Re-surface cylinder head for more compression?

Discussion in '2-Stroke Engines' started by fairracing31, Jul 23, 2007.

  1. fairracing31

    fairracing31 Guest

    I used to re-surface my cylinder heads on my dirt bikes and quads when I was a teenager to bump up compression. It really helps on low and mid range on the powerband. Has anybody done it on a chinese motor? I'm afraid to try it, I'm not sure these motors can handle it.
     

  2. Herrmanator8

    Herrmanator8 Guest

    i would say no...we have a few polaris snowmobiles and the heads have been shaved by a professional, it works great i must admit, but if its not done by a professional and its not precisionly flat, if it is somewhat on and angle, it will lose compression. too much compression is bad for these tiny putt putt motors, theyhave thin walls which only have a thin chrome layer then cast iron, the engine is so small that the pressure will cause extreme wear and will dramatically reduce the life of the engine.

    but, maybe you should refer to a professional. i would not recommend it, i think it would risk the life of your engine. cool ideal though.

    jon
     
  3. fairracing31

    fairracing31 Guest

    My uncle owns a machine shop, so thats no problem. I've never been inside my motor yet so I have not seen how well its built internally. If its anything like the outside No way will I try it.
     
  4. HI,

    I like the idea of a little boost in performance So I am just curious but does anybody have any idea what the compression ration of a stock 70 CC engine is?

    I have heard that when an automotive engine has its compression raised, that only a very minute amount of material is actually removed...Any Idea on how much material would have to be removed from the head to increase the compression ratio by 1 point....(probably mere thousandths of an inch I would imagine?)....and if the head is made of as soft a material as the motor mounts I would imagine it would be fairly easy to shave off using some fine grit sandpaper attached to a smooth level piece of glass?.

    Thanks

    Andrew
     
  5. fairracing31

    fairracing31 Guest

    I used to use a file and a straight edge, but dont do it!! I did that when I could not get to the shop, Take it to a machine shop, must shops will do it between $45-75. You have to mesure between the head and piston to see how much you could take off, its usually a few thousands of a inch. A good machine shop can help you with that. We do it all the time for are race cars, but we take it right to the edge, on our race motors we take out alot depending on how we want to tune the motor.
     
  6. rcjunkie

    rcjunkie Guest

    You can shave off up to 12 thousanths of an inch on the head. To play it safe, try 10 thousanths first.

    Engine compression ratio is about 6.5:1 or so.
     
  7. srdavo

    srdavo Active Member

    can you estimate what 12 thousanths, shaved off, would do to the compression ratio?
     
  8. gone_fishin

    gone_fishin Guest

  9. srdavo

    srdavo Active Member

  10. fairracing31

    fairracing31 Guest

    Sorry guy's, leaving for Mexico tonight for my race Saturday. Dont have enough time, be back somtime next week. Will be happy to have all that info for you when I get back.
     
  11. Have a good trip.....Looking forward to all the info! ;-)

    Personally if our engines are 6.6 to 1 compression....I'd like to bump it 1 point which is still pretty conservative I think.

    Andrew
     
  12. skipu

    skipu Member

    Yup !!!!!!!
     
  13. caddymankc

    caddymankc Guest

    What does a machine shop use to shave the head?Does anyone know of thinner head gaskets being sold? That would bump up compression a bit and you could easily go back if you dont like it.
     
  14. Pablo

    Pablo Motored Bikes Sponsor

    Forget about milling it. Just get a GOOD sheet of flat glass, 12"x12" or a bit larger. Get several grades of wet/dry sand paper. Mix up a fairly strong dish soap/water solution (NOT dishwasher detergent powder or liquid). Wet the paper, glass, head.....start with the roughest grade (maybe 180 grit?) and go in a orbital pattern. Rinse well between paper grades to get the larger chunks out. Go to a 220-300-400 (I can't remember the exact grit numbers)

    I took off about 0.008" this way. Make sure you use a new head gasket, and I used the aluminum flake paint coating on the new gasket as well - installed wet. It does make a difference in low end torque and medium high power. Not huge, but noticeable and not such a huge difference that reliability is sacrificed.
     
    Last edited: Jul 29, 2008
  15. ibdennyak

    ibdennyak Guest

    Good simple and to the point suggestions. Used to flatten warped heads on some of my small engine equipment that way. Worked well.

    You could be a redneck if you use sandpaper too..............:smile:
     
  16. Pablo

    Pablo Motored Bikes Sponsor

    This thread. It has drifted.

    Back to the head. Polishing. Head. OK enough.

    I'll just add mine doesn't leak, but I did replace my khrappy studs last go around.
     
  17. speedrider

    speedrider New Member

    I believe a machine shop would use a milling machine and a larger end mill to do this. the hardest part would be settin it up so it's level, but it could be done.
     
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