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Heavy flywheel engine for Lawnmower???

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  • #16
    Back when I was young, I fixed lawnmowers by the score. Jacobsen's had a Briggs with a dished shaped rotor, and 4 little triangle shaped cutters. Also some flaps for airflow. These were as good as it got in the 60's. At the time, vertical Briggs had an Aluminum flywheel, too light for naked operation. They depended on the added weight of the blade to make up for the too light flywheel. These old jakes with the heavy rotor/blade assy. were as smooth as it got. I tried the iron flywheel from a Horizontal shaft on a couple rotary mowers, they ran so smooth I couldn't believe it.

    At that time Briggs was careful to use an aluminum key for the flywheel. Prevented crank damage most of the time. I don't have a clue what they do now. I have a John Deere mower with a Briggs on it now, it's 20 years old and it never has missed a lick. Maybe I'll need to pull it down sometime, and then I'll know. Get this engine on your mower, you'll love it.
    I cut it off twice; it's still too short
    Oregon, USA

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    • #17
      I ran a 3.5hp Tecumseh mower engine with no blade,just a 2.5" pulley on my self propelled ladder.You really had to pull quickly on the rope to start it.This past fall changed it out with similar version of that engine,but added a 6" pulley for some rotating mass ,starts easier now.

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      • #18
        My favorite small gas engine of all time is the Kohler K series. Restoring an old John Deere 140 with one... Anyhow, back to the discussion on Briggs, if you want a really good Briggs, you need to buy one of their Vanguard series engines. These are popular for repowering old garden tractors. They are up there with the Honda's. I've only heard good things about the Predator engines from Harbor Freight, mainly from the Go cart crowd. I do know of some people taking the twin cylinder Predator and repowering older tractors with. I have no idea how long they last. I know a K series can go 30 years before needing to be rebuilt.

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        • #19
          Actually, there IS some added risk with a heavy flywheel, depending somewhat on the key system.

          The flywheel is normally on top, while the blade is on the bottom. In between is the crank, which transmits torque well to either top or bottom, but less well THROUGH itself from top to bottom.

          The blade is normally the dominant "flywheel" in the system, with the light aluminum unit functioning to operate the magneto, and act as a fan. it has far less effective rotating mass than the blade.

          If the blade is stopped abruptly with the light "magneto disc" flywheel in place, probably not much bad happens. May partly shear the key, I've seen that to the point that the timing is messed up, but you do not really see a key problem visually. With a heavy flywheel, a sharp stop will certainly shear the key, if it is an aluminum shearable key, and the torque on the crankshaft as the flywheel slows down due to friction may distort the crankshaft and/or score the flywheel seat. If the key is not a designed-to-shear key, there may be serious crankshaft distortion, depending on the flywheel mass, and the actual rate of stopping.

          I've seen the problem with built-up crankshafts, like Gravely, where the pin is held by friction, no splines. That starts the back of the crankshaft wobbling as you look at it, due to the pin shifting in the two discs. The problem seems to be the electric start gear on the back side, which is pretty big and has a fair amount of rotational inertia.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #20
            A hundred bucks later and now it's mine!


            I really like the electric start feature of this engine due to my disabling neck condition that makes pull rope starting not only difficult for me but in fact quite risky. From what most are saying here I think it might be a good idea to remove the flywheel before I install this thing to make certain the key is shearable aluminum, also whenever I have a flywheel off of one of these engines I like to coat the shaft end with antiseize compound to make it easier to disassemble later if the need ever arises. I have done this many times in the past and the compound has never caused a problem, in this case I am thinking it might even prevent damage if the key shears and the flywheel turns on the shaft, well maybe anyway. That's interesting about the taper securing the flywheel in place and although I had never thought of it that way it makes perfect sense, I guess that because I had never thought about it one way or the other I had just always "assumed" the key did that job also along with properly aligning the flywheel for timing.

            In any case I should have my mower going by this afternoon.

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            • #21
              Originally posted by J Tiers View Post
              Actually, there IS some added risk with a heavy flywheel, depending somewhat on the key system.

              The flywheel is normally on top, while the blade is on the bottom. In between is the crank, which transmits torque well to either top or bottom, but less well THROUGH itself from top to bottom.

              The blade is normally the dominant "flywheel" in the system, with the light aluminum unit functioning to operate the magneto, and act as a fan. it has far less effective rotating mass than the blade.

              If the blade is stopped abruptly with the light "magneto disc" flywheel in place, probably not much bad happens. May partly shear the key, I've seen that to the point that the timing is messed up, but you do not really see a key problem visually. With a heavy flywheel, a sharp stop will certainly shear the key, if it is an aluminum shearable key, and the torque on the crankshaft as the flywheel slows down due to friction may distort the crankshaft and/or score the flywheel seat. If the key is not a designed-to-shear key, there may be serious crankshaft distortion, depending on the flywheel mass, and the actual rate of stopping.

              I've seen the problem with built-up crankshafts, like Gravely, where the pin is held by friction, no splines. That starts the back of the crankshaft wobbling as you look at it, due to the pin shifting in the two discs. The problem seems to be the electric start gear on the back side, which is pretty big and has a fair amount of rotational inertia.
              In the case of the Briggs engine the key is a designed to shear key made of aluminum. They will shear if the flywheel isn't tightened properly or if the blade end comes to a sudden stop causing the inertia of the flywheel to shear the key. That is not to say that then crankshaft will never be damaged. Even with lightweight flywheels on Briggs engines abuse such as sudden stops can twist or even break the crankshaft, at times even at the blade end or at the counterweights.
              On older Briggs engines, flywheels were heavy regardless of intended use. That changed over time probably more due to cost effectiveness than any mechanical reason.

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              • #22
                Originally posted by firbikrhd1 View Post
                ...
                On older Briggs engines, flywheels were heavy regardless of intended use. That changed over time probably more due to cost effectiveness than any mechanical reason.
                yes they did....easy die casting, far cheaper than cast iron. But the shear key is certainly, as you point out, not a guarantee. Much less so in the case of the heavier flywheel, as I mentioned.

                The heavy flywheel increases the risk for devices subject to sudden dead stops like a direct-drive mower with no intervening belt.

                Briggs was not interested in what happened with a sudden stop. Not their concern, but that of the OEM customer. Mower companies could have put in a slip clutch to prevent that damage, but chose not to, again for cost reasons. If their consumer customer was blame fool enough to jam the blade, that's outside of the "proper usage" and not warranted.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #23
                  I may be off base, but I would tend to think that the lighter flywheel on a mower engine is part of the quick engine stop on consumer grade mowers. I remember mowing my parents yard 40+ years ago and when I ran the mower too fast into a thick clump of grass, and the mower was about to stall, just pushing down on the handle to lift the mower deck up (and clear out the excess grass) the mower would speed right up to normal RPM. Its rare that I am able to do that today w/o the mower stalling anyway. I like the old unsafe way (without the quick engine stopping) much better. I bought a pressure washer (expecting it to have a bad pump) to use the engine as a push mower engine. Turns out the $75 pressure washer works just fine so I never got around to swapping the motor onto a spare push mower deck.
                  Metro Detroit

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