trimming the piston skirt

Discussion in 'Performance Mods' started by unraw, Oct 7, 2008.

  1. unraw

    unraw Member

    What are the advantages/disadvantages? Has anyone done this before?

  2. mountain80

    mountain80 Member

    I just did it to mine. It revs like their is no tommorow. Make sure you make it nice and round and radius and polish the edges.
  3. I did too. Much smoother........
  4. Mountainman

    Mountainman Active Member

    I think this will

    raise your rpms

    but - lower your torque

    maybe I am wrong -- hoping to here more from others

    always fun when we Ride That Thing Mountainman
  5. Ghost0

    Ghost0 Guest

    It will not change torque or rpm because it is not altering the port timing but it is reducing blockage on the intake side which can make it run smoother and maybe increase power slightly.
  6. unraw

    unraw Member

    So would it really be worth the time unless I had it apart already?
  7. Ghost0

    Ghost0 Guest

    In my opinion no, there are so many other things that you can do to increase performance before you start cutting on your engine. Have you done your intake and exhaust manifolds? Gaskets? Is you engine even broken in yet? Do you know that your carb is tuned correctly? How does your plug look? When you have taken care of all the basics then you can start doing some of the more radical stuff, tuned pipe, shaving the piston skirt, changing your port timing. Most things will effect other things so start small and work your way up. Just like putting a tuned pipe on there won't be near as effective if you haven't done your gaskets and manifolds or have your carb tuned correctly.
  8. MotorMac

    MotorMac Member


    This looks like it will be a never ending ride to perfection!! this is a great hobby!!!
  9. Egor

    Egor Guest

    Actually it is the poor mans way to increase port timing. Think about it the skirt gets out of the way sooner and closes later, increase of time open. When we were kids we had no way to port but we had a hacksaw so you guessed it we used the saw. I would not recommend it, the pistons were cheep and we had all the time in the world, ours was an experiment and the porting was better on those Hodaka's. The happy time has gotten better in the last few years, I got an engine from Dax and the exhaust was right on 1.1" from the top. I never match ports on these engines they just run too slow in my judgment to warrant the extra work. I do look to see if I can find a thick gasket to insulate the carb though. I have never used an expansion chamber yet, and that was a must for our bikes back in the day, I guess the reason is that my bikes will pull from any stop without my having to pedal so what is the need! Also I am running a 39 tooth sprocket. I have a Whizzer now and the happy will still out pull and climb the Whiz. The Whiz has a higher top speed but I never go that fast. Have fun, Dave

    PS:This is a barrel off a Honda CR 250, Look at the exhaust ports and how they go over the Scavenging ports! I would think that the charge would just go out the port.

    Attached Files:

  10. Ghost0

    Ghost0 Guest

    I stand corrected, I was just thinking that the intake port height itself in relation to the piston didn't change so there was no net effect but it is true that it will start to open sooner and close later so yes it does effect the timing. Thanks Egor for the clarification. I still believe there are a lot of simpler things to do to improve basic performance before we start hacking on our engines though.
    Last edited by a moderator: Oct 10, 2008
  11. unraw

    unraw Member

    I have all the Gaskets and ports matched up as best as I could. I also bored out the intake manifold as far as I could. Then I o-ringed the carb. Those things together made a big difference. Does SBP carry expansion chambers? Ive looked on their site, but I haven't seen one.
  12. Ghost0

    Ghost0 Guest

    No tuned pipes yet. 4 Stroke shift kit is next on the priority list.
  13. unraw

    unraw Member

    OK. i don't think I can afford that right now anyway.