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Thread: Head Gasket thickness selection

  1. #111
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    Default Re: Head Gasket thickness selection

    Ahh damn, well maybe a couple of weeks lol, I think I have to go to Ballarat end of next week for work for a few days anyway.

    I have stuff I am doing on the Hilux at the moment anyway so a bit of extra time will be fine
    1980 Hilux 4x4 7mgte, r150f trans, lockers - Broken
    1974 Mazda 4 door 1300 drag car, 4 linked, caged, 13b bridge port turbo, ta 51/76 turbo, microtech, soon to be full chassis - In the build

  2. #112
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    Default Re: Head Gasket thickness selection

    i soon will be taking my block an head to be machined for a mhg what Ra values should be done. and what exactly are Ra values? and i am planning on running probe pistions and saw on the thread that dish changes compression also. i guess im just confused on the compression ratio i would like to stay as close to stock to prevent problems. sorry if this is off subject

  3. #113
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    Default Re: Head Gasket thickness selection

    Needboost: it's a arithmetic mean value of the surface roughness.


    Now I need a little help to decide on a MHG.

    As far as I know there is no MHG size between 2.0mm and 3.0mm, is this correct?

    My block is now: 197.4 or 197.6mm (I don't have the exact number in my head, but somewhere in between).
    My head is now: 116mm (unsurfaced so far)

    Stock piston protrusion: 0.5mm
    My protrusion value: 0.5+ (198.5-197.4) = 1.6mm (Max. value)
    My protrusion value: 0.5+ (198.5-197.6) = 1.4mm (Min. value)

    Combustion chamber volume: 37.5cc (forums) or 40cc (Arias)
    I have been grinding away some hotspots so my combustion chamber is a bit larger than stock.
    And I'm not sure which stock value is correct? On the piston paper from Arias, they say 40cc

    Let's say that during surfacing of my head I'll end up removing 0.5mm
    Let's also say that the combustion chamber opening is the same as the stock bore: 83mm. Just to keep it simple.
    The volume removed will be:
    pi*r^2*removed material = pi * (83/2)^2 * 0.5 / 1000 = 2.705 cc

    So 2.705 cc is removed if the head is shaved 0.5mm
    Let's say that my grinding has added 1cc/cylinder
    This gives me:
    Min : 37.5-2.705+1 = 35.795 cc
    Max: 40-2.705+1 = 38.295 cc

    Piston Dish: 19cc (stock)
    My piston dish: 16.09cc (Arias)

    If I would use a 2.0mm MHG I will have the following clearance between piston and head:
    Min = 2-1.6 = 0.4mm
    Max = 2-1.4 = 0.6mm

    And with a 3.0mm MHG:
    Min = 3-1.6 = 1.4mm
    Max = 3-1.4 = 1.6mm

    The stock clearance is 1.37-0.5 = 0.87mm

    How much does the Eagle rods stretch during max rpm?

    The cylinders are now 83.49 mm

    gasket bore is 86mm

    With this calc: http://www.csgnetwork.com/compcalc.html

    and these values:

    Enter Cylinder Bore Size 83.49
    Enter Piston Stroke Length 90.98
    Enter Head Gasket Bore Diameter 86
    Enter Compressed Head Gasket Thickness 2
    Enter Combustion Chamber Volume In CCs 35.795
    Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') -16.09
    Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : -1.6

    I get the maximum static compression of
    With a 2mm MHG 10.10526513241693 : 1
    With a 3mm MHG 9.231210911947521 : 1

    and these values:

    Enter Cylinder Bore Size 83.49
    Enter Piston Stroke Length 90.98
    Enter Head Gasket Bore Diameter 86
    Enter Compressed Head Gasket Thickness 2
    Enter Combustion Chamber Volume In CCs 38.295
    Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') -16.09
    Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : -1.4

    I get the minimum static compression of
    With a 2mm MHG 9.543791078210202 : 1
    With a 3mm MHG 8.769627288645353 : 1

    Since the combustion chamber, according to arias, is 40cc and that is bigger than my max
    I added a static comp ratio for that too:

    I get the minimum static compression of
    With a 2mm MHG 9.30101738082517 : 1
    With a 3mm MHG 8.568338122041688 : 1

    Until I have measured my final combustion chambers, these are the values I have to work with.

    My max. static comp. ratio with a 2mm MHG is 10.105:1
    My max. static comp. ratio with a 3mm MHG is 9.231:1

    My min. static comp. ratio with a 2mm MHG is 9.301:1
    My min. static comp. ratio with a 3mm MHG is 8.568:1

    I'm not planning on using huge amounts of boost.
    I will use my supra mostly on the street and on the occasional track-days.
    The largest turbo it will see is probably a GT-35 or BW S200.
    It's already stand-alone
    BC stage 2 cams, BC valve springs+retainers and 1ZZ-FE COP's are on it's way.

    What's the closest piston to head clearance I can run?

    Should I just go with a 3mm MHG and be over with it?

    What's your suggestion?

    Thanks/Marcus

  4. #114
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    Default Re: Head Gasket thickness selection

    Bump for answer

  5. #115
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    Default Re: Head Gasket thickness selection

    I went with a 3mm MHG.

    The easiest equation is stock block + stock head + stock gasket - new block - new head = new HG
    This should give a comp ratio slightly higher than stock and since I'm using 9:1 pistons so I should be around that.

  6. #116
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    Default Re: Head Gasket thickness selection

    ok bare with me here, my heads already been shaved .020 from PO and is probably going to need another .015-.020 taken off thats almost 1mm of thickness problem is i wanna run a composite head gasket because pulling the block isn't an option right now, but with 1mm of thickness to make up due to machining whats my CR going to be at? also can i even run the car like that and what would i need to do to actually get it running properly

  7. #117
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    Default Re: Head Gasket thickness selection

    ok i have had my head and blocked machine and decked. block is bored .20 over

    HEAD BLOCK stock head gasket 1.37mm before total- 315.87mm
    before-116mm before- 198.5mm after total- 313.918
    after- 4.560in. = 115.824mm after 7.799 in.-198.094mm equals = 1.952 mm gasket that i need?

    if i have the caculations correct what head gasket will i need?

  8. #118
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    Default Re: Head Gasket thickness selection

    2mm will do it fine if your math is correct.

  9. #119
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    Default Re: Head Gasket thickness selection

    Quote Originally Posted by IJ. View Post
    2mm will do it fine if your math is correct.
    ok i have had my head and blocked machine and decked. block is bored .20 over

    HEAD BLOCK stock head gasket 1.37mm before total- 315.87mm
    before-116mm before- 198.5mm after machine total- 313.918
    after- 4.560in. = 115.824mm after 7.799 in.-198.094mm equals = 1.952 mm gasket that i need?

    if i have the caculations correct what head gasket will i need? the machine shop did the measurement for me but i feel like i have made a misake some where

    i added the stock head+block+headgasket size to get my total of 315.87mm
    then dded the after machine work of head and block then subtracted the amount 313.918
    to get 1.952

    head
    before- 116mm
    after 4.560in = 115.824 mm

    block
    before-198.5mm
    after- 7.799in= 198.094mm

    stock headgasket. 1.37mm

    before- 315.87mm
    after machine work- 313.98

    315.87mm-313.98mm= 1.952mm

  10. #120
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    Default Re: Head Gasket thickness selection

    Quote Originally Posted by IJ. View Post
    2mm will do it fine if your math is correct.
    with the cylinders being bored .20 over what difference does that make on compression and do i have to get a certain kind of gasket since the block was bored. sorry for the ignorance on this just trying to get everything correct for the first build and learn in the process thank you.

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