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Thread: Boring....

  1. #1
    Canadian Beaver Inspector jkindt's Avatar
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    Cool Boring....

    When you bore out a cylinder for larger pistons, do you also need to bore the head? I noticed the head's squish zone ends in line with the cylinder wall. By overboring the cylinder this would leave an edge where the heads bore is smaller than the cylinder's bore. What is the usual practice with this? I am most likely going to bore out some 750 jugs to 71.5mm (1.75mm oversize) to give me a total of 783cc's with the low/midrange porting of the 750! I have also thought about just boring the things out all the way to the max at 72mm (2.25mm oversize) to get to 795cc's! Either way, this SL650 is gonna rock!!!

    I have often heard "there is no replacement for displacement"


  2. #2
    AWA Member 32DegH2O's Avatar
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    Yes, with the size bigger you want to go to, the heads need to be adjusted.

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    ujustlost's Avatar
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    Quote Originally Posted by jkindt View Post
    When you bore out a cylinder for larger pistons, do you also need to bore the head? I noticed the head's squish zone ends in line with the cylinder wall. By overboring the cylinder this would leave an edge where the heads bore is smaller than the cylinder's bore. What is the usual practice with this? I am most likely going to bore out some 750 jugs to 71.5mm (1.75mm oversize) to give me a total of 783cc's with the low/midrange porting of the 750! I have also thought about just boring the things out all the way to the max at 72mm (2.25mm oversize) to get to 795cc's! Either way, this SL650 is gonna rock!!!

    I have often heard "there is no replacement for displacement"
    Xlint89 has a set of pistons for sale. I believe they are 71.0mm Good price too.

  4. #4
    Canadian Beaver Inspector jkindt's Avatar
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    Quote Originally Posted by ujustlost View Post
    Xlint89 has a set of pistons for sale. I believe they are 71.0mm Good price too.
    I know I think they are actually 71.5mm, but he may have more sets than I remember!

  5. #5
    Hayabusa's Avatar
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    Quote Originally Posted by jkindt View Post
    When you bore out a cylinder for larger pistons, do you also need to bore the head? I noticed the head's squish zone ends in line with the cylinder wall. By overboring the cylinder this would leave an edge where the heads bore is smaller than the cylinder's bore. What is the usual practice with this? I am most likely going to bore out some 750 jugs to 71.5mm (1.75mm oversize) to give me a total of 783cc's with the low/midrange porting of the 750! I have also thought about just boring the things out all the way to the max at 72mm (2.25mm oversize) to get to 795cc's! Either way, this SL650 is gonna rock!!!

    I have often heard "there is no replacement for displacement"
    I have never "adjusted " the head for an overbore on a non race engine and I have not had a problem. However, you should use the proper head gasket as they do come in a couple of slightly different sizes based on the amount of overbore. Overbore will also effect the compression so be sure you are still within the spec that you want it to be.

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    You should be able to get away without boring the head up to .5mm or so.

    Anything over that I would think the head needs to be addressed as the piston may start to make contact with the lip hanging over into the combustion chamber.

    As Hayabusa said, the larger bore with stock size head will also raise compression. So you need to check squish and compression after assembly to make sure you're not going to have detonation issues.

    1.75mm OS would DEFINATELY need to be bored to size

  7. #7
    Canadian Beaver Inspector jkindt's Avatar
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    So...boring the 750 jugs out to 71.5mm (1.75mm over), will require head work. Is this a standard machine shop job or do I need to have them done by one of our well known pro's? What would happen if I just used 780 heads on the bored out 750 jugs? Then the head is only 0.20mm over what the stock 780 piston diameter is, that should work shouldn't it? What's the difference in the volume of a 750 head compared to a 780 head? Will the compression go up or down?

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Maintaining the angle of the squish band, and enlarging the head bore shouldn't be too difficult for a regular machine shop.

    Now the questions are:

    Will that be too much of a squish band?

    What's the compression going to be with the stock sized head volume and larger bore?

    These 2 Q's I think would be left for the pro shops.


    The 780 heads was really good thinking on your part. However, the 780 bolt pattern is ever so slightly larger than the 650 or 750. You can hog the holes out a bit to make it fit, but I'd have to wonder how well the seal will hold. (it may actually work)

    Things I do know:

    The 780 has less squish clearance machined into the head vs. the 750. DEFINATELY needs to be checked. Ideal squish would be between 50-55 thousandths.

    750 head volume is 30.3cc

    780 head volume is 28.5cc

    So, a 780 head would be a good contender for a 71.5mm 750 engine.

    Squish clearance and the head sealing would be the only possible issues I see with that set up.

  9. #9
    Canadian Beaver Inspector jkindt's Avatar
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    Guess I have some calculations to figure out. Good thing I paid attention in math class!
    Last edited by jkindt; 11-06-2009 at 04:53 PM. Reason: Spelling

  10. #10
    Canadian Beaver Inspector jkindt's Avatar
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    Has anyone actually tried fitting the 780 heads on a 750 cylinder? From what I understand, the squish zone clearance can be adjusted with different head gaskets, as long as it is not too far out. The compression ratio would be a concern, and that would require modifying the center area of the head, correct?

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