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Interchangeability of Cylinder Heads

Discussion in 'XJ Technical Chat' started by drdee.1, Feb 17, 2013.

  1. drdee.1

    drdee.1 Member

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    Anyone know if I can fit Xj650 mid 80's Cylinder head on my 85/6 xj700....I know this will offend the purists, but, needs must as the devil drives
     
  2. day7a1

    day7a1 Member

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    Cylinder head???? Valve cover yes, the cylinder head is a totally different size.

    At first I was tempted to just say no, that's absurd. But after looking at the parts fiche it does look like that it MAY bolt up....but then the question remains regarding the gasket size (probably wouldn't match up and seal), compression ratio (you may blow up your cylinder with a too small combustion chamber, 50 cc is an almost 8% difference in size), cam timing (which camshafts are you going to use?), and a whole host of reservations about what could go wrong!

    In short....can you fit one, probably yes. Would it actually run....wouldn't count on it, mostly due to gasket size.
     
  3. quebecois59

    quebecois59 Well-Known Member

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    Here are the cylinder head parts numbers according to the diagrams:

    mid '80's Maxim 650: 5G2-11101-00

    85-86 Maxim 700: 41Y-11101-01

    Now the head gasket part numbers:

    5N8-11181-00

    1FG-11181-00

    Central digits are always the same, is that enough to say if it will fit or not, I don't know...
     
  4. MiGhost

    MiGhost Well-Known Member

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    Just to be clear. The 50cc difference is in the dispacement of the engine, and has nothing to do with the combustion chamber sizing. The 700 engine is actually a destroked 750 using a shorter crank stroke with longer rods. To meet the import restrictions of the time.

    Taking a step backwards, and checking the base gasket shows that the 650/700/750 all share a common base gasket. This would show that mounting studs through the cylinder are the same. Allowing the 650/750 cylinder head to be mounted to the 700.

    On the cubustion chamber sizing. The 82-83 650/750 engines share the same cylinder head. This allows the combustion chamber sizing to be correct for the 700 also.

    The big difference is going to be the intake camshaft. The 750 camshaft has .050 higher lift than the 650 camshaft. This will only make a difference if you are looking for higher top end power. For normal operation. It should barely be a noticable difference.

    Would a 650 head work on the 700?
    I would have no problems with using the combination with the noted difference in cams. There should be little if any noticeable difference in operation.

    The gaskets actually work out to be the same parts under different part numbers. Possibly due to a change in materials, better manufacturing processes, etc...

    One major item that needs to be an absolute must. The 700 was a YICS engine. This limits the 650/750 cylinder head to the 82-83 models that were also YICS engines. The 80-81 650 engines were NON-YICS, and will not work on a YICS engine.

    Ghost
     
  5. day7a1

    day7a1 Member

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    Doesn't the 750 have a 65 mm bore and the 650 have a 63 mm bore?

    I don't see how those gaskets could be the same. I also don't see how removing 2 mm from the diameter of the cylinder, which the combustion chamber matches up to, would not decrease the diameter of both and therefore reduce combustion chamber volume proportionally.

    Of course, if the 650 and 750 have the exact same bore size, then I agree.

    If you do swap it, you can leave the camshafts with the correct bike....
     
  6. RickCoMatic

    RickCoMatic Well-Known Member

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    You might be able to get a definitive answer by calling Customer Service.

    Yamaha Motors USA
    California
    (800) 656-7695
     
  7. day7a1

    day7a1 Member

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    I think the better question is....WHY do you ask? Maybe there is a better solution. Maybe there is no problem. Maybe there's pride and a beer on the line....what's really at stake here?
     
  8. drdee.1

    drdee.1 Member

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    We have discovered after rebuild of our xj700 and some short distance running that the a previous owner had helicoiled a spark plug hole.

    Fine not a problem...cept the idiot had helicoiled on top of a helicoil....after we had a run a short distance the plug blew out complete with helicoils...thus leaving a hole thats too big to helicoil.

    In order not too lose what is an excellent head we have comissioned an insert with 14mm metric thread and an external diameter of 18mm but turned with a wierd AF type non metric threading......We will then have to tap a thread for the outer threading andscrew in the insert

    Xj650 head is a back up in case the operation falls on it's bum
     
  9. day7a1

    day7a1 Member

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    Oh....I guess pretty soon you may be able to answer your own question. I was thinking you were going to buy one. Have you visually compared the two?
     
  10. drdee.1

    drdee.1 Member

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    We are going to have to buy one......should have said that it's a back up plan.

    Thanks for the info everyone
     
  11. RickCoMatic

    RickCoMatic Well-Known Member

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    You might be able to save that Head by using an OVERSIZED "Insert" from the
    "TIME-SERT" Brand of Spark Plug Repair --> Sleeves <--

    Similar to HeliCoil, the Time-Sert Sleeve is like a Bushing with BOTH the O.D. and I.D. threaded.

    Time-Sert offers an Option called: "Big-Sert"
    Its an Oversized Insert with Standard Plug I.D.

    Here's a Link to their site:

    http://www.timesert.com/html/bigsert_sparkplug.html
     
  12. drdee.1

    drdee.1 Member

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    Thanks for that Ric...we have an agent for them in the UK.

    We got all excited ....then discovered their fitting kit....not the big serts but just the fitting kit is....wait for it.....£300......

    OUCH or what
     
  13. drdee.1

    drdee.1 Member

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    Thanks to MiGhost for the ins and outs, do's and don'ts. The post is extremely imformative....

    I'm putting my xj for dummies head on now....

    Why does it matter if we fitted a non yics head onto a yics engine.....sure the yics would not function because the appropriate connecting chambers wouldn't be there.....but it wouldn't stop the engine from functiong, surely
     
  14. murray

    murray Member

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    Any specialist head service / rebuilder should be able to do the job with a time insert, Ive had it done on a xj650 head and a xj900 head, and it was only $ 25 each time Its not a big deal for the specialists,
    Cheers Murray
     
  15. RickCoMatic

    RickCoMatic Well-Known Member

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  16. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    All sorts of stuff won't line up correctly; I don't think it would actually "fit" even if the bolt holes line up.

    The "connecting chamber" is on the cylinder casting, it would still be there and possibly get in the way.
     
  17. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    It MAY be possible to put a YICS head on a non-YICS engine if you also swap the jug/cyls.............. not sure, just seems it back in the foggy recesses of the brain, though.

    Dave F
     
  18. drdee.1

    drdee.1 Member

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    The latest

    After talking this over the light suddenly came on....

    We didn't take the head off due to the £70 plus cost of the head gasket....

    Now we know an xj 650 head gasket fits and we can buy it for £20...the head is coming off... spark plug holes welded and retapped as required and the head back on.. annual MOT test and back on the road

    Thanks everyone....I'll put piccies on when it's finished
     
  19. drdee.1

    drdee.1 Member

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    :D I was absolutely sure that I had posted the final part,(we hope),of our cunning plan to get this animal up and running......obviously not.

    We did not remove the head at any stage of rebuild due to the fact that head gaskets are an arm and a leg.....£70 plus taxes... $120

    However now that we know that the xj650 gasket is a good substitute.....£20 inc taxes....$30

    The head is coming off.....weld the offending plug hole and retap...

    Head back on....sorted

    Thanks for the help will post piccies
     
  20. drdee.1

    drdee.1 Member

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    Something strange going on ....I had posted but it wasn't there now it is
     

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