Mill head on 91 CR250????

Discussion in 'Bike Mods, Performance, Maintenance & Repair' started by puggerton03, Aug 2, 2010.

  1. puggerton03

    puggerton03 Rookie DRN Member

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    I have a 1991 Honda CR250. I just had to pull the top-end yesterday because it has a fresh rebuild and the person that put it back together didnt do it right and bent the cam spindle and rack for the power-valves. They werent working at all. While I have it apart I was thinking of milling the head. How much should I take off if I use 93 octane gas? Will I notice a differance? It was rebuilt using a sleeve. I dont like those but it looks to be a good job and was running good before except the power valves didnt work. Thanks. Jim.


  2. puggerton03

    puggerton03 Rookie DRN Member

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    Bump, nobody knows?
  3. dirt bike dave

    dirt bike dave Sponsoring Member

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    No direct experience with the '91, but I'd stick with the stock head if you want to continue to use pump gas.

    BTW, if you mill the head, the squish band may also need to be altered for optimum performance, which is going to add to the cost.

    FWIW, increased compression is usually most noticeable at low rpm.
  4. whenfoxforks-ruled

    whenfoxforks-ruled Old MX Racer

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    Milling the head was vague, and not recommended by itself. By raising the compression, your 7 hour top end will be less, and you may need race gas to get it that far. Still game? Vintage Bob
  5. plynn41

    plynn41 Rookie DRN Member

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    No one can answer the question about how much to mill the head without measuring the current squish clearance, which can vary quite a bit on a stock engine because of machining tolerances. Your squish band clearance ought to be somewhere between 1.0 and 1.4 mm for optimal performance. If you mill the head and set the squish band where you want it, it could increase your octane requirements, but most likely it won't be a radical change. And sometimes the engine will run better on the same gas because the combustion chamber will be more efficient.. And Dave is right, you'll feel the difference in the bottom end.

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