Flywheel Modification

Discussion in 'General Questions' started by BigBlue, Dec 25, 2013.

  1. BigBlue

    BigBlue Active Member

    I have a 72cc Zenoah 2 stroke leaf blower engine that I'd like to use on my project. The engine is brand new and is rated at 4.6hp. I added the 54mm air filter. The intake tubing was rotated 90 degrees.

    Here's the engine:
    HPIM0961.jpg HPIM0962.jpg

    I need to mount a clutch to the flywheel and make an adapter plate, so I can mount the engine to my Staton gearbox. This is similar to what I'd like to do. I found this 72cc project from GoPed Nation. I am not going to use the gearbox or relocate the ignition coil, as posted in the 2nd picture. The GoPed nation poster also removed the bottom half of the engine mount which I don't plan on doing.

    72cc clutch.jpg

    If possible, I'd like to keep the factory ignition, so I don't have to have a battery powered ignition and/or alternator to charge the batteries.

    What I propose doing is making a spacer to add onto the face of the existing flywheel. I'd machine the lip off the face of the original flywheel, so that the adapter and flywheel have maximal surface contact. On the spacer, I'd drill and tap 4 holes that would line up with the existing holes on the original flywheel. I am just not sure how many threads would be left after machining the lip. I would then segment the flywheel circumference into 3rds. I'd then have the spacer tack welded onto the original flywheel with a Tig welder. The length and place to weld the spacer and flywheel would be marked for the welder.

    The reason to divide the circumference into 3rds would be to alleviate any balance issues and minimize contact with the flywheel magnets. My concern is that the welding may damage the magnets. Welding a continuous weld around the circumference would be preferred. If the magnets aren't damaged by the heat and only need to be re-epoxied, I may follow that route.

    By the way, does anyone know the width of the flywheel magnets and a source to purchase?

    If not feasible, I'll machine a new flywheel with the proper depth and use an electronic ignition with dual pickups. I found CDI unit from D&B Engines that uses less power and has a built in voltage regulator:

    AKA: BigBlue

  2. HeadSmess

    HeadSmess Well-Known Member

    the magnets....they arent separate. as far as im aware, theyre cast in situ, then magnetised after everything has been done... look very closely at them! they are not simply pushed in and glued! theres even part of the casting that passes THROUGH them! (in my experience)

    they wont be neo types, just plain hard iron. (alnico, more than likely) really really hard. laminated too, usually. at least they always have been for me. try hacksawing through one. you could, maybe...just weld em, after noting down which is north and south, then remagnetize need something much like an MRI machine for that bit...a really really strong electromagnet!

    be aware that there may be welding issues simply because you have no real idea what alloy is used in that flywheel... so no idea on how it will react with whatever filler the welder uses, and whatever you choose to use for the spacer... or how to heat treat it afterwards... an invisible crack is still a crack...

    my real advice? do NOT TOUCH THEM, OR TRY WELDING. spin it at 9000 rpm and if they let go, they will go through the shroud(if alloy), through you, and through anything else in the immediate vicinity! say 6000 rpm. thats 100 revs a second. put 10 grams on 2 inches or so of string, and spin it at 6000+ rpm. stand behind a brick wall when doing this... could do the maths on centrifugal force but why bother?

    that, straight away, is one of the nicest lil flywheels ive ever seen on a small engine, other than retrofitted RC aircraft. im assuming they bypass some of the blower air for cooling? read...jealous :)

    what i would do, if i was to use a spacer... tap the holes out to TWICE the size, with a nice fine thread(m10x1?). then drill THOSE bolts out, and tap them... bolt the spacer on with the big hollow ones, bolt whatevers going on afterwards to them... if youve seen the adapter nuts they supply with wire brushes for angle grinders...something like that :) internal and external thread.

