Rebuilding bottom end crank

Discussion in '2-Stroke Engines' started by Pottus, Jul 21, 2014.

  1. Pottus

    Pottus New Member

    Ok well I had some issues with my engine with the cylinder being damaged a piece of metal broke off of the transfer port and ruined the cylinder. I noticed when I put my finger in the connecting rod and pushed my bike there was a clicking noise and and a bit of metal on the fly wheel so I decided I had to do a complete tear down and clean it out.

    I noticed that the cheap china bearings were not smooth I have already bought high quality replacement bearings. Now another issue is was the needle bearing on the bottom end crank broken and needs to be changed but I'm not sure how to get the crank wheel apart any suggestions? I know that I need to pop the pin but it's really in there methods?
    Last edited: Jul 21, 2014

  2. HeadSmess

    HeadSmess Well-Known Member

    buy a new crank...

    the crank needs to be heated up to about 150 degrees C, then pressed apart. the service will cost more than a new crank.

    and it requires a special mandrel to hold the two halves in line when they re-press the thing. notice how theres two small holes just around the shaft itself? ahuh. two guide pins go in there when they press them, so the halves will line up again. ANY inaccuracy, no matter how small, and it will just destroy the crank bearings, or the crankcases, or both. and the whole assembly needs to be heated up a fair bit to do it without damage (AKA shrink-fit)

    and it has to be pressed back together to within a thou or two of the original spacing...

    simply not worth it :) not impossible, just not WORTH it.

    at this point, a new engine is only a few dollars more usually... ;)
  3. Fabian

    Fabian Well-Known Member

    Buy a new crankshaft and connecting rod assembly, and make sure you get a crankshaft with the crowded needle roller bearing system.
  4. HeadSmess

    HeadSmess Well-Known Member

    just to make sure it fails if you dare rev it over 5200 rpm ;)
  5. Fabian

    Fabian Well-Known Member

    It's heartening to see you offering such sensible advise HeadSmess: to keep engine rpm under 5,000 rpm to ensure good engine reliability.

    Was it "you" that made a video showing how reliable your bike is?

  6. butre

    butre Well-Known Member

    if you gear it right and do mods to improve low end there's nothing wrong with only revving to 5200. I'd love to have an engine that makes peak power at about 3000 rpm
  7. HeadSmess

    HeadSmess Well-Known Member

    actually, it was a video to show it went fast. so it was a bit of a stuff up :jester:

    still, my crank didnt seize...didnt blow up, didnt have major catastrophic failures, and was a good reason to get off my butt and finally make a proper bracket for the exhaust like ive been meaning to for...mmm...a year now? :jester:

    im failing to see how a broken exhaust bracket has any relevance to the performance of my small poxy lil engine?

    oh it was struggling... i pedalled to get it started. thats now a crime. had to carefully predict all of, gear changes to maintain the carefully gained 1/2mph...despite it being a single speed, and running a 36T on a 700c type wheel!

    or a flat tyre due to hitting a POTHOLE? because skinny racer tyres have SO MUCH rubber to protect from sudden impacts, of course...

    maybe it was shrapnel from my grenading engine that caused the puncture? :wacko: the pothole being a figment of my imagination?

    question...wheres your video of your bike doing anywhere near that speed, when its not dragging ten trailers up a 45 degree slope? im waiting.... or maybe....maybe you just talk talk and talk some more :rolleyes: you also have to do it in top gear thankyou, dont pedal, and take off like that. both you and the engine in top gear. remember, you need 36T on a 27 inch wheel... or its cheating :p

    i did take your advice last night... finally fitted a rear view mirror, so no freakin headchecks anymore!
    Last edited: Jul 23, 2014
  8. HeadSmess

    HeadSmess Well-Known Member

    two strokes have no bottom end, well documented. theres a reason MOST are rated for around 7000 rpm +
  9. Fabian

    Fabian Well-Known Member

    None of these things happened because your bike wasn't reliable enough to travel any distance long enough for those things to happen. Parts were falling off your bike after not more than about 2 miles, which is a fortunate thing in the prevention of blowing your crank.

    Maybe it's best that your bike is built to such precise levels of unreliability.
  10. Fabian

    Fabian Well-Known Member

    Once you've installed rear view mirrors, you'll only realise how important they are when riding a bike without them.
  11. HeadSmess

    HeadSmess Well-Known Member

    i needed some milk, i forgot my wallet when i went to work this morn.

    so i chanced it, because, y'know, this (stock standard) 48cc thing of mine is always breaking down, to ride to the shops and get some milk.

    so yes, its nighttime. its mostly a video of black :) it wasnt for me, riding, but this camera...well... so much for Hewlett Packard :jester:

    mind you, i rode to work this morning, now i have a new rear tyre and tube. so thats 50km plus this 5km so far today.

    and in about half an hour, part two, the ride home will upload. making 60km today. which is about the norm for a daily ride.

    im loving this new exhaust bracket :)

    and yes, rearviews are quite important. they are mandatory on real motorcycles for good reasons ;) shame noone passed me today :(

    good thing i posted that last video. made me make one or two NECESSARY modifications. unlike "crowded needle bearings".

