New Piston Question...(?)

Discussion in 'Motorized Bicycle General Discussion' started by mapbike, Jul 19, 2010.

  1. mapbike

    mapbike Active Member

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    HELP NEEDED....Piston Issue

    I am in the proccess of rebuilding a 66cc slant head engine that thrashed on me a while back, I ordered a new piston for it a few weeks ago and today as I was getting my parts together and looking them over I noticed that the new piston has a lot less dome on the top of it than the old piston.

    I will be making a .016" thick head gasket instead of using the stock .027" gasket but even at .011 reduction in gasket thickness my compression ratio will be way to low if I use this piston with the angle plug head.

    Question: Do the straight plug engines normally have the flatter top piston?

    Question: Is it the 6MM head that is for the Flatter top piston or is the 8MM straight plug head also for the flatter top piston?


    Using a caliper I measured from the top edge of the wrist pin hole to the center top of the pistons, the original piston from the angle plug engine measures .965" from this point on the new flatter piston it measures .907" that is a huge .058" difference so I know I need a different head in order to use this piston but I'm not sure which head I need, I would love to have one of the heads that Jim @ Manic Mech. makes but I think they were set up for correct compression with the domed piston and not the flatter one I have.

    The pics I have included show the differences in the 2 pistons, I need some solid facts on this so I will not order the wrong head, I'm not sure who all stocks the head I need, but if anyone has the head I need for this build please let me know either by reply to this thread or send me a PM, make me a fair deal and I will just buy the head from one of you guys that know for sure that you have the right one.

    Thats DAX sells the straight plug head in 6MM & 8MM but I dont know if either one is right for the piston I have.
    Thanks in advance to all and any that can help me on this......!
    Shan
     

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    #1 mapbike, Jul 19, 2010
    Last edited: Jul 19, 2010
  2. demac

    demac New Member

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    im not really good with measurements but clearly one piston is much smaller i think you may have gotten the wrong piston?
     
  3. mapbike

    mapbike Active Member

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    The piston is not smaller in DIA. it just has a shorter dome on top of it, some of the 66cc engines came with a semi flat top piston rather than the higher domed piston, but as far as I know all of the slant head engines use the domed piston.

    You are right in that I did get the wrong piston for the slant head, but all I need to do is find the head that is used on the engines with the flat/Slightly domed piston and I will be good to go with it.
    Thanks for the input.
     
  4. corgi1

    corgi1 New Member

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    being shorter would make the port duration a little longer,that might be a good thing
     
  5. Flight Risk

    Flight Risk Member

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    It looks like only the dome is different. The side skirt distance is the same.
    Can you mill your head down?

    Rod S

    PS. You could always put a little JB weld in the head or on top of the piston to decrease volume. hehe.
     
  6. mapbike

    mapbike Active Member

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    I've shaved about .008" off my head and I plan to use a head gasket that is .016 thick in place of the stock .027 thick gasket that comes on these motors, these 2 things will decrease my combustion chamber and add a little extra compression as long as the head I use matches my piston.
    I have about come to the conclusion that maybe I just need to get the domed piston designed for my angle plug "Slant" engine and that will resolve my issue with this engine rebuild, I ordered the new piston I have off of ebay from BGF and wouldn't you know it is an odd ball that probably isnt even being used in these engines any more, I wont make this mistake again......!
     
  7. mapbike

    mapbike Active Member

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    I have thought about cleaning the inside of my head up real good and then giving it a good acetone bath and then welding more material into the chamber with Alumi-Weld Rod, there is room to add a pretty good amount of material, but I'm not sure what other effects this could have on the way the engine runs, I may give this a try some day just to see how it turns out on one of my other engines.
     
  8. mapbike

    mapbike Active Member

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    I have desided to change the piston instead of changing the Head so this post is no longer an active issue........................we are done with this thread......Thanks for all the input guys.
    Shan
     

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