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Thread: 81.5mm oversized Pistons

  1. #11
    MR2 Obsessive superwhite90's Avatar
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    42mm cranks are common, 40mm are getting rare.

  2. #12
    MR2 Fan markjane35's Avatar
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    The guy who rebored the engine overbored it to much so it was burning oil. Had to find another block and rebuild the engine but did not need the pistons as I used the existing ones.

  3. #13
    MR2 Fan markjane35's Avatar
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    I also have both crankshafts 40 and 42 mm

  4. #14
    MR2 Fan markjane35's Avatar
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    If it’s a 42mm crankshaft these will not fit.

  5. #15
    MR2 Fan Donesy's Avatar
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    Quote Originally Posted by markjane35 View Post
    If it’s a 42mm crankshaft these will not fit.
    I’ll have to make certain exactly what we have and get back to you. I don’t think we have the budget but how much for the 40mm crank.

    Also, what did your engine builder bore it out to so we can avoid the same mistake.

    Any benefits to the smaller stuff? Lighter I suppose but not so good if your chasing big bhp?


    Sent from my iPhone using Tapatalk

  6. #16
    MR2 Fan markjane35's Avatar
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    Hi Dave
    I think that he just over bored it because when I checked the ring end gap after the problems with burning oil it was way out of spec. He should of only bored it out to take the 0.50 pistons. He has since closed down so can’t go back to him.
    The 40mm crankshaft is 1kg lighter than the 42mm one.
    Would sell the 40mm one for About £75.

  7. #17
    MR2 Obsessive superwhite90's Avatar
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    Quote Originally Posted by markjane35 View Post
    Hi Dave
    I think that he just over bored it because when I checked the ring end gap after the problems with burning oil it was way out of spec. He should of only bored it out to take the 0.50 pistons. He has since closed down so can’t go back to him.
    The 40mm crankshaft is 1kg lighter than the 42mm one.
    Would sell the 40mm one for About £75.
    The 40mm cranks are lighter but weaker. They were found in the 3rib blocks which were also revised after 86 with 7 ribs along with the bigger crank.

    The price offered is excellent and you will not find one cheaper in the condition it's in. I would have had the 42mm crank off of him if my crank was damaged.

  8. #18
    MR2 Fan Donesy's Avatar
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    Quote Originally Posted by superwhite90 View Post
    The 40mm cranks are lighter but weaker. They were found in the 3rib blocks which were also revised after 86 with 7 ribs along with the bigger crank.

    The price offered is excellent and you will not find one cheaper in the condition it's in. I would have had the 42mm crank off of him if my crank was damaged.
    I thought I might be able to find out what we have without having to wait until tomorrow via the miracle of the internet but there's way too much conflicting info out there.

    What I can tell you is we have a 7 rib block and and a big-port head. I'm pretty sure that makes us a 42 but it seems there are few exceptions to this around the 86/87 model year.

    One final question, do you know the compression ratio on those pistons? it seemed that changed around quite a bit too. but again, lots of conflicting info

  9. #19
    MR2 Fan markjane35's Avatar
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    I think they are the lower compression ones as the crown is not raised.

  10. #20
    MR2 Obsessive superwhite90's Avatar
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    Quote Originally Posted by markjane35 View Post
    I think they are the lower compression ones as the crown is not raised.
    AW11 MR2 and ae86 corolla 4AGE with the big port heads have 9.4:1 CR Pistons. So All mk1 mr2s.

    AW11 MR2 4AGZE big port come with 8.0:1 Pistons

    Corolla ae92 4AGE with smallport has 10.3:1 pistons

    Corolla ae92 4AGZE wig smallport has 8.9:1 pistons

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