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Help Overboring an FZR/FZX. Options.

Discussion in 'Yamaha 250cc In-Line 4's' started by Linkin, Sep 1, 2016.

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  1. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    OK @Linkin that can help to clarify things, is it possible to measure the valves from that one?
    I didn't realise they produced 2KR's with EXUP valves, learn something everyday
     
    ruckusman, Sep 11, 2016
    #61
  2. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    They are 3HX1/3 models. I believe only produced in 1988 and 1989 then the 3LN's came along.
     
    Linkin, Sep 11, 2016
    #62
  3. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Specs are as advertised... 49.5mm with suitable rings, 14mm little end, 5mm longer skirt and 1mm higher compression height. I still think trimming the skirts would be a good idea even if they clear the crankshaft. As for weight, my untrained hands cannot tell a difference, so I'll have to find some scales. I'm off to the garage to see if it clears the crank and bottom dead centre.

    [​IMG]
     
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    Linkin, Sep 12, 2016
    #63
  4. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    OK so when the GSX250 piston is at bottom dead centre, rocking it side to side on the little end makes the skirt touch the crank counterweight. Would probably be fine when aligned with the bores, but I don't want to take that chance.
     
    Linkin, Sep 12, 2016
    #64
  5. gregt

    gregt Well-Known Member Premium Member

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    Looks like some work on the domes is needed. Check the dome width in line with the pin bores. I had to narrow the domes on the GSXR400 pistons. Exhaust cutaways need moving closer together too - and lose the bumps in the cutaways as they won't line up with the valves.
    I would have a good look on the inside of the piston and see where the ribs are before cutting. Mine I'm sure are longer skirts (not how it sounds lol..) and don't touch the crank at bdc. I didn't cut because there were ribs there bracing the skirts which i didn't want to lose. Bore clearance will be only around .002in so very little rocking is going to happen.
    FWIW 1mm more compression height would be accomodated on a stock camchain.
     
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    gregt, Sep 12, 2016
    #65
  6. gregt

    gregt Well-Known Member Premium Member

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    Have a look at the domes on here and see how much work has been done. Sides cut bumps removed and ex cutaways moved closer together

    FZR150 pistons.JPG
     
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    gregt, Sep 12, 2016
    #66
  7. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Here's a shot of the underside.

    [​IMG]

    Did you do the cutting yourself or have someone else do it?
     
    Linkin, Sep 12, 2016
    #67
  8. gregt

    gregt Well-Known Member Premium Member

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    Looking at those most of the piston weight is concentrated around or above the pin. I wouldn't cut the skirts.
    Compared to the GSXR pistons those aren't very nice...They have plenty of safe room for lightening IMO.

    I did the piston crown work myself - by hand. I could have paid a mate to set up a CNC mill to get the same result.
    I'm very experienced in porting work and fabrication - most of which is hand held tools.
    The big thing is holding a piston securely to do the work. I'll get a pic of my simple - and safe - way to hold one tomorrow.
     
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    gregt, Sep 12, 2016
    #68
  9. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Get ready to hit those piston crowns with a file and bring out your inner Burt Monroe ;)
     
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    ruckusman, Sep 12, 2016
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  10. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    I was thinking more along the lines of piston in a vice and a dremel :)
     
    Linkin, Sep 12, 2016
    #70
  11. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    that's supposed to be used for your toe nails prior to going out on a date :imtheking:
     
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    ruckusman, Sep 12, 2016
    #71
  12. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Been doing some reading. Overboring usually results in increased compression, assuming the oversized pistons are the same compression height, correct? The increasing the compression height will also raise the compression... I'm thinking I may have to run a spacer or several base gaskets. This would also help keep the piston crown modification to a minimum.

    I want to keep this as cheap and as reliable as possible so I think 2 or perhaps 3 base gaskets would do.

    In any case I need to have a set of cylinders bored & honed to do a mock fit, which should make it obvious on which way to go.
     
    Linkin, Sep 13, 2016
    #72
  13. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    All else being equal, increasing bore size must increase compression ratio. Lowering the piston is not the correct way to compensate as the squish band is designed to be an appropriate thickness. Doing a bodgy thick base gasket will ruin this and defeats the whole purpose of trying to increase performance.
     