    OR, now i look at it again... make the spacer to fit just inside that flywheel lip.Youll still have to trim the flywheel a bit on that internal face... notch spacer so it will fit, and be driven by existing lugs, with the flywheel nut holding it to the flywheel. maybe stick two lil m5 bolts on either side of that nut, just to be sure, but...its not experiencing that type of load, only sheer... and if the flywheel lug bits are driving it...theres not even a sheer load! you can make the spacer just fat enough to take the clutch bolts threaded section. then space out from the engine exactly as youve done, keeping in mind that the closer to the crank bearings you can get everything, the stronger it will be...and the narrower it will be! a win win situation, methinks :)

    and, if they be long hex nuts for spacers...weld them together! for all the torque it makes, to drive YOU, is equally and oppositely acting on those mounts... personally, i would make that section from one big lump of alloy, bored out, drilled...SOLID! you could actually get it profile cut from some 50mm plate, only cost say, $20 (depends who does it really... i know the big suppliers here dont charge cutting fees-you just pay more for the material in general.) and then, its just drill holes and face it down to the right thickness!

    machining your own is also a good option, but finding the right magnets will be tricky. just making the taper and keyway is going to be a challenge in itself! once again...9000 rpm is going to let you know if anythings going to break very very quickly... and i believe (i could be wrong!) that the flywheel is balanced... also hard to get right if you make one. (whipstaff balancing works)

    usually one side has a bit more meat on it than the other, or only one side has magnets... iunno. not with that specific engine. just going by what ive dealt with before...

    pop it off, make a mandrel for your drill or something, and see if it feels balanced or not?

    you have to get the holes for the clutch shoes SPOT ON. maybe use that flywheel that takes the shoes as the spacer, with some machining?

    remember, its a diaphgram carb. you can mount that engine at any angle you want, other than cylinder vertical... may make set up easier. i would leave the coil where it is, unless your willing to use a battery powered one and install your own trigger. you will never get the timing right if you move the coil and try a different flywheel. who is to say they key in the same relation to TDC? checking will answer that one, you need a compass! you might get lucky .iunno.

    grrr. been behind a lathe most of today, cutting threads for the tractor implements. nasty stuff. stupid holiday season and everyone being shut! i had to make my own nuts as well! meh. 1"x 8tpi is easy :) my first internal threads :) (yes, i myself am still on a learning curve!)
    Last edited: Dec 26, 2013
  3. keatonx

    keatonx Member

    Why weld it, just use those 4 bolts to hold the spacer on.
  4. BigBlue

    BigBlue Active Member

    After your responses, I'll probably just bolt the spacer on with the four bolt holes in the flywheel. I am thinking of replacing the flywheel nut with a class 8.8 or 10 hex coupling nut. One side of the coupling nut will hold the flywheel onto the crankshaft. The other side, use a bolt to hold the spacer against the flywheel.

    I plan on making my own spacers from steel rod for the adapter plate. I then plan on welding some side plates to the adapter and spacers.

    I plan on adding DRO to my lathe and mounting a edge finder into a boring bar holder that mounts onto the tool post. This will get me spot on for the clutch mounting holes.

    If I need to make my own flywheel, I can use the compound or use a boring head and live center in the tailstock to make the tapers and keyway in the flywheel. Never done it, but what the heck, it's worth the try.

    Calculating tapers:
    Cutting tapers:

    AKA: BigBlue
  5. HeadSmess

    HeadSmess Well-Known Member

    all sounds good. further you can recess that spacer adaptor into the recess in the flywheel the better :)

    a quick sketch (alright, it took half an hour of swearing... im used to MSpaint, this GIMP thing is a pain in the .... ill get it one day! no straight lines! ellipses! or anything! but it has everything else! layers. appendices! smudge blur blend! i just want a rectangle :cry: )


    hope it makes sense, dont need ANY bolts or nuts other than the crank nut you already have ;) just notch out the spacer at the four points on the circumference. (not shown, obviously)

    tapers, theoretically, are a cinch :)

    measuring whats on the crankshaft proves to be slightly harder in reality! ive found around 8`(or 16` included?) is about standard but each manufacturer has their favourite. some scrap, if you do find yourself going down this path, and keep adjusting, until you can get a taper that "locks" onto the crank ... then do the flywheel to that setting ;) when its right, youll barely be able to get it off, the taper holds the flywheel on, the key simply locates it.

    heavier the flywheel, the smoother the power delivery but the slower the accel will be...its 76cc! who cares! vroooom!
  6. BigBlue

    BigBlue Active Member

    I get your idea! Thanks for your time and the drawing.

