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Help FZR250 Rebuild but cant source conrod bolts?

Discussion in 'Yamaha 250cc In-Line 4's' started by ironmonkey, Jun 6, 2021.

  1. ironmonkey

    ironmonkey Member

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    Hi,
    There were a couple of people who rebuilt their FZR250s with new pistons and bearings. However typically on an engine rebuild the conrod bolts require replacing too. But the FZR250 bolts are discontinued on all the major parts sites and cannot source them on ebay, etc.

    I bought some FZR400 ones hoping they will fit.

    If not, does that only leave the option of reusing the old ones?
     
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  2. gregt

    gregt Well-Known Member Premium Member

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    From memory, they're 6mm bolts. Honda uses 6mm bigend bolts on quite a few engines - some of which are more recent than the FZR.
    The important dimensions you're looking for with replacements are - underhead length, head shape and shank diameter.
    I'd ask about CBR bolt dimensions - 250 400 and 600. VF400 and VFR also. The R3 would also be worth a look.

    I've recently been through this with Honda RC30 rod bolts. I found another model bolt 1mm shorter but otherwise identical - and available. They are unusually long due to the large section of the Ti rods.
     
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  3. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Depending on how determined you are with respect of replacing the bolts, which think is very sensible practice, you could investigate getting custom ones made by ARP - their catalogue is huge, they may even have some that are close enough as is.

    ARP do bolts with all of the right design and engineering features, whether or not all of the right features are available in the one suitable bolt will be the question, therefore custom may be necessary.
    I was investigating ARP with respect of a planned FZR250 engine build that never gets initiated...but I think about it lots :prankster:

    @gregt has the right idea also with trying the Yamaha R25 or R3
     
  4. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    If not, does that only leave the option of reusing the old ones?

    Big end bolts on the ZXR250 are TTY, so can't be reused. Not sure if the same applies to the FZR250?
     
  5. gregt

    gregt Well-Known Member Premium Member

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    Motorcycle engines tend to use the bolt shank as the locating method for the rod cap. This makes the shank diameter critical.
    Most ARP bolts are for rods with separate locating dowels. Hence the shanks are waisted, Been there, looked at those.
    Yes, they can do specials - but at a cost.
     
  6. Linkin

    Linkin The Apprentice Premium Member Contributing Member Dirty Wheel Club

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    From what I recall the shanks are spiral fluted, if it matters. Never reused any but I did take a good crank and rod set into another set of cases with new main bearings
     
  7. ironmonkey

    ironmonkey Member

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    Well the FZR400 ones dont fit. They appear to be 7mm thread and different shank diameter too.

    The FZR250 bolts are likely TTY. I cant believe there's all sorts of spare and aftermarket parts like pistons, bearings, etc for these engines but not a disposable item like conrod bolts.
    A rookie at engine building and did not know at the beginning these bolts are disposable items.

    The engine will be used for a little moped anyway, not a proper motorbike, and will rarely see above 10K rpm. I think I can risk reusing them - looks like from the replies that the only way is custom and I'm not throwing that kind of money at this engine.

    Abit strange in all the engine build threads I seen nobody mentions this issue.

    I have another spare fzr250 engine; I wont even bother inspecting its big end bearings if I cant assemble it back together without doing more harm than good.
     
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  8. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    That's a bummer. I was going to suggest using maybe the bolts from a Zeal, but it looks like they're NLA too.
     
  9. Linkin

    Linkin The Apprentice Premium Member Contributing Member Dirty Wheel Club

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    Good thing about Yamaha part numbers, the part number stays the same and the model prefix changes.

    Part no is 25G-11654-00

    I have tried 1HX, 3LN, etc. Nothing. Then I tried 5EB, the YZF-R6 prefix and it appears they are available. Do not know if they will work or what size they are, but they visually appear similar to the 250 ones.

    Have a look: https://www.ebay.com.au/itm/152760049120

    Could also try YZF600/YZF750 / Thundercat prefixes but I don't know the model codes
     
  10. ironmonkey

    ironmonkey Member

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    25G-11654-00's head shape is off - the FZR250 is a narrower oval with a shorter 'straight edge'. Although if its 6mm and the right shank, can one gamble just grinding the head until it fits?

    Although grinding introduces heat which I presume will stuff up the metal?

    25G-11654-00 is also another contender but cannot find a front on image of the bolt head
     
  11. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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  12. gregt

    gregt Well-Known Member Premium Member

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    The R6 ones look possible. Anyone in your area racing one ? It's the off season so someone will have an R6 apart.
    Head shape might not be too much of a problem, there's a bit of room around the OE heads on the 250 rods.
     
  13. thebeefsalad

    thebeefsalad Well-Known Member Premium Member

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    FML. Are all conrod bolts supposed to be swapped out? Just dug through the mc19/22 service manual and don't see any mention of it. Don't mind spending the money but hate waiting on stuff to be shipped overseas.
     
