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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. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    @Linkin
    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.

    I do not know why you want to fit a thicker base gasket. Is it to lower compression or to suit the different crown height of the larger pistons?

    If you are thinking to fit a thicker base gasket to compensate for increase in compression then you are doing the wrong thing. 1.3mm is a huge difference in deck height and the burn properties of the combustion chamber will be drastically altered. After you have modified the piston crowns you use a burette to measure the assembled cylinder volume or some other liquid displacement method and then you can calculate the compression ratio. There is no quick, dirty and cheap way to do this correctly.

    If you are compensating for extra height on the pistons then, as Gregt suggested, aluminium is the best bet.
     
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    maelstrom, Sep 13, 2016
    #81
  2. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    The OEM FZR base gaskets are alloy coated in that black gasket material... one I pulled off the 3LN7 block had been stripped back to the alloy and coated in silicon, clearly it had been reused. The only paper ones I have seen can be bought as a pack from aliexpress and are asbestos. Tried one, it was a poor fit and leaked. I'll stick to the OEM ones.

    Yes to suit the different crown height & avoid the piston hitting anything it shouldn't, not to lower compression. Will see if I can get some sheet aluminium for a spacer.

    Going to dig through the service manual now and gather the clearence specs needed to have the cylinder boring done.
     
    Linkin, Sep 13, 2016
    #82
  3. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    @gregt
    Greg I don't know if this information is any use to you CBR250 valves have 3.5mm diameters
    Some specs are posted here
    http://cbr250.com/forums/cbr250-forum/21113-valve-diameters-lengths-~3-5mm-stem-valves.html
    Measurements taken from used valves
    inlet :- 3.48mm stem dia. Head 18.95 mm. o/all length 80mm

    Exhaust:- 3.46 mm stem dia. Head 16.52 mm o/all length 79.4 mm

    I suspect that extra length won't allow shortening and cutting a new collet recess, but I don't want to pre-empt your genius in any way
     
    ruckusman, Sep 16, 2016
    #83
  4. gregt

    gregt Well-Known Member Premium Member

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    77.5 Xs 79.4.....it's probably do able. Trim the exposed stem above the collets and possibly take a little out of the inside of the buckets. If the cams are reground as my inlet, no probs at all, that base circle came down just over 1mm.
    Might need to thin some shims but...
     
    gregt, Sep 17, 2016
    #84
  5. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    @gregt Well there you go, looks like you're across any potential necessary changes. I need to re-read your original thread, however from what you've got here, the total lift was increased by grinding the back of the cams.

    Would you need to redo the hardening on the valve stem tips?

    What about shim washers below the springs to get back the spring installed height? Necessary?
    Anyway precisionshims.com.au has a full selection of shims, really reasonable prices.

    Should be no issues getting some MC22 valves to at least inspect, I've asked about different outer diameters between various models, seems not to be any, however on this page you'll see different stem diameters listed, just something to be aware of
    http://2fiftycc.com/index.php?threads/honda-cbr250rr-mc22-specifications.2521/

    BTW have you got your bucket racer firing, noticed any excess vibration?
     
    ruckusman, Sep 17, 2016
    #85
  6. gregt

    gregt Well-Known Member Premium Member

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    I wouldn't try hardening the tips - even putting a stellite overlay on something that small risks overheating the material. Once it's overheated, it'll break. The loadings on the tips are pretty small anyway.
    Yes, shims under the springs to regain correct fitted height
    Going North next weekend to collect a chassis for the motor. Once i've got something to carry it, then i can look at getting it running. Aim is Southern Classic festival early December.
     
    gregt, Sep 17, 2016
    #86
  7. gregt

    gregt Well-Known Member Premium Member

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    Actually a couple of memory neurons have collided, Collets...
    I know from past experience that in the 5.5mm stem size, between Honda, Suzuki and kawasaki you can find collets with the raised groove engagement bump at top, middle and bottom of the collet bore.
    I've used this way back to solve a spring retainer/cam follower clearance problem.
    Worth having a look at the 3.5mm Honda collets you have and comparing them to yamaha ones to see just where they engage the groove in the stem. Ideally you want ones with the bump at the top end so as to get the retainer as far down as possible.
     
    gregt, Sep 17, 2016
    #87
  8. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    I haven't got any valves/collets as yet, just scoping potential parts. OEM honda valves are very expensive indeed ~$480 for a set of 16

    It's very good to know about the collet differences as that's something which may come in very handy indeed
     
    ruckusman, Sep 17, 2016
    #88
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