    I just thought of another possible way to mount the clutch that may be simpler. Remove the flywheel nut. Make a threaded tapered shaft extender (some pocket bike engines with alternators have a shaft extender). Screw it onto the crankshaft instead of the flywheel nut. Install a clutch holder to the shaft extender.

    Here's a picture of the clutch holder. I found a post here: I have a PM to dchevygod to get a contact from Goped Nation.

    Super clutch.jpg

    AKA: BigBlue
  7. HeadSmess

    HeadSmess Well-Known Member

    i wouldnt try driving 4hp through that crank thread! its ok for an alternator, but any real load will stuff it :(

    unless you make the "nut" from steel bar, with a wide enough "flange" to pin and or bolt it to the flywheel...

    will also tend to loosen off on the compression braking unless pinned.
  8. keatonx

    keatonx Member

    I haven't had a problem with putting power through the crank but on weed eaters etc., and never had one loosen off on me. Think of all toes guys on youtube with friction drive weedeater bikes, holding a bike peg drive roller to the flywheel with the crank nut.
    But that's a weedeater, and that shaft doesn't look all that thick for the engine power, so I would pin it in place for sure.

    That flywheel nut looks about flush with the end of the flywheel; why not weld (or get it welded) the nut to the centre of that adapter thing that holds the shoes in place. Then just play around with washers on the flywheel threads until the adapter is touching (or almost touching) the flywheel. Then you can drill small holes through both ends of the adapter and into the flywheel. Then screw the adapter to the flywheel with a small sheet metal screw through each hole
  9. BigBlue

    BigBlue Active Member

    Keatonx, I like your idea, but I am not thrilled about using sheet metal screws at something spinning that fast or have the clutch destruct out in the middle of nowhere.

    I removed the flywheel to get an idea of what the back of the flywheel looked like. I knew that it wasn't going to be solid. It is approximately 12.10mm or 0.150 inches in depth in the area with the letters OPPAMA. The other areas have more depth, but are drilled for what I think is for balancing the flywheel. Also the profiles are different. One area definitely has more depth than the other.

    The lip is approximately 1/4" or 6mm in height from the face of the flywheel. The 4 holes are 6mm X 12.10mm. The two profiles have different depths.

    The shaft extends approximately 0.413inches or 10.50mm from the face of the flywheel.

    HPIM0974.jpg HPIM0975.jpg

    More than likely, I'll use the 4 bolt holes on the flywheel to bolt a spacer that holds a clutch. I'll then replace the flywheel bolt with a coupling nut and bolt the spacer into the coupling nut.

    AKA: BigBlue
  10. keatonx

    keatonx Member

    Now that I see the back I see what you mean, it's too thin for screws. (I was picturing 3/4" of aluminum to put the screw into, and using the long sheet metal screws)

    You COULD just have use small (ie 1/8") nuts and bolts instead of the screws, and position the clutch holder thing so that the holes go through the thin part of the flywheel (so you don't drill into a magnet!).

    Keep in mind the flywheels on lawnmower/harbor freight engines have massive magnets that are 1" in diameter and 1" long (so really heavy) and those only have epoxy preventing them from coming out of their grooves. That flywheel may spin at a lower speed, but the diameter is MUCH more (so more centrifugal force)

    If your worried you can put a blob of weld on the nut to stop it from loosening (I would do this, but I weld everything lol). I would definitely do something so that the nut doesn't loosen tho, ie crush the threads. The nut and bolt doesn't have all the friction holding it in that a screw has.