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Thread: Help with piston size in the KTM 65

  1. #1

    Default Help with piston size in the KTM 65

    So I have done heaps of searching all over the net and have no conclusion.

    The KTM Manual lists 2 cylinders and 2 pistons. Size "A" bore is 45.025 - 45.037mm, Piston is 44.995 - 44.965mm. Size "B" bore is 45.037 - 45.050mm, Piston is 44.965 - 44.975. I am sweet with this and if the bore was brand new the piston choice would be easy.

    Aftermarket Vetex (KTM OEM), Wiseco and Wossen all list 3 piston sizes. 44.96mm, 44.97mm and 44.98mm.

    KTM Manual states piston to bore clearance should be 0.060 - 0.085mm so choosing an aftermarket piston to suit a brand new bore would again be easy.

    Now. I have purchased this bike with an unknown past. The previous owner told me he had brought it new and it had never been rebuild.

    So my question is. Does a Nikasil bore not wear and I should just order the original size piston that matches my bore? (As in check the piston stamp and cylinder markings). Or does the bore really need measuring as the it will have some wear?

    If the I should really measure the bore, how do I measure that accurately? I mean most everyday micrometers only go to 0.01mm accuracy. So I would first have to put a Telescopic gauge in the bore, lock it off them measure that gauge with a micrometer. I'm just not sure measuring a measurement, getting the gauge dead square in the bore and all will give me accuracy that fine. I mean if I get it 0.01mm out. I'm as far out of spec as I can go. There is only 0.02mm from the largest to smallest piston.

    Suggestions please. Wondering what others do and what is considered normal? I have rebuilt motors before but never a 2 stroke and never a nikasil bore.

    Thanks

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  3. #2

    Default Re: Help with piston size in the KTM 65

    Today's progress.

    I have measured the bore the best I could with telescopic gauge and micrometre. As per the manual, measured in 6 places. All but one came in at 45.04 with one at 44.045.

    Now since the manual states piston to bore clearance as 0.06 to 0.085 (I'm rounding this down to 0.08). So with a bore of 45.04, if I take away 0.06 to 0.08, I get 44.96mm to 44.98mm. I'm laughing now because that the exact available pistons. So which one do I choose? Is it better for the piston to be tighter or will that cause drag and less power and wear the bore more? Or if I choose the looser piston will it not last as long before it becomes too loose? What's the point of there being choices even?

    Anyway. I have ordered the 44.97mm (the middle ground choice).

  4. #3

    Default Re: Help with piston size in the KTM 65

    I was always told to go with the "B" piston on your first and every other rebuild after that. The dealer told me the "A" piston is only used when the bike is built or you put a brand new cylinder on it. I put a "B" piston in my sons bike every rebuild and have done it twice now without a problem. I also did the same thing on his 50 and rebuilt that thing about 5 times

  5. #4

    Default Re: Help with piston size in the KTM 65

    Check out "2019 85, size 1 or 2 piston" thread on pistons and read Post #6 by Roland3.
    Last edited by David 37; 02-13-2020 at 12:55 AM.
    2010 KTM 65 SX
    2009 KTM 450 XCF
    2007 KTM 950 Super Enduro
    2009 Honda CRF 450F - Sold
    2008 KTM 250 XCF-W - Sold
    2005 Yamaha YZ450F - Sold
    2002 Yamaha YZ250F too many mods to list - Sold
    2001 KTM 300 MXC - Sold
    1997 KTM 300 MXC - Sold

  6. #5

    Default Re: Help with piston size in the KTM 65

    Quote Originally Posted by snaggle View Post
    I was always told to go with the "B" piston on your first and every other rebuild after that. The dealer told me the "A" piston is only used when the bike is built or you put a brand new cylinder on it. I put a "B" piston in my sons bike every rebuild and have done it twice now without a problem. I also did the same thing on his 50 and rebuilt that thing about 5 times
    Do you measure the bore or is it just a guess?

  7. #6

    Default Re: Help with piston size in the KTM 65

    Quote Originally Posted by David 37 View Post
    Check out "2019 85, size 1 or 2 piston" thread on pistons and read Post #6 by Roland3.
    Thanks mate, I'll check it out.

  8. #7

    Default Re: Help with piston size in the KTM 65

    Honestly when I did his 50 I never measured the bore and didn't have an issue ever. I have only rebuilt the 65 twice now but I will most likely measure the bore next time now.

  9. #8

    Default Re: Help with piston size in the KTM 65

    Quote Originally Posted by snaggle View Post
    Honestly when I did his 50 I never measured the bore and didn't have an issue ever. I have only rebuilt the 65 twice now but I will most likely measure the bore next time now.
    As far as I am aware, the bikes can come with an "A" or "B" piston. They correspond to cylinders marked "1" or "2" as specified in the repair manual.

    As per my first post, "A" or "B" piston or cylinder is a size range and not a specific size. Those figures I have written are from the KTM Repair Manual. After market manufactures (including Vertex who make the KTM Piston) label their pistons "A", "B", "C" and "D" or some use "A/B" and "C/D" but behind those labels if you check the specs is the actual piston size in MM. For example Wossner lists "A", "B" and "C" for 44.96mm, 44.97mm and 44.98mm.

  10. #9

    Default Re: Help with piston size in the KTM 65

    Quote Originally Posted by joel0407 View Post
    As far as I am aware, the bikes can come with an "A" or "B" piston. They correspond to cylinders marked "1" or "2" as specified in the repair manual.

    As per my first post, "A" or "B" piston or cylinder is a size range and not a specific size. Those figures I have written are from the KTM Repair Manual. After market manufactures (including Vertex who make the KTM Piston) label their pistons "A", "B", "C" and "D" or some use "A/B" and "C/D" but behind those labels if you check the specs is the actual piston size in MM. For example Wossner lists "A", "B" and "C" for 44.96mm, 44.97mm and 44.98mm.
    My sons 65 had an "A" piston in it and the dealer told me to put the "B" in it cause "A" was just for brand new engines. Guess I'll measure next time to see

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