    a spring mount for the exhaust, so it wont vibrate and break its mount again!

    a heavy duty 700c tyre and tube!

    a mirror!

    oh, the joys of being a chronic procrastinator :) seriously kids, dont. it bites you on the derriere! get up and do it! NOW!!!!

    so fabian... as the old TAB ad said...put ya money where ya mouth is...waiting for YOUR video still?
    Last edited by a moderator: Dec 15, 2015
  12. HeadSmess

    HeadSmess Well-Known Member

    oooh, the congestion cleared up and it suddenly went turbo mode!

    the next 5km... took the back route home, the way with that evil pothole :jester: even darker! so a boring video, it is! more audio than anything.

    yes, my "quick run" to the corner shop is 5km. and when i go do grocery runs, its about 12km... or approx 8 miles.

    im over this. time to get back onto some serious projects :jester:

    like that dyno that still hasnt gotten a bench yet!

    (its nothing personal, i just dont like seeing...well, not BAD advice. just advice that suits one particular style of riding. once again, i feel no need to drag dead horses or other heavy items up steep bush tracks. nor do i feel that a twostroke should be run UNDER 5000 rpm unless idling. i havent had to buy any engine parts in about two years. at least, not for this particular motor. go figure. stop telling people that your way, and ONLY your way is correct. it isnt. neither is mine, truth be told.)

    sorry OP :) i hijacked :) i simply dont want to see people being misled. a common practise on forum boards. constant :icon_bs:
    Last edited by a moderator: Dec 15, 2015
  13. Fabian

    Fabian Well-Known Member

    Long term reliability under arduous conditions isn't going to happen revving your engine well over 5,000 rpm.
    Sure, if you just want to make a complete nuisance of yourself in the neighbourhood revving the rings out of your engine for a 400 yard dash to the corner store, you don't need the engine or the bike to be reliable.
  14. Fabian

    Fabian Well-Known Member

    There is enough info out there that graphically shows the reliability of my bike, not to mention it's load carrying capacity.
  15. jaguar

    jaguar Well-Known Member

    I think there is a "sweet spot" for rpm range for engine reliability, not too low and not too high. Higher rpm spreads the load out more per cycle but raises the inertia forces causing stress to the metals. Low rpm has more load per engine cycle but less inertia forces.
  16. butre

    butre Well-Known Member

    I never said anything about 2 strokes. something like a predator 79cc, which is rated at 3 hp at 3600 rpm would be perfect. a 2 stroke is normally built to be revved to the moon
  17. Fabian

    Fabian Well-Known Member

    That statement is a complete load of rubbish as a generalisation.
    The port shape and port timing of a 2-stroke engine determine where maximum torque and maximum power will be generated.

    A 2-stroke engine can be designed to give truck loads of torque at low rpm if so desired.
  18. HeadSmess

    HeadSmess Well-Known Member

    hmmm, this is the same engine, when it was all new and shiny...well, at least the head nuts were!

    whats the date? bit over a year ago now... new since i made that pipe...

    though i have forgotten a few spat the pinion gear bolt out through the clutch cover. so i had to replace the cover.
    i guess a CR machined head would have prevented that from happening!

    finally got around to checking the clutch last night. so, thats another 15 parts. pads worn down to the slotted disc.

    of course, thats because it only ever gets ridden less than 2 miles...actually, make that 400 yards to the shops.

    5km IS 400 yards, right? im still sorting out basic arithmetic, though you definitely have a firm grasp on it :)

    there was the crank seal, that stopped it dead... well, stopped it from starting again. thats because its a highly unreliable engine :) only took over a year to show up how unreliable it really was. a shift kit would have obviated that problem instantly, of course. as would reed valves and a walbro carb.

    17 parts in a year. oh, what an unreliable POS it really is! and yet i ride it...wait...PUSH it to work and back home again almost every day because its always breaking down, and i just love pushing my motorised bicycle upwards of 50km every day. it saves on fuel this way, not actually starting the engine, just in case.

    maybe you are right after all :jester:

    Fabian likes this.
  19. Fabian

    Fabian Well-Known Member

    I love your humor. It hits my funny bone every time.
    If only we lived somewhere near each other, we could organise a ride together to see who hauls the other persons bike home, or drags it home on it's side, should the wheels or tyres or pinion gear eject themselves from the frame, and by the sound of it, i'll be hauling your backside home, if you even make it anywhere near the destination:

  20. HeadSmess

    HeadSmess Well-Known Member

    no no no! not the GEAR!

    the retaining bolt :) it does pay to at least have a look and make sure things under covers have been tightened before operating these things, i will admit that! it was just sitting there idling at home, making a nasty grinding noise then pop! out it came. i was surprised, to say the least :jester:

    (only a month old!)

    i reckon i would make it to melbourne but ive stated my theory on travelling south before. they get narky down there. and this is a racer, not a semi trailer... pffft, fancy putting a lambo up against a...well, another lambo... :jester:


    oh. someone beat me to it... :wacko:

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