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Thread: 37/31 heads ?????

  1. #1
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    Cool 37/31 heads ?????

    Are all the 888/926/916/955 heads from the same basic casting with different machined inlet/exhaust porting and appropriate valve seats etc ??

  2. #2
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    Pretty much all the same castings apart from the 916SP/955 Corse heads, these have a lower height over the exhaust cams to clear the front wheel under braking.
    888 "G" Corse heads have a poor exhaust port with a low floor and very little radius from the valve seats, to overcome this I fitted 32mm exhaust valves to gain a little more radius.
    Probably the best castings to start with for tuning are early 851 parts, these came with 32/28 valves leaving you with more material to play with.
    It's a lot easier to remove material than it is to add it. But...
    888SPS/SP5,916SP and Corse 888 "G" heads have seats that are soooo large at their outer radius, it's OK to open them out to accept 37/32mm valves without needing to fit larger items.

    Steve R
    Last edited by Mr.R; 11-29-2019 at 06:05 AM.

  3. #3
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    It's not only a question of valves but also of model of combustion chamber and heigh of the head or then are trouble to make appropriate squish and compression ratio

  4. #4
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    Quote Originally Posted by workslorenz View Post
    It's not only a question of valves but also of model of combustion chamber and heigh of the head or then are trouble to make appropriate squish and compression ratio
    As he's said...
    They'll be a need to reshape the combustion chambers to unshroud the new bigger valves.
    ...and there's more than one way to skin that particular cat.
    I have 6 pairs of Desmoquatro heads in my workshop, apart from the early '91 851 32/28 valve combustion chambers all the others had the same shape comb' chambers originally.
    These are '92 851, 888, 916 (these have 33/29 valves), 916SP and 888 "G" Corse heads that had 34/30 valves originally.
    My modified 916SP heads now have 36/31 valves with "ROTAX" shaped comb' chambers to unshroud the bigger valves, I then skimmed them by 1mm to restore most of the comp' ratio lost by the unshrouding, they now run 11.8-1 comp' ration using flat top 94mm 916cc pistons with the valve pockets opened out to suit the big valves.
    My modified "G" heads now run 37/32 valves with a comb' shape very similar to 999 Testastretta heads, using these with 96mm raised crown PISTAL pistons for Corse 955cc, resulted in the need to remove 1mm from the PISTAL's crowns and opening out the valve pockets to suit the new valves, still resulted in a comp' ratio of 12-1.
    I like around 12-1 because it works well without the need to run race fuel, running quite happily on Super/Shell V Power 98/99 RON street fuel.
    I run my heads at 0.8mm squish clearance to be on the safe side.

    Steve R
    Last edited by Mr.R; 11-29-2019 at 10:00 AM.

  5. #5
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    Cool Heads

    Just been in the shed the heads are stamped Ď92 and valves are 34/30 and both have the machine area below the Desmo logo ?????

  6. #6
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    Cool

    So Steve looking at your post 888 G Corse ????

  7. #7
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    Cool

    92 Corse heads ?
    Attached Images

  8. #8
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    It certainly looks that way to me.
    What cams are you going to use?

    Steve R

  9. #9
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    Cool Heads

    Steve

    Got G cams, but I planned to build a spare motor for the 916R, Iíve already got a spare 926 full spec motor. Does the fact the valves are smaller affect the performance monumentally ? Or would 37mm be a better option ? 30 v 31 probably not too HP unfriendly ?? Iíve got a great machinist, but living in Oz minimal access to spare parts. I reckon 95% of parts I import.

    All that said if anyone needs a pair of 34/30 heads and have 37/31 spare get in touch super quick.

  10. #10
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    Quote Originally Posted by Ducal View Post
    ... have 37/31 spare get in touch super quick.
    I though Tom Irvine had a set.

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