66cc rebuild hurdle =piston contacts head

Discussion in '2-Stroke Engines' started by djase10, Jun 20, 2008.

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  1. djase10

    djase10 Guest


    struck a couple of hurdles during my 66cc rebuild ,

    1st problem =Gudgeon Bush to big for conrod
    No worries..
    rotated over a file & sandpaper.
    had to take off about 0.75 mm
    took a while , but got it done.

    2nd problem
    is a concern!!
    the top of piston
    is making contact
    with base of head,
    by a good 2mm
    stopping rotation.

    As a quick solution,
    Using the old PITTED head,
    i've ground out possible contact points,
    using a grinding stone on a drill ..

    not the prettiest of milling jobs
    might just work..

    pictures of the scene so far.
    not finished grinding as yet


    Attached Files:

  2. Jase .

    You are one determined Man..

    Any thoughts on getting or making a thicker head gasket. Also you may want to use some clay on the top of the piston to view clearance to the head. Also putting a small chamfer on the ends of the bushing wouldn't hurt as well.

    You are one great example of ingenuity at its best.

    good luck.

  3. djase10

    djase10 Guest

    Gday bluecollar,
    Yep tried two head gaskets, no good.

    I may try to cut a slither off the top of an old barrel to make a
    "thick head gasket" to get the gap i need.

    keeping it leak proof may be a drama,
    but may also-- just do the trick.

    if i try the slither trick , i'll be smoothing out the barrel before i cut.

    It is strange to see this miss match of parts???


    Attached Files:

  4. Man yeah that piston sticking out a bit, make a thicker base gasket.
    Last edited: Jun 20, 2008
  5. djase10

    djase10 Guest

    After quick discussion on the matter with WANGER during our ride today,
    I think the BASE is the part to play with..
    That is ..
    (If the head trick is a no go??)..
    thanks guys..
  6. I would agree as well.
    As you know by moving the combustion chamber up ( jug and head ) You may reduce the chance of leaks.

    But you would need to keep the ring clearance in mind.

    It seems you got a handle on this and I'm anticipating your Outcome.

    I have a feeling your going to Succeed no matter what .



  7. djase10

    djase10 Guest

    revolution= removed 6 grams from head..

    Hi Guys & girls

    Thanks Bob for your words of encouragement,

    I persisted with the slow process of grinding the inner head , and i won..

    I used texta color to find contact points, done this about 5 times,
    & removed a total of around 6 grams of ALLOY from inner head , before revolution was had..

    Still lots of meat there to hold up to combustion forces ( my nuts hope!!)

    i havnt fitted/started
    it as yet, but im pretty confident all will be cool.

    I will update this once its running.


    Attached Files:

  8. You've done a neat job, I'm not trying to be a smart arse but theres a couple of things you need to consider.

    1st you have made the circumference of the head larger than the top of the cylinder so a portion of the head gasket is going to be exposed in the combustion chamber and it will burn and cause detenation and melt a hole in the piston.

    2nd you will find you have shortened the length and amount of time the exhaust and transfer ports are open which will limit performance.
  9. djase10

    djase10 Guest

    Keeping a close eye on the head gasket.

    Cheers phil,
    yeah i'll be keeping a close eye on that head gasket, ,

    if exhaust is over restricted, i'll have to do a slight grind on the top of exhaust port.

    heres the interesting stuff!!

    For those that may be wondering why this even happened??,
    it's to do with a different gauge GUDGEON PIN.

    my original was a tiny 9.5mm

    and the new piston kit is 11.5mm

    there are two sizes of bush to cater for each one.

    be aware of your pin size
    before ordering a replacement bush.

  10. KiDD

    KiDD Member

    I had this problem also, I just used a thicker base gasket
  11. djase10

    djase10 Guest

    Thanks kidd,
    after 24 hrs of thought, i will do the same ( thicker base gasket)..

    my brides gonna freak out that all the grinding was for nothing :smile:.

    the combustion forces wont be properly utilized, and blown head gaskets will be my new life..

    kidd what material did you use for your base?
    How does a rubber base gasket sound ??