  14. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    As it happens I've got a set of YZF600R conrods on the table in front of me - the bolts will be too long by 3mm at least as the conrod journal is 27mm for the FZR250 and 30mm for the YZF600, BUT the threads look to be longer
    http://www.firstclassmotorcycles.com.au/part/yamaha/4JH1165400#content

    4354febb-9fea-4c3d-a02b-904ee0bf05b7_1.f11ae349ec2e9b44d88786f1e43b3622.jpeg

    Same design features, can you put some calipers on the FZR250 conrod bolts, give me length, diameter of the machined part and thread diameter, I'll get another battery for my calipers and we can then compare measurements

    This might save you from getting bolts that you cannot use

    EDIT - although the YZF-25 and YZF-R3 bolts use a different form of machining/knurling as per the post from @gregt suggesting them a candidates, IF the diameter is the same the 5SL-11654-00 may be suitable also, used in quite a few bikes - see the list here

    https://shop.northstaryamaha.com.au/part/yamaha/5SL1165400
     
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    Last edited: Jun 8, 2021
  15. jmw76

    jmw76 Well-Known Member Premium Member

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    Refer my posts in the other thread on torque to yield bolts.
    Conrod bolts should be designed/selected to not stretch under normal use.
    It is wise to check them for stretch (longer than they should be and/or damaged threads).
    The nuts are generally softer and likely to fail first, so check these also for thread damage.
    I see no harm in reusing "good" components.
    Conrod bolts are swapped out for upgraded units in high performance applications where the load is higher.
    For classic, standard applications, the original bolts should be just fine. Unless there is a known design flaw.

    I would be more concerned that the operation of pressing out old bolts and replacing with new is more likely to introduce other stresses/variations to the conrod itself resulting in poorer clamping of the big end bearing.
     
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  16. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    About the stretch/torque issue: from
    https://www.cp-carrillo.com/ft-2429-frequently-asked-questions-rods.html#q27213
    The fastener system of a high performance connecting rod is most likely the most critical unit of the assembly. As a result it is important that the fastener is installed per instructions and with recommended lube. CP-Carrillo has close to fifty years of experience of working with what have become the most recognized fasteners in the connecting rod industry. Much of this has come from experience which has been tested and validated repeatedly.
    Each CP-Carrillo fastener design is based on a known yield load. During linear stretch of the bolt this load should never be reached. The recommended stretch value is 80% of yield load calculated in a tension – torque diagram.
    Torque figures are the more commonly used method for tightening fasteners, however this is not CP- Carrillo’s preferred method and we will explain why: Torque is affected by numerous variables, all of which can alter the true preload figure you are trying to attain. Particularly, the lube utilized, the condition of the threads, the spot face, dimensional integrity, as well as other variables have an effect. Nearly all of these elements are alleviated if the assembler uses a “stretch” method. CP-Carrillo found that the consistency of high quality fasteners offers the least deviation when using the stretch method, with the result that the fastener will exhibit proper preload using designated stretch.


    The Honda uses nuts. There is no way on God's earth that I would re-use those.
     
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  17. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Not being argumentative, all the points you make are good, @maelstrom may know this off hand, I 'believe' the recommendation in the FZR400 manual is to replace the conrod bolts, on that basis I'd seek to replace them in the 250.

    The bolts themselves don't need to be pressed out, them are a perfect fit into the conrods themselves, we're talking no wobble or wiggle because they serve the purpose of locating dowels also.

    It may come to be necessary to reuse them because there aren't any suitable alternatives, in which case I'd be replacing the nuts as a matter of necessity.

    It's just a bugger that Yamaha haven't kept up production or stock of an essential item.
     
  18. gregt

    gregt Well-Known Member Premium Member

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    The style of the shank surface finish isn't as important as the measured OD. They can be spiral flutes or straight up so long as the OD is correct. They won't be a press fit in the rod, they're supposed to be a firm push fit. I push them in as far as they'll go then draw them into place using a nut - not necessarily the one I'm going to use either.
    The fluting should carry on past the rod dividing line. The lower end of the bolts is the locating means for the cap.
    A couple of MM excess length on the thread won't be a problem.

    If you do have to reuse old bolts and want to be really anal, magnaflux them.
     
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  19. ironmonkey

    ironmonkey Member

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    below are approx. as i'm using a cheap caliper (in milimetres):

    outer threaded diameter = 5.85
    shank diam=5.95
    shorter diam of bolt head = 10.7
    longer diam of bolt head = 13.9
     
  20. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    OK, I don't like to have to be the bearer of bad news, the YZF600R conrod bolts - 4JH-11654-00 are:
    ~6.96mm shank diameter
    ~6.85 thread diameter

    So that rules them out

    Don't take this is as gospel but I suspect all of the R6 bolts will be the same diameter, so too the other models which use the R3 conrod bolts as they're close to 1/2 of a 600 with respect to some parts
    Drats

    What about the conrod bolts from the other 250's?

    This one from a CBR250rr looks interesting - I know that @Gen has some MC19 or MC22 engines partially merged, he may be able to take some preliminary measurements for you
    https://www.cmsnl.com/honda-cbr250rr-mc22-1990-l-japan_model14665/bolt-connecting_13213kaz003/

    Also note that these are used on the 2007 CB250F - so they should be available
     

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