1998 KTM 300exc Top-End Rebuild

Discussion in '2 smokers' started by TexasNoah83, Sep 23, 2013.

  1. TexasNoah83

    TexasNoah83 Noah

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    I recently purchased a 1998 KTM 300exc from a friend who bought it then decided maybe it wasn't the best beginner bike for him lol. I've been out of the dirtbike scene since highschool but this KTM has really gotten to me and I find that I'm spending every waking hour fixing,tuning,massaging,cleaning,riding and re-tuning the old girl,so I decided a top-end rebuild was the next order of business. The other day I noticed that the base gasket looked a little bit thick compared to the one included in my recently purchased moose gasket kit,so I tore into my new love to discover that there were actually 4 base gaskets installed and I got all excited thinking about all the compression I was about to gain by pulling off the 4 and replacing them with my 1 new gasket that I just got...... then something dawned on me. With all 4 on the bike the piston comes right up where it needs to be at TDC,which I assume means that with just 1 gasket it will travel too high in the cylinder,right? What gives? Any opinions and/or advice will be greatly appreciated. :ear
    #1
  2. nevgriff64

    nevgriff64 Super Moderator Super Moderator

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    Welcome to the forum, TexasNoah.

    As good as our leader is, I'm not quite sure posting this in Ask Baldy, Blame AceRPh will get you the right answers. :lol3

    Let me move this over to 2 Smokers for you, I'm sure there will be some answers along shortly. :thumb
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  3. Spikester300

    Spikester300 Roll Tide!

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    I had a '98 KTM300 a few years ago, great bike. About every KTM I ever had had different thickness base gaskets in the kit and you are correct, the piston will contact the head without the proper thickness of base gaskets. If it has a flat top piston all you need is a straight edge on top to see if you have any clearance issues. The older ones with domed top pistons had a curved gauge to set the gap on those.
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  4. Pmason

    Pmason Been here awhile

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    The term KTM uses is the X dimension which is how you select the correct base gaskets you need when installing a new top end. There is no set number or thickness for any given size bike.
    I would look for a thread on KTM talk by supertrunk who details just about everything on how to rebuild a pair of ktm 200's.
    What I did was put the cylinder on without gaskets, placed a straight edge across the cylinder (head off), and used feeler gauge to measure the required gasket thickness (by adding more thickness until you hear the piston just touch the gauge). I have a .04 & .02 on my 2008 300.
    This could be over simplifying what you need to do if the head has been modified or if you are shooting for an "x" dimension other than zero.

    On my bike I don't think there is a clearance issue (the piston won't hit the head even without a gasket) but rather a larger x dimenion normally means less performance or compression.
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  5. Grreatdog

    Grreatdog Long timer

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    What I did was put my 200 motor in a box and ship it to Mike at Cycle Playground. :lol3
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  6. TexasNoah83

    TexasNoah83 Noah

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    Thanks a lot for the insight guys:D, I guess I was just wondering if anyone knew why there would be a need for 4 base gaskets to get the proper X dimension? A longer rod perhaps? BTW my 300 has a flat-top piston. I've also had hell trying to adjust the carb on this thing so it won't spooge so much and one day it'll run great then the next day it's off concerning the bottom end power, like it's running fat down low but then cleans up once I start climbing into the top of the powerband? If I had the extra cash I'd box it up and send it to Eric Gorr or a similar engine shop but I just bought a new Renthal Twinring,renthal countershaft sprocket,a throttle tube and grips, so I'm tapped at the moment lol. It had a 13/52 set-up and once I get the new stuff on it will be a 14/50, hopefully that will give it some more umph down low.
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  7. Spikester300

    Spikester300 Roll Tide!

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    There is a preload adjuster for the power valve that makes a big difference in power delivery. You can have a no hit powerband or a sudden hit by adjusting the screw. i don't remember anything about how or how many turns, maybe the manual tells where to set it back stock.
    #7
  8. barnyard

    barnyard Verbal tactician Super Moderator

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    If you google search KTMtalk, you should be able to find your jetting answers. You cannot do a JD Jet kit with a 98, but JD does have some very good jetting info. Using his info, I was able to jet my 98 250 to run much cleaner than it did stock.

    While you have it apart, google 'starter pawl spring' or something like that. There is a very common issue with a starter pawl. I do not remember exactly what, but when mine broke, it turned up with a search. It is an easy fix and my local dealer had the parts.
    #8
  9. Navin

    Navin Long timer

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    IIRC there were a few years with goofy pistons that are no longer available for the 300cc bikes. That might be one and a different piston used and shimmed to function?
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  10. barnyard

    barnyard Verbal tactician Super Moderator

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    I do not remember that. I did not have any problems finding parts when I did the topend on my 98 250.

    98-99 though are the only 2 years that share engine parts. Kind of a short run for KTM.
    #10
  11. Navin

    Navin Long timer

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    Looks like it was the 1995 model 300s.


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  12. TexasNoah83

    TexasNoah83 Noah

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    Thanks for the heads-up Barnyard, that's definitely a solid tip for anyone with a '98 because it was a big issue on that year model from what I hear. The starter pawl on this bike has already failed and been replaced.
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  13. TexasNoah83

    TexasNoah83 Noah

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    So you don't think my clearance issue has anything to do with the piston Navin? A friend of mine has been good enough to help me out with any questions I've been having (and there have been many) bc he and his dad have been running KTM's for years but this whole top-end issue with having to use 4 base gaskets to give me the cylinder height required to keep the piston from hitting the head has us completely stumped? :baldy
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  14. Navin

    Navin Long timer

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    I'd guess it is the wrong part or somebody milled the cylinder and went too far? I had a 99 300 that we rebuilt, single base gasket was fine and it was not a problem to jet. it ran great in fact after the crank/top end. Get it together to measure squish. Eyeballin it is OK, but really measure the thing, might be fine. Just don't force it if it tops out!

    I had a 96 360SX that ate the starter pawl and associated parts on a once a month schedule. PITA. Check ignition timing, mine ran backwards a few kick-backs too.
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