Bought bottom end from Boy Go Fast, should I be worried?

Discussion in 'Motorized Bicycle General Discussion' started by fischer550, Apr 23, 2014.

  1. KCvale

    KCvale Active Member

    Joined:
    Feb 28, 2010
    Messages:
    3,890
    Likes Received:
    19
    Sorry, I can't help but snicker at even mentioning 80cc talking about these little 2-stroke engines.

    A CC measurement is Cubic Centimeters of displacement of the piston in the cylinder from TDC to BDC.

    Exact mm measurements of cylinder bore x piston stroke movement = total Cubic Centimeter Displacement and nothing is 80cc.
     
  2. Theon

    Theon New Member

    Joined:
    Jan 20, 2014
    Messages:
    1,440
    Likes Received:
    0
    your right MAP 110mm is from end to end.
    Approx 90mm is from center to center.
    And your right KC.
    I believe the 40mm stroke gives approx. 69.5cc
    and the 38mm gives approx. 66cc.

    I know not everyone wants to build a race motor, however most of us want the most we can get out of our China girls with in our capabilities.
    A 38mm stroke is possibly able to rev higher than a 40mm stroke motor, except the port timing is usually wrong on the 38's, the crank balance isn't good enough, the rings are too wide...
    You can all have your 38mm stroke motors.
    And your angle fire heads.
    But not me.
    And it's not all about the highest reving motor, only one of my 4 motors is going to be doin over 9000 RPM, It's about torque.
    With only one gear and wanting to be able to do minimal peddling, I want my bikes to pull well from just off idle all the way to 8500 RPM. I don't ride my bikes around at 80 K all the time, however there aint a lot of intersections, other traffic or cops were I live. The traffic we do have is doing 80 K + so if I'm doin' 75K I feel that I'm not holding anyone up that might try and do an illegal pass on a blind corner and end up pushing me off the road.
    I think a lot of us want our bikes to do over 40MPH and I'm only trying to help.

    And larger displacement as well as longer stroke both give more torque.
    As does higher compression and correct squish which are easier to get with a 40mm stroke motor.
    I've only had one 38mm stroke motor, my 40mm stroke motors piss all over it.
    The 38mm crank is a paper weight for me.
     
  3. fischer550

    fischer550 Member

    Joined:
    Mar 24, 2014
    Messages:
    234
    Likes Received:
    0
    Ok, well I just want to know when I get my bottom end if it is a 38mm or a 40mm. What do I measure to know which one I have?
     
  4. Theon

    Theon New Member

    Joined:
    Jan 20, 2014
    Messages:
    1,440
    Likes Received:
    0
    Easiest way is to measure from the main bearing pin on the crank to the edge of the big end pin.
    4mm means you have a 40mm crank, 3mm has you a 38.
    [​IMG]
     

    Attached Files:

  5. fischer550

    fischer550 Member

    Joined:
    Mar 24, 2014
    Messages:
    234
    Likes Received:
    0
    there's now way of knowing without taking the crankcase apart?
     
  6. Theon

    Theon New Member

    Joined:
    Jan 20, 2014
    Messages:
    1,440
    Likes Received:
    0
    If your piston comes to the top of the deck on your barrel it's probably a 40mm stroke.
    If it sits down in the barrel at TDC a 38mm crank.
    But this will depend on deck height, height of barrel and base gasket.
    But generally if you want to set your squish to less than a mm on a 38mm stroke motor you will have to shave the barrel.
    Again this is why the 38mm stroke motors have an angle fire ('high compression') head, to compensate for the lack of stroke not bringing the piston to deck height.
    A 40mm stroke motor should give correct squish of around 1mm with a base gasket of around 0.4mm and a head gasket of around 0.5mm with a decent quality barrel. Barrel heights also differ, but are generally suited better to the 40mm crank.
    A motor with properly set squish is likely to find the standard angle fire head to have a little too much compression.
    A 38mm stroke motor with a standard barrel and straight fire head is likely not to give enough compression.
     
  7. fischer550

    fischer550 Member

    Joined:
    Mar 24, 2014
    Messages:
    234
    Likes Received:
    0
    But don't most all kits (not all, but most) come with the slant head?
     
  8. Theon

    Theon New Member

    Joined:
    Jan 20, 2014
    Messages:
    1,440
    Likes Received:
    0
    That's cause most cheap kits are 38mm stroke.
    the 40mm stroke motors will have a straight fire head.
    The straight fire is a better head, and also gives more torque.
    Everyone wants the cheapest kit they can get, that's why the 38mm stroke motors are everywhere.
    But if you pay an extra couple of bucks you get a better motor.
     