    ( this stuff is heat - acid- & oil resistant industrial rubber mat),
    ==kindly sent to me by [fatboy67] -for sound deadening the sides of motor)

    Thanks to all that have been of help, appreciated !

    regards Jase

    Attached Files:

  12. Rubbers a poor sealer and you only have a surface around 6 -7 mm wide so trying to keep it in place when you tighten it down will be a issue, it will keep trying to push out of the join.
    The best option would be to make a shim from bakelight or similar and glue a gasket to each side with contact adhesive.
    You need to come up a fair bit by the look of it, you would get away with sticking 2 gaskets together with contact adhesive, more than 2 you may have problems sealing, any leaks and it won't run or cause it to run lean.
    Last edited: Jun 25, 2008
  13. KiDD

    KiDD Member

    I just used high density gasket material sourced from the local Kragen. Then I dressed it with some high temp RTV. One gasket was too thin, two was perfect.
  14. djase10

    djase10 Guest

    shaved out head lack any torque or power.

    I tried the rubber gasket and the material just squashed, and metal to metal contact remained.

    I could'nt source base gaskets (late shipment)
    So i bolted on the "SHAVED OUT" head,

    And gave it a test.

    took a while to start (compression was high)

    once running it
    idles lumpy
    lacks any torque (slightest hill & almost dies out)
    minimum acceleration
    struggles to reach 24km/h on flat ground with tail wind.
    go against the wind & its more like 16-18km/h..
    Embarrassing to ride..

    Didnt blow the head gasket, and does'nt seem to be leaking pressure from it.

    My thoughts are that the piston isn't getting the DIRECT push down it requires , and force is being lost in side way pressure within the head.

    Just have to strip it down and wait for the shipment of base gaskets.

    Had to try anyways..
  15. Skyliner70cc

    Skyliner70cc Active Member

    Use factory gaskets and double up on them. Many of the HT engines have double gaskets on the base of the jug.
  16. Willy

    Willy New Member

    It seems as that you have a piston with an incorrect compression height, ( centerline of piston pin to top of piston). This will cause, in addition to piston to head contact, a detrimental change in port timing. You now have a very high compression motor that is not timed properly, hence the poor performance.

    I know you have a lot of time into this project but I believe a new, proper piston with correct pin, bushing, and rings, as well as a new head will go a long way to setting things right again.
  17. djase10

    djase10 Guest

    2mm or so makes SO much difference to timing!!

    Spot on Willy,
    it really is amazing how much timing difference that 2mm can make.

    this was a strong motor before the (mismatched part) problems

    Because of my motor being of old stock ,none of the newer parts match up properly, and my supplier carries none of the older parts.

    Yep just gonna have to double or tripple the base gasket (shipment arrives soon, i'm told)
    30 day shipments suck, but patience is a virtue.
  18. Willy

    Willy New Member

    Jase, not sure of the difference in piston shape, (skirt, shape, compression height) in relation to the old one. Try to keep port timing as close to stock as possible.

    You will also be dealing with a lower compression head now that you have removed material to gain clearance. You may be able to regain some compression by using a thiner head gasket. I have used thin copper sheeting successfully in the past. It may take a little time to cut one out with tin snips and a file or grinder, but they do seal very effectively.

    Don't forget to anneal the copper gasket when you are done making it as it does tend to work harden. Annealing it will leave it nice and soft so that it can conform to the mating surfaces. All you have to do is to get it red hot and then allow it to cool slowly and it will be dead soft.
    Good Luck gettin the old girl running again.
  19. djase10

    djase10 Guest

    Base gaskets, doubled & trippled

    Ok ,
    Received my parcel from zbox
    3 X 70cc base gaskests

    also a pair of main seals for my 50cc =(blew the main seals, =in another thread)
    & bearing chain roller &pin..

    glued two gaskets together,
    but i may need 3....

    to elaborate on the problem=
    the 2mm or so comes from the gudgeon pin size
    old was 9mm
    new is 11mm
    piston height was identical as was the barrel height..

    I hope that this brings the 70cc to its former glory..

    Attached Files:

  20. djase10

    djase10 Guest

    Yes!Yes! Yes! 3 base gaskets does it >>

    # done it.. NO contact!!

    Runs like it should, Great!!

    A bit of a high pitch noise coming from somwhere, nI'll check it out further tomorrow..
    But yeah ..
    What a relief !! ..
    Port timming seems perfect!!..

    thanks for reading ALL...
    . Regards Jase

    Attached Files: