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Thread: 83 to 87mm Piston Considerations

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    83 to 87mm Piston Considerations

    When going from 83 to 87mm pistons, what modifications (if any) are required in parallel to ensure a good running engine? I would like to keep the intake manifold original and not chamfer or port the head, but am concerned this might be too restrictive for the increased capacity. I understand the higher compression ratio will result in increased heat and premium fuel may be required, but other than that are there any real concerns? FYI, I have a 3A with an early high port head. I do plan to convert to an electric fan/alternator/narrow belt, but everything else will be original.

    Just a quick background - the engine was rebuilt about 15 years ago with 87mm pistons and setup on the frame without the body. It seemed to run hot during initial startup back then, and the engine has not ran since. Due to sitting such a long time, I recently disassembled to clean. If multiple other modifications will be required with the 87's, I will probably just get new liners and rings for my original 83's.

    Joel

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    Re: 83 to 87mm Piston Considerations

    I've installed 87s with no other changes. Ran fine, no more overheating than with 83s. Unless there is some other problem, you should be fine. Yes, the stock manifolds won't give you all the power you could possibly get, but they will work well enough. Don't forget, the TR2 originally had smaller carbs and a more restrictive head (with 83mm liners).
    Randall
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    Re: 83 to 87mm Piston Considerations

    I installed 87mm liners and pistons in my 64 tr4. It still my favorite car ever.

    Old 64 1.jpg
    Tom
    76 TR6
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    Re: 83 to 87mm Piston Considerations

    Me too - from 83 to 87 in a TR3A. Nothing else changed, it's all good. I still run regular gas and get no problem pinging. Never overheated before or since.

    Quote Originally Posted by Joel M View Post
    ...I understand the higher compression ratio will result in increased heat and premium fuel may be required...
    Does this change increase the compression ratio?

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    Re: 83 to 87mm Piston Considerations

    Quote Originally Posted by Geo Hahn View Post
    Does this change increase the compression ratio?
    Yes, a little, at least with the sets I've had. The larger bore gives you more displacement (each cylinder goes from 498cc to 547cc), but the 87mm pistons have the same height and flat top as original 83s. So the compressed volume remains substantially the same.

    Compression ratio is (displacement + unswept volume) / (unswept volume), so if displacement goes up and unswept volume doesn't, the ratio goes up.

    I can't find my notes from measuring a stock TR3 head (and they weren't all the same anyway); but if we assume the 8.5:1 number is correct with 83s and work the equation backwards, the unswept volume comes out 66.4cc. That would make the new CR 9.24.
    Randall
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    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Re: 83 to 87mm Piston Considerations

    Thanks for the feedback all. I will just reuse the 87's.

    I do have a somewhat related question. The County pistons have 4 very rough holes below the oil ring, that appear to be covered from inside the piston. Does anyone know the reason for these holes? The look of them makes me question the quality of the pistons, and wonder if they will last long term. I have only rebuilt a few different engines, but have not seen this anywhere else.

    Piston Outer2.jpg
    Piston Inner2.jpg

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    Re: 83 to 87mm Piston Considerations

    Those holes are likely not blocked. They go up to the back of the oil ring, so that oil which is scraped from the cylinder by the ring is transferred lower down the piston skirt for lubrication. Sometimes they will just go inside the skirt to reduce/increase the amount of oil on the cylinder wall. Those pistons are just fine.
    John

    1955 TR2

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    Re: 83 to 87mm Piston Considerations

    That makes sense, although I don't understand why the holes look like they have been busted out. What kind of manufacturing method is that? I guess one that is cheaper than just drilling a clean hole.

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