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any rule of thumb regarding milling and combustion chamber cc reduction ?

Discussion in 'The Garage' started by R72K5, Aug 10, 2005.

  1. R72K5

    R72K5 Banned

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    does anyone know if any kind of general rule of thumb exists regarding milling cylinder heads and combustion chamber cc volume reduction on say a set of 3973493 1971 400 heads ? they are 76 cc and id like to resurface them at least, but if i could mill them down enough to knock the cc's down quite a bit without any problems with stock 350 pistons then id like to go for it

    if anyone has any clue as to how much should be milled off them ?

    cuz i have a set of these heads id like to run on a 1977 LM1 350 engine,

    they are in nice shape and everything

    thanks
     
  2. LongIslander26

    LongIslander26 1/2 ton status

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    Head milling

    Depends on what your cam specs are, Lift AND duration. You may have better luck having the block zero decked, as this will essentially do the same thing as milling the heads without reducing you chamber size, but you'll still see the change in Compression your after.
     
  3. R72K5

    R72K5 Banned

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    good point but im pretty sure neither of the two local small town shops here can deck mill

    but who knows,

    plus it may cost me more than im willing to spend, just for that one labor item,

    what is fair cost for milling block decks ? so that i can find out

    is there any general rule of thumb i can follow to know how much milling to do to shave off about 10 cc`s(each cylinder) of the block on a stock original LM1 350 ?


    theres gotta be something i can follow/use here...

    just blindly shave the decks ?

    but still... how much ??

    stock LM1 cam.. stock 1.94 and 1.5 valves,

    hmm.,.




    thanks
     
  4. LongIslander26

    LongIslander26 1/2 ton status

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    Machine work

    Prices vary so much from place to place, even shop to shop, but I'm sure if you brought your heads in JUST to be milled, it wouldn't be more than 150 bucks or so. As far as how much to take off, you still need to know your cam specs. If you have alot of duration or alot of lift, that's going to limit how much you can mill and still not have piston to valve interference problems. The machinist should be able to advise you. With flattop pistons, you should be able to mill quite a bit, but you also need to know the deck thickness of your heads, so you don't get into a water jacket, or so they don't get too hot and crack.
     
  5. jarheadk5

    jarheadk5 1/2 ton status

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    IIRC, when you remove material from either the block decks or the heads, you need to shorten the pushrods by the same amount.

    I don't understand why a shop could mill heads, but not blocks. Same machine, no? (something about a "Sunnen" machine is tickling my brain here, but it's late...)
     
  6. R72K5

    R72K5 Banned

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    i have no idea, but i got an amount on chevytalk by motorman he said i thnk .015 per .001 shaved ? for heads*

    i forgot now ill have to look again

    thats general rule of thumb anyways, since head castings vary, ya know,

    i replied there and asked him about same with a stock 4" bore 350, general rule of thumb regarding cc removal when milling decks of such an engine


    cuz ill go that route if they can shave the decks instead,

    yeah i know on the pushrods, thats no big deal though, i know how to adjust rockers properly per GM specifications, they are simple as cake to do ill get new ones anyways and springs and rockers and such, :D

    jsut want ot jump the CR up to around 9 or 9.5 if possible

    ill ahve to do some math with the general rule of thumb number when i get it and have time to and then go talk to machine shop



    thanks
     
  7. kgblazerfive

    kgblazerfive keymaster Premium Member GMOTM Winner

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    You can mill them alot a whole lot, the only problem that you run into is then you have to mill the intake or intake side of the head also cause they won't line up any more. If you do the block then you don't really have that problem but you can go .010 with out any problems with the heads.
     
  8. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    be aware that the more you deck the heads the less valve lift you can run and you have less piston to valve clearance. the more you deck the closer it gets.
    Measure your piston to valve clearance with some modeling clay on the top of a piston. :thumb:
     
  9. beater_k20

    beater_k20 Banned

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    no, you should have to adjust pushrod length, unless your geometry is off. there's a reason that rocker arms are adjustable.

    as for a machine shop being able to mill heads, but not blocks, that's BS. it is all done on the same machine.
     
  10. beater_k20

    beater_k20 Banned

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    Randy, in order to run the stock LM-1 cam and those heads with 9-9.5:1 CR, you're going to need to run something other than 87 octane. 9.5:1 would be pushing the limits of 92 octane VERY hard. need to stick to around 8.8:1 to keep it running good on 87. switching to a better head with an upgraded combustion chamber design (ie Vortecs) you could probably pull off 9:1 with some careful tuning of the ignition timing.
     
  11. Mad-Dog

    Mad-Dog 1/2 ton status

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    If you mill the heads, deck the block, change cams or rocker ratio then the pushrod geometry is changed so you must check and adjust as needed.
     
  12. beater_k20

    beater_k20 Banned

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    agreed, the geometry has changed, but it could have changed for the better. one cannot say "you must shorten your pushrods" without actually testing the geometry on that particular engine. if the engine had pushrods that were too short to begin with, you could actually end up putting longer thank stock pushrods in an engine that had been milled. there is no single cut and dry answer, it all varies from engine to engine.
     

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