounting the fuel tank externally, like an old John Deere LP tractor on the front, but figured that would be tantamount to giving up. Instead, I decided to mount the fuel tank where the generator regulator used to be.

For an air filter mounting bracket, I just cut the old mounting bracket where it bolts to the carburetor, turned it around 180 degrees, and used a hose clamp to secure the air filter.


Next: the fuel tank. I had a spare tank laying around from another project, so I hit it with some CC yellow paint and made a bracket from some flat aluminum I had lying around. This, too, would require some "massaging" later to get it to fit under the hood correctly. More on that later.


With everything pretty much in place, I moved on to electrical wiring. Not much really out of the ordinary here, other than I needed to upgrade to a key switch with a ground pin for the magneto in the "OFF" position. I found one on Amazon for less than $20. I used the original starter solenoid and ran all new wiring for the whole tractor. I used copper impact crimp connectors for everything high-current, as well as for the battery and frame ground connections.


I neglected to take pictures of the new wiring... sorry. Let me know if you want pictures and I'll take some. I added a plug for a battery tender, too.

Moving on, I finished the wiring harness (sans lights - will do those later) and finished massaging everything to fit under the hood without rubbing. The result:


Last thing I got done before totally running out of daylight was the marking of where the gauges will go (center punched), and the installation of the "Check Engine Light" for the oil pressure sensor. This light will come on when I turn the ignition on, then go off once the engine builds oil pressure. A matching (blue) light will go on the other side as a headlight indicator.


That's all for now. Hopefully I'll have the air filter connected today, and have it running and driving!


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HI. One thing I think I would have done different was built a gas tank to sit where the battery is and extend it out over the engine. I have a 27 hp Kawi in my zero turn, if you pull them hard they are thirsty! My zero turn has two 7-1/2 gallon tanks, takes about 3-1/2 to 4 gallons of gas to mow my 2.3 acre yard, I haven't put any gas in the left tank in years! I'd have mounted a full size car battery back under the seat and run heavy cables forward. On my model #72 I've run wires all over the tractor for lights and my AM/FM cassette radio and use headphones to listen to it. The added weight in the back would help Traction.
Midwest SuperCub would have everything you need for a heavy-duty clutch and driveshaft. I don't think the center differential from a 9" Ford traction-loc axle will fit in the Cub Cadet rearend, an 8" out of an old Falcon or 6 cyl early Mustang might be easier to make fit. The garden tractor pullers use a Dodge Dart differential that fit and withstands a lot more horsepower than you have. The bearing retainers that hold the differential are the weak links, they crack, break, and the ring gear and pinion lose proper mesh and destroy themselves. MWSC sells alloy aluminum bearing retainers that are much stronger that the cast iron retainers.
That 2-stage dry pleated paper air filter on that Kawasaki engine is a GREAT filter, my 27 hp has a similar filter. My zero turn stirs up a LOT of dust when I mow, every spring I've replaced the primary outer filter with a new one, think I've replaced it 3 times in 6 years now, have NEVER replaced the inner filter, my parts guy told me "Maybe next year" when I bought the primary filter last spring. The little dry type filters Kohler used on the 7 hp engines was dinky, the 8 hp had a slightly wider filter, same filter as my air compressor! And the 10,12,14 hp had a bigger filter and when used with a foam filter wrap did a good job of filtering dust & dirt, but no where's near as good as that 2-stage filter. I want to replace the oil bath air filter on my two FARMALL'S, Donaldson makes a large family of vertical mounted filters that would be perfect. They advertise them removing 99.95% or better of the dust. Best I've ever seen stated on an oil bath is around 95 percent. I had a '96 F-250 Diesel pickup for 20 years, Stock air filter was pleated paper, I put a K&N in it one year, I did oil analysis on the engine about every 3 oil oil changes, with the K&N the silicon, meaning DIRT, was 5 PPM in the oil, with the OEM pleated paper filter it was Zero or One.
Anyhow, good luck with the rest of your build!
looks awesome. you might want to consider duals in the back and run ags. i would think that would definitely help traction for what you want to do.

now the most important part; what color scheme?
Hey everybody, it just occurred to me that I finished this project and then never updated this thread... the mortal sin of online forms. Sorry! I'll do my best to bring y'all up to speed on this below:

OK, so what's changed since I abandoned this thread...
- Turned my brain back on and added a driveshaft pilot bearing
- Finished wiring harness (and CEL)
- Finished front electric PTO clutch. Having a push-button PTO is really nice. It barely fits.
- Added CHT sensors to detect cooling air intake blockage due to leaves and such
- Added tach to dash
- Added hour meter to dash
- Rear weight rack for snow plowing
- Fenders (Yes, I know they are on backwards - I need to switch them around. Doesn't bother me enough to warrant me doing it at the moment)

As far as performance goes, this thing rocks. The original mule drive / mower deck fit without any modifications. The snow plow fits without modifications too. Currently looking for a creeper gear and a snow plow. I'm totally satisfied with the bump in power and ease of use!

Future mods (to do):
- Find some wheel spacers for duals. I have the second set of tires... just need to find the wheel spacers
- Add creeper gear
- Fabricate live rear PTO run off front PTO clutch instead of transmission. More thoughts on this below.
- Either add a sleeve hitch or fabricate a 3pt. Not sure what I'm gonna do here yet
- Somehow: hydraulics

My idea for a rear PTO:

I want a live rear PTO. I'm never gonna use the mower deck and the rear PTO at the same time, so my plan is to use the front (electric!) PTO clutch to run a driveshaft that runs to the rear of the tractor, which then runs a full-size 540 PTO mounted to the center of the rear diff plate in a pillow block bearing with a V-Belt pulley. For sure this is more complicated than just installing the factory PTO, but it will spin the right direction, and in the end be more useful I think.

Anyhow, photos:





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