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Thread: Minimum compression height for a 4AGE piston.

  1. #1
    SC14'd Member Domestic Engineer nick.parker's Avatar
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    Question Minimum compression height for a 4AGE piston.

    Hi,

    I should definitely ask a piston maker this question. But thought I'd try here first. I was thinking of a 7AGZE..(just transfer my 4AGZ twin screw setup to 7A block) and add a worked smallport cylinder head. But I want to make the rod/stroke ratio better than stock which is rod 132.5mm, stroke 85.5mm, R/S = 1.55. Reason being I have a 7A motor and custom rods cost the same as after market standard dimensioned rods. FYI 4AGE is 122/77= 1.58. I was think 132.5+4 / 85.5 = 1.6 is better than 4A motor = sweet!

    I have seen that its quite possible for oil rings to go over the gudgeon pin hole, and read that 4mm height increase is usually possible. Is their any downside to this?

    Cheers, Nick.
    == 4AGZE SC14 Supercharged ==
    Now flogg'n the SC14 @ 18psi....

  2. #2
    Balloon Slayer Backyard Mechanic jimmmayyy's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    Unless i'm understanding wrong what you're describing, if the oil ring goes over the gudgeon hole wouldn't you possibly get a lot more blowby than normal?

  3. #3
    doctor ed Conversion King ed's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    i reckon youd get 4mm without going near the oil ring land
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  4. #4
    SC14'd Member Domestic Engineer nick.parker's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    For a street engine, my recommendation is that you use the best rod ratio that is practical. I'd also stay away from oil rings that intersect the wrist pin. You'll find that race motors don't particularly matter, but when it comes to long-term durability, you'll want the better sealing and stability of a full circumference oil-control ring.
    I found this on a website. Will have to check out a 4AGE piston I guess.
    == 4AGZE SC14 Supercharged ==
    Now flogg'n the SC14 @ 18psi....

  5. #5
    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    not best pic but these are stock late model GZE..
    what about single compression ring?
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  6. #6
    doctor ed Conversion King ed's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    theres HEAPS of materialbetween land #1 and the crown, plus between compression ring #1 and #2, AND between the #2 and oil control. 1mm here and there adds up
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    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    Having the gudgeon pin as high up as the oil ring in no way increases blow by, for starters its the oil control ring, not a compression ring. Secondly, the ring still has a total circumference around the piston(besides the gap) and gudgeon is just made shorter. Sometimes they use specially designed button style inserts between the ring and the pin to fill in this space and hold the gudgeon pin in instead of a circlip.

    Having the gudgeon up this high is done ALOT and has little afffect to the sealing or control of the oil.
    Last edited by jeffro ra28; 02-01-2008 at 10:04 PM.

  8. #8
    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    Ok so i took some snaps for you.





    Notice there is no reciever groove for a circlip? It is still a full floating piston tho, but as mentioned above. This is a piston where they use buttons on each end of the gudgeon pin to hold it in and stop the gudgeon pin from hitting the bores, instead the buttons do, but are made as such it actually lubricates the bores.

  9. #9
    SC14'd Member Domestic Engineer nick.parker's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    Why do pistons have such a big gap between compression #1 and #2 rings?
    Why is the first compression ring down so far from the crown? is it safer from heat / detonation? What is the downside to the oil ring over lapping the gudgeon pin hole?
    Would the oil ring normally be squashed between the inside of the oil ring groove and the cylinder wall? I dont fully understand the job of the oil ring I guess.

    Cheers, Nick
    == 4AGZE SC14 Supercharged ==
    Now flogg'n the SC14 @ 18psi....

  10. #10
    Junior Member Carport Converter Billzilla's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    Quote Originally Posted by nick.parker
    Why do pistons have such a big gap between compression #1 and #2 rings?
    Why is the first compression ring down so far from the crown? is it safer from heat / detonation? What is the downside to the oil ring over lapping the gudgeon pin hole?
    Would the oil ring normally be squashed between the inside of the oil ring groove and the cylinder wall? I dont fully understand the job of the oil ring I guess.

    Cheers, Nick

    The higher the top ring is, the better (less) the emissions the engine makes, and a touch more power.
    But the higher it is, the more likely it is to have the top ring land crack and fail. So they have to have them down a few mm's to get it all to hang together. Similar reason for the 2nd ring, though the closer they are to the top ring the better as there's less volume for any by-passed gas from the top ring to get into.

    There's pretty much nothing by downsides to having the gudgeon pin overlap the oil ring groove. It'll increase the engine's tendency to detonate and it makes it harder (albeit only slightly) to pull the piston & rod apart.
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  11. #11
    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: Minimum compression height for a 4AGE piston.

    Nick, the above is a top-fuel piston to mate, normal pistons do not have anywhere near the same size ring land spacings.

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