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Thread: 1200 head skim

  1. #1
    Join Date
    Nov 2012
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    Bracknell Berkshire
    Posts
    1,238

    1200 head skim

    Been thinking about skimming the 1200 head as looks like I'll be having the top end off soon in order to do big end 3 bearing, seems too much trouble doing it purely through the sump. I already have no base gasket on the bike to up compression a little, is it safe to have the head skimmed as well as running with no base gasket at the same time? ZZR pistons in the bike already. I read 0.2mm from the head is about as much as you can take off, thing is the base gasket is 0.15mm apparently so if its necessary to refit the gasket after skimming surely you end up with less compression hike anyway as then its only 0.05mm lost! Haven't encountered any issues without base gasket so far. Don't really fancy the spend for high compression pistons so if its worthwhile doing this I'd like to...

  2. #2
    Join Date
    Oct 2015
    Posts
    193

    Re: 1200 head skim

    I'd be cc'ing the head and top of pistons to make sure it wouldn't mess up.

  3. #3

    Re: 1200 head skim

    0.2mm from the head is as much as PDQ was prepared to do on my ex-1224, mainly to clean up gas flow to/from the valves.

    Main benefits are from the high comp JE 1224 pistons and gas flowing the exhaust ports, plus some work unshrouding the area around the valve pockets.
    Don't take much off the inlet ports apart from cleaning them up, as you need to keep the gas velocity high....apparently advice from the ex-Cosworth technician who did the porting for PDQ for me.

    CVK40 ZZR carbs or FCR / TMR flatslides make a further 3 to 4 bhp over standard carbs with stacks / pod filters.

    I retained the base gasket on advice from PDQ, as the 1200 piston / head tolerances are closer than with the 1100s.

  4. #4
    Join Date
    Nov 2012
    Location
    Bracknell Berkshire
    Posts
    1,238

    Re: 1200 head skim

    So do you reckon the 0.05mm difference between a skimmed head or running without a base gasket is going to be noticeable? The bike has been run with no base gasket and zzr pistons the last 30k with no issues arising from that but sounds like skimming head and no base gasket isn't going to be a cheap alternative to higher comp pistons. The head has been ported and tidied up by Pal in the past which I'm sure is helping things.

  5. #5

    Re: 1200 head skim

    Doubt you'll notice it, only on a dyno if at all.

  6. #6
    Join Date
    Nov 2012
    Location
    Bracknell Berkshire
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    1,238

    Re: 1200 head skim

    Quote Originally Posted by Purge View Post
    Doubt you'll notice it, only on a dyno if at all.
    Was thinking that'd probably be the case! Reckon continuing with the ZZR pistons and no base gasket will be the way to keep it for now. I managed to get the #3 rod bearing shells replaced through the sump last night anyway so will put it back together and see how things go. Was in no rush to pull the top end just seemed it might be a good time to play with things if I couldn't change the shells over from down under.

  7. #7
    Join Date
    Jun 2014
    Posts
    8,427

    Re: 1200 head skim

    If a shell has let go, you sure the crank doesn't need grinding Chris?
    What Goes Around..........!!!

  8. #8
    Join Date
    Nov 2012
    Location
    Bracknell Berkshire
    Posts
    1,238

    Re: 1200 head skim

    Quote Originally Posted by ZRXNige View Post
    If a shell has let go, you sure the crank doesn't need grinding Chris?
    Well the surface didn't seem too bad to be honest I've cleaned it up as well as I could from underneath the sump there are no real deep marks I could feel on it. The marks on the shells around the tabs I took out make me think there was a little too much crush which caused it. I had the engine out about 500 miles prior to do #1 which went due to running too low an oil level as far as I can tell (the spark plug well seals were leaking) and inspected all of them at the same time, the others all looked and measured fine. I think this one has gone due to overtightening a little I've been really careful with marking the cap up this time to do the 120 degree turn you're supposed to do on the nuts after initial torquing. Anyway I bought some new oil pump rotors so I can make sure that's as good as it can be and I've done the little filter bolt mod which is supposed to allow more oil flow hopefully that'll help but I think keeping the oil brimmed to the top mark is the best thing you can do!

  9. #9
    Join Date
    Jun 2014
    Posts
    8,427

    Re: 1200 head skim

    Good as gold Chris, was just thinking out loud I guess
    What Goes Around..........!!!

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