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    maelstrom, Sep 13, 2016
    #73
  14. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Ok, trying to wrap my head around this...

    Looking at the GSX pistons, the overall dome shape is fairly close, but the valve relief cuts are different. To initially accomodate the Zeal valves I have to:

    Remove most of the ridge between intake valve relief cuts
    Thin out the ridge between the exhaust valve relief cuts (from 6mm to 4mm)
    Make relief cuts on the exhaust side above the top compression ring

    Then I have to worry about the increase in compression height. Compared to the fiddly work of using a die grinder or dremel tool on 4 individual pistons, or getting appropriate conrods made, it seems easier to raise the deck height of the cylinder block by using a spacer like @gregt made, or use multiple base gaskets (not head gaskets) if the appropriate increase in height can be achieved. Squish height would not change from standard.

    To clarify, the spacer would go at the bottom between the cylinder block and crankcase, not between the cylinders and the head.

    Why would raising the deck to match the new piston (with appropriate valve relief cuts) be bad? 1.3mm isn't a drastic change in deck height. The standard cam chain and cylinder liners should be able to accomodate it.
     
    Linkin, Sep 13, 2016
    #74
  15. gregt

    gregt Well-Known Member Premium Member

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    What you need is a cylinder head...Just offer the pistons into the combustion chamber of the head to see what on the dome needs changing. Yes, you can measure the OE piston and duplicate the valve pocket locations. Depth usually won't need changing as the bigger motor they came from should use more valve lift than the FZR.
    Read my thread again - specifically setting the squish.
    You do not want to have to get into machining the squish lands on the pistons - believe me please.
    For a base spacer I'd probably use 1.6mm aluminium sheet - easily worked with hand tools.
    Final adjustment of squish clearance is done by cutting the top of the barrel which is going to be needed anyway to true things up after boring.

    Burt did a lot of things right that are forgotten now. Doing things by hand as he did gives time to look and think about what you're doing - less likelihood of making an error IMO. Once you press go on a CNC you'd better have it right...
     
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    gregt, Sep 13, 2016
    #75
  16. gregt

    gregt Well-Known Member Premium Member

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    As promised. Disc with hole in center. Block on threaded rod. Gudgeon pin goes through hole in block and the piston is then pulled down onto the disc.
    It's a disc because i use it a lot to machine ring grooves to take readily availble car ring sets and it gets set up in the 4 jaw chuck to do that. If it's going in a vice, use a block with parallel edges.
     

    Attached Files:

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    gregt, Sep 13, 2016
    #76
  17. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Guys if the pistons you have are too much work check this site

    http://www.hispeedpiston.com/ the site is good, most of the pistons give pin diameters, deck height and crown heights
    There are pistons in an enormous range of diameters, I really can't overemphasise that...

    specifically from this page

    http://www.hispeedpiston.com/Yamaha/ThaiYamaha/SPARK135/index.html
    The top right one looks perfect for 54mm bore, specs are missing, however it looks almost identical to the single ring Pro-X below
    hispeedpiston_stacks_image_18383.jpg
    hispeedpiston_stacks_image_12216.jpg
     
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    ruckusman, Sep 13, 2016
    #77
  18. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    All things being equal, if the piston contours between both sizes were identical, then the swept volume increase would be compressed into the same head volume of the combustion chamber so there would be an increase in compression ratio.

    Interested to know what it costs you to get the boring work done
     
    ruckusman, Sep 13, 2016
    #78
  19. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    My machinist usually charge's me around $50 per bore, he might be a little cheaper doing 4 though
    I have seen an Z900 inline 4 he repaired that had dropped a rod (ex drag bike), it had smashed the sleeve and needed new barrel's, but he made up a new one from a solid steel cylinder and fitted that to repair it.
    The owner was stoked to get his original barrel back and was able to use it again

    Linkin most bearing shop's and good car part's place's sell 1.6mm thick gasket paper
     
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    my67xr, Sep 13, 2016
    #79
  20. gregt

    gregt Well-Known Member Premium Member

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    Do NOT use a paper base gasket. You want a repeatable precise squish clearance which means a metal spacer and/or the OE shim alloy gaskets.
    If you've got a base gasket which is compressible, cam timing changes too besides the squish and valve to piston.
     
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    gregt, Sep 13, 2016
    #80
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