  9. fischer550

    fischer550 Member

    Joined:
    Mar 24, 2014
    Messages:
    234
    Likes Received:
    0
    I looked around ebay and I could not find a single 40mm stroke engine, even the few straight heads all said 38mm
     
  10. Theon

    Theon New Member

    Joined:
    Jan 20, 2014
    Messages:
    1,440
    Likes Received:
    0
    #30 Theon, Apr 25, 2014
    Last edited: Apr 25, 2014
  11. Greg58

    Greg58 Well-Known Member

    Joined:
    May 1, 2011
    Messages:
    2,881
    Likes Received:
    63
    Since you have already ordered the bottom end install it and give it a try, it could turn out to be a good engine.
     
  12. KCvale

    KCvale Active Member

    Joined:
    Feb 28, 2010
    Messages:
    3,890
    Likes Received:
    19
    I like the Skyhawk 38's with a slant head, you can use an Iridium spark plug which are too long for a straight head 40, and have a new 2 piece more balanced crank so vibration is not a main factor.
     
  13. mapbike

    mapbike Active Member

    Joined:
    Mar 14, 2010
    Messages:
    5,380
    Likes Received:
    1
    go here to this link and scroll down you will see the lowers I run built and run on two of my bikes so far and I have another just waiting to come together, smoothest and best performing engines I 've run hands down. and these are 40MM stroke engine lowers for $69.99 + shipping

    Neil aka ( motorbicycleracing ) here on forum also sells these lowers and complete engines he calls his GenV engines, he sells his cheaper than dax and does his sales with money order or check, great fella and he will help you out any way he can.

    http://thatsdax.com/ENGINE_KIT_PARTS_PAGE_3.html

    here is a link that will take you to one of neils threads that you can use to click on his username and send him a private message if you are ever wanting one of his GenV kits, engines or Lowers.
    http://motorbicycling.com/showthread.php?t=48391

    Map
    .wee.
     
  14. fischer550

    fischer550 Member

    Joined:
    Mar 24, 2014
    Messages:
    234
    Likes Received:
    0
    Well thanks map bike, but I already got the bottom end from bgf in the mail today. Looks nice, has a big front mount and I don't know if this matters but the piston does measure 1 1/16, not 15/16. I am going to put it on tomorrow and see how it pulls. I asked bgf again about stroke size, and he said 40mm. However, I really do not trust him on that one.
     
  15. mapbike

    mapbike Active Member

    Joined:
    Mar 14, 2010
    Messages:
    5,380
    Likes Received:
    1
    Bgf sells three different lowers so you may have a 40mm stroke engine there, the bgf ngines I have are 40mm stroke engines and two of them are the engines with the wide front mount... I just posted those links for anyone that might be interested and knew you had bought the engine already, I say build it up good and enjoy it, I hope it is a good one all the way around and brings you smiles and fun everytime you start it up and take to the road......

    Fred Chelminiski has said that he had got a bgf engine that is a 38mm stroke and that was the basis for my comment originally, but now that I think about it since bgf sells more than one type of lower that could very well mean some are 38mm. And some or one is 40mm stroke.
    Map
     
    #35 mapbike, Apr 26, 2014
    Last edited: Apr 26, 2014
  16. fischer550

    fischer550 Member

    Joined:
    Mar 24, 2014
    Messages:
    234
    Likes Received:
    0
    So if I indeed it is a 40mm, I have a milled slant head. Will it be too much compression? I don't have a compression tester
     
  17. Theon

    Theon New Member

    Joined:
    Jan 20, 2014
    Messages:
    1,440
    Likes Received:
    0
    can you check the squish?
     
  18. fischer550

    fischer550 Member

    Joined:
    Mar 24, 2014
    Messages:
    234
    Likes Received:
    0
    I don't know what that is, haha
     
  19. Theon

    Theon New Member

    Joined:
    Jan 20, 2014
    Messages:
    1,440
    Likes Received:
    0
    You need to measure the distance from piston to head once the engine is assembled.
    I have one engine running a 40mm crank and anglefire head, but I have taken quite a bit out of the chamber to lower compression and have a squish of around 2mm.
    I use solder to measure the squish.
    stick it in the plug hole and turn the motor over, makesure you get the solder right to the edge of the chamber.
    My experience with to high compression and angle fire head had my motor running a little warm and pinging above 8000 RPM.
     
  20. mapbike

    mapbike Active Member

    Joined:
    Mar 14, 2010
    Messages:
    5,380
    Likes Received:
    1
    Theon why do you insist on all the talk of checking squish and things like that with people that dont know what that stuff is, im wondering hy you cnt seem to help yourself from always being over twchnical about things when hat noobs need is just plain and clear simple advice about how to have a good running reliable safe motorized bike, not meaning to offend here but you'd be much better served to share your advi e and opinions on all the high performance stuff in a thread dedicated to that stuff, isn't it obviouse by now that fischer550 doesnt understand what you keep talking about? Most people dont know or care about these things when all they are trying to do is get a bike up and going.... nothing at all wrong with all the knowledge you claim to have, but most of us try to help people think simple and safe and help them to understand the basics of these engines and how to do the things that help the most for noobs like jetting the carb correctly .

    Map
     

Share This Page