gt5 new jug and piston

Discussion in '2-Stroke Engines' started by IbedaYank, Aug 21, 2011.

  1. IbedaYank

    IbedaYank Member

    gt5 new jug and piston now the piston hits the head and locks up the motor.
    The fix for this is? I dont want to start doubling/tripling up headgaskets because these motors ar low enough compression already. stack the bottom cylinder gasket till it no longer locks up?

  2. HeadSmess

    HeadSmess Well-Known Member

    usually when the piston hits the head, a low comp ratio is your last concern...

    start measuring bits and find out what bit isnt the same as the original ;)
  3. FelipeCobu

    FelipeCobu Member

    This is just my 1 cent:

    You can ask a industrial gasket supplier and get some "asbestos" covered metallic gasket (similar to a automobile head gasket) to use under the jug. Doing this you can increase the piston deck measurement, mine engine I found it to be 0.7 m.

    I will also raise the ports too, so you have to check the amount off raise w/ this gasket. Based on my inspections raise the jug can also increase the "overlap" time among transfer and exhaust ports, if I'm right it can make the engine runs stronger at high RPM.

    Also the cylinder liner is conical, the more it raise worse the rings will seal (due to increased gap at the ring ends).

  4. IbedaYank

    IbedaYank Member

    no really i would not have guessed
    being that these are the only replacement parts available theirs not much choice is there? Now the question is how to get around it.
  5. IbedaYank

    IbedaYank Member

    jugs/cylinders match but the pistons are slightly off on the new piston.
    The wristpin is just slightly lower causing there to be just enough increased stroke to lock the motor up
    the joys of made in china quality control....
  6. FelipeCobu

    FelipeCobu Member

    I got, maybe they changed the connection rod length, in this case you will have to shim up the jug. But take care and check to ports timing and position.

    Felipe Cobu
  7. IbedaYank

    IbedaYank Member

    .175 of a inch on wristpin location
  8. HeadSmess

    HeadSmess Well-Known Member

    any access to a lathe? trim the crown of the piston... but 4mm is heaps! down to the top ring land! that cant be right? if you turn it over without the head, how far does the piston stick out? just remove that much... :p might only need a trim... a bit of modelling clay helps to check clearances ;)

    gah! its 4mm matey! :p these are metric motors and should be measured as such! gripe over... it might actually be 3.5...? :eek:

    thats a lot. and well, shimming up the cylinder is probably the only answer. if turning the piston is out of the question. this can be tricky. it has to be 4mm( or 3.5?:p) ali plate cut precisely where needed and dead flat.

    using anything "squishy" at that thickness is a no no. ali or steel plate. repeat after me. ali or steel plate. no matter how hard it is when you squeeze it, you are not a running engine! it is made from ali and steel for a reason!

    you can do it if you have a fret saw file drill etc, combined with top notch metal fabrication skills,but its hard to not warp it. get it machined...made of cash? could get fancy with CAD and get it laser cut in various thicknesses...and materials! not terribly expensive and saves a lot of frustrations.

    if the pistons 4mm higher, shimming the head alone will reduce your port timings to something ridiculous where it wont rev over 3000rpm or so... regardless of your comp ratio which will have increased anyway by having reduced the exhaust port timing...(if you measure comp ratios from the ex port closure)

    you have to get the edge of the new piston to the same point in the new cylinder as where the old piston and cylinder were at TDC. stands to reason. um...iunno! id be getting a new one by now... :whistling: gawd im lazy...
    Last edited: Aug 23, 2011
  9. FelipeCobu

    FelipeCobu Member

    It's a tremendous error even for chinese stuff.

    Probably your engine is the 70cc (40 stroke x 47 bore) and you get the jug of a 66cc (same bore x 38 stroke) and that why you piston is jumping out the liner.

    Felipe Cobu
  10. IbedaYank

    IbedaYank Member

    yeah im sure its a 66 cc motor

    ordered the piston from
    we will see what their tech line says later this afternoon

    Attached Files:

  11. HeadSmess

    HeadSmess Well-Known Member

    felipe. check your ruler. 48's and 66's both have 38mm strokes. all mine have anyways... though back in the early days i DID measure 40mm, even on the 48's... i checked my ruler!

    ok. anyways.

    the 66 cylinder will not fit a 48. the "spigot" at the base is bigger.

    a 48 cylinder will not fit a 66. the stud spacing is different, and 48's use m6, 66 uses m8 threads.

    but. as you said, the wristpin/gudgeon is out on the piston by 175 thou... so that is the problem. the piston. NOTHING ELSE BUT THE PISTON. so, either machine the piston, machine up a "shim" plate, or return the piston and get a replacement and hope that yours was just a QC evader... :) tech line could easily measure and tell you if so...
  12. IbedaYank

    IbedaYank Member

    pistons are within .05 in height
    cylinders are within .25
    will post pics of gasbikes solution and all parts
  13. IbedaYank

    IbedaYank Member

    quote from gasbike email........

    Sent: Wednesday, August 24, 2011 5:47 PM
    Subject: gasbike

    Hey fred this is josh from gasbike, send that piston back and i will send you the correct piston the one with the lower hole, and it is the piston for the super rat/gt5a motors write your order number on the box and note that you want to exchange for super rat piston
  14. HeadSmess

    HeadSmess Well-Known Member

    ^^^^theres the answer there^^^^^
    ^^^^and there^^^
  15. IbedaYank

    IbedaYank Member

    problem is the original piston the wristpin is higher in the piston making it have less stroke than with the replacement piston
    I have the regular GT5 not the superrat or GT5A

    would make it a WHOLE lot easier if grubee gave all the parts a specific part number
    to order replacement parts

    measured from top of piston
    motor is at work because i order all my stuff from online/ebay through work
    i will try and get pics of both pistons next to each other today
    Last edited: Aug 25, 2011
  16. HeadSmess

    HeadSmess Well-Known Member

    cool :)

    its something to keep in mind, that theres two styles of engine out... "deck height" of the piston is different... a useful thread :)

    which one has the longer conrod? or is it just that the cylinder sits higher/lower on the crankcase?

    longer conrods equal more power :)

    reminds me of this thumpstar i had. went to soup it up and found that it had a conrod 10mm longer than any similar engine. therefore the cylinder and studs where 10mm longer... guess what i snapped? :rolleyes7: then the following tapping out to m8 studs and dealing with oil galleries :(...grrrr.

    stroke stays exactly the same. thats equal to the crank circle. unless thats been changed?

    these things must be found out and stickied somewhere!
  17. IbedaYank

    IbedaYank Member


    as far as i can tell there are 2 cranks for a grubee motor one uses a 202 bearing the other a 203

    the anglehead motors have a higher compression due to the combustion chamber being smaller
    i do have the anglehead GT5
    as far as i can tell by using a dialcaliper there is no differance in the cylinders
    Boss already sent the piston back this morning so no pics...

    guess the smart thing would be if ordering a piston would be to measure it before ordering
  18. HeadSmess

    HeadSmess Well-Known Member

    two cranks and two crankcases

    6202 15 35 11

    6203 17 40 12

    so once again... is it the stroke, conrod, and/or cylinder to crank height thats different? and what other changes are there? hmmmmms.
  19. IbedaYank

    IbedaYank Member

    without a few to tear down and mic not sure how we can answer
    As far as ordering replacement pistons. measure old one and contact before ordering to make sure you get the right one
  20. IbedaYank

    IbedaYank Member

    cylinder is the same... heads and pistons are differnt