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Thread: Ca18det squish / piston to head clearance

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    Ca18det squish / piston to head clearance

    Can anyone tell me what the oem clearance between piston and cylinder head is?

    Replacing the headgasket and wish to achieve oem spec

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    I can't answer your question directly. I can only say that, if the head has only had a light skim (or hasn't been touched), you should just invest in an OEM head gasket. As soon as you are into MHG or doubling up, its less likely to be fit-and-forget.

    I am a little bit fascinated as to why you are wanting to get OEM-spec compression, Rich

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    Quote Originally Posted by Jonny Wilkinson View Post
    I can't answer your question directly. I can only say that, if the head has only had a light skim (or hasn't been touched), you should just invest in an OEM head gasket. As soon as you are into MHG or doubling up, its less likely to be fit-and-forget.

    I am a little bit fascinated as to why you are wanting to get OEM-spec compression, Rich
    Min head height is 125.9 i am currently at 125.96.

    Currently have a cometic 1.6mm but had no end of issues with it so looking at tomei but these are only available in 1.0 1.2, 1.5, 1.8

    Speaking to my tuner he said if I can get it back to close to oem squish height/clearance there is potentially a good amount of power/torque that could be gained.

    Any opinions on this?

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    Think Sideways Danny did something similar years ago. If you're on the Facebook group it might be worth tagging him in the question.

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    I'm assuming you know this already but if you don't then the standard head gasket is a 1.2mm if I remember correctly. My tuner (Jez - Horsham Developments) also said the same thing for my forged build and said if you skim the head then ensure you use a gasket to try and achieve a 8:1 ratio.

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    Not sure if this helps. See pictures.

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    Just been looking at my notes from a convo I had with Jez. So the piston is suppose to sit flush with the top of the block. If ur pistons are flush and the head was untouched then you want a 1.2mm gasket. If your pistons stick out of the block because of machining then you want 1.2mm + how much they stick out. If the head and the block have been machined then... The head should be 125.9 to 126.1. work out what that is or how much u need to add to get it to that figure and then add 1.2mm + how much they stick out.

    Hopefully that makes sense.

    Also this may help.

    https://forums.nicoclub.com/sos-deck...t-t512656.html


    In my standard engine I have a cometic HG but Jez also said he has heard of too many issues so said either OEM gasket or.... Go with a jap branded one like Tomei or hks.

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    Last edited by Asad.C; 25-02-2021 at 22:25.

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    Dorset & Hants Rep pointz's Avatar
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    Yes piston to deck height is 0.00mm so I am checking this at the weekend, I have already had the head height measured and as stated in above post I am at 125.96, manual states minimum is 125.9

    Yes so oem hg is 1.2mm so I'm probably going to require a 1.3mm however these only seem available mostly by cometic so trying to source a tomei

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    Last edited by pointz; 26-02-2021 at 06:29.

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    Surely being slightly higher compression would help with power or at least need less boost to hit the same levels?
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    Quote Originally Posted by Chriscooke View Post
    Surely being slightly higher compression would help with power or at least need less boost to hit the same levels?
    But can induce det from my understanding.

    Raising or lowering compression can have detrimental affects, lower it = more boost but more chance of knock

    So atm I run a 1.6mm but I am going to try get the comp around oem so In my situation I will be raising compression

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    Now with 400bhp....
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    It can make det more likely but also livens the engine up. Don’t think 1mm difference on the head gasket would raise it enough for it to be an issue surely.
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    At 125.96, you are well within tolerance to fit an OEM HG of 1.2mm. Yours is 0.04 below centre of tolerance so you will have a slightly raised compression but still well within tolerance.

    The 1.6mm MHG would have taken you outside the tolerance quoted in the manual and given you lower compression.

    My view would be fit a genuine OEM HG and you will get close to perfect compression.

    Anyone agree ?

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    Quote Originally Posted by Jonny Wilkinson View Post
    At 125.96, you are well within tolerance to fit an OEM HG of 1.2mm. Yours is 0.04 below centre of tolerance so you will have a slightly raised compression but still well within tolerance.

    The 1.6mm MHG would have taken you outside the tolerance quoted in the manual and given you lower compression.

    My view would be fit a genuine OEM HG and you will get close to perfect compression.

    Anyone agree ?
    I think I'm going to have to go 1.3mm as the block was decked for the mhg but I am going to measure this today, dont think the oem gasket will hold up at over 400bhp?

    Also with a big turbo surely dropping the compression ratio slightly would be more beneficial than raising it?

    I am at present just going to go off my tuners advice to get it as close as possible to oem spec altho this means a retune of the ign map which isn't ideal

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    15 years ago every tuner pushing past 400 bhp removed the squish from the head to drop compression.

    Getting the squish wrong >1mm means there is a pocket of un-burnt mixture waiting to det.

    Getting the squish wrong <0.? means the piston hits the head.

    Maybe they were just lazy bast'ds and couldn't set squish and dish the pistons.

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    Quote Originally Posted by skyshack View Post
    15 years ago every tuner pushing past 400 bhp removed the squish from the head to drop compression.

    Getting the squish wrong >1mm means there is a pocket of un-burnt mixture waiting to det.

    Getting the squish wrong <0.? means the piston hits the head.

    Maybe they were just lazy bast'ds and couldn't set squish and dish the pistons.
    Can you elaborate a little more please

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    All the old threads on SXOC have lost the photos due to photobucket being nobneads.

    https://www.sxoc.com/vbb/showthread....R-on-a-CA18DET

    Squish is the area on inlet and exhaust where the head is flat. This concentrates the combustion chamber into a better shape. The gap between the piston and head should be 1mm, the mixture in this space is quenched by the large surface to volume ratio and won't burn but more importantly resists det. If it's more than 1mm the volume of mixture gets bigger and it's more likely to det before the flame reaches it. If it's less then at redline the piston can get closer to the head - the oil film in bearings thins, rod stretches, piston expands. If there is too much gap a thinner head gasket is needed. Too little gap a thicker head gasket is needed.

    When high power levels are targeted, high boost is required and a drop in compression ratio. Just putting a thick gasket in stops the squish working, so it's removed. The alternative and better technically is to get custom pistons with a thicker crown that can have a dish machined in them to reduce compression and retain squish. Because the squish is still working this allows a smaller reduction in compression ratio, better off boost performance and better spool up.

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    I had a cometec and it leaked but I put it down to the fact id not skimmed the block. TBH I would not be worried about raised compression as modern fuel is much better that old 1990's stuff.

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