X dimension for KTM newbie?????


9 replies to this topic
  • rpt50

Posted November 13, 2010 - 04:28 AM

#1

I've done a ton of top-ends over the years, but this is my first KTM. I'm unfamiliar with this "X dimension" setting. Is this something KTM guys really measure and worry about, or do you just get enough gasket thickness to insure the piston does not totally top out the bore? I'm running a 250 SX for woods racing, so it's not like I need to wring every ounce of power out of the motor. Any advice?

  • gmoss357

Posted November 13, 2010 - 05:37 AM

#2

I've done a ton of top-ends over the years, but this is my first KTM. I'm unfamiliar with this "X dimension" setting. Is this something KTM guys really measure and worry about, or do you just get enough gasket thickness to insure the piston does not totally top out the bore? I'm running a 250 SX for woods racing, so it's not like I need to wring every ounce of power out of the motor. Any advice?


X dim at 0 is the piston level with the top of the bore. positive X is above and neg is below. Tolerances are generally loose, so tightening up X to 0 gives you better squish and compression...power. Having the head cut helps that much more.

  • Byte

Posted December 04, 2010 - 03:51 AM

#3

..... positive X is above and neg is below.........



GMoss - I am not doubting you as I still consider myself a newbie, but I have read the opposite (+ve is below top and -ve is above top).

Can you you possibly confirm this ?

byte

  • gmoss357

Posted December 04, 2010 - 06:17 AM

#4

GMoss - I am not doubting you as I still consider myself a newbie, but I have read the opposite (+ve is below top and -ve is above top).

Can you you possibly confirm this ?

byte


could be. I am not concerned about it though, as I set my X as close to 0 as I can. if you find out different for sure, post up. but, does it really matter as long as you know if it's above or below and what the effects are?

  • Gary jp4

Posted December 04, 2010 - 06:51 AM

#5

Gmoss has it backward and so does MikeS on the thread in KTM Talk you were involved in, at least the way my KTM shop manual and the others I have seen state it. I can't speak for all models maybe they turned it around on newer ones. On my 2003 (2004-2006 manual) and the 125/200 manual below it is the other way. It is stated both ways all the time in forums, but I usually understand what the poster meant by context. It doesn't really matter and when MikeS mentions a positive X with a meaning contrary to the manaul he usually adds a note to the effect "the piston sticking out of the top of the cylinder" or something like that.

A plus dimension is having the piston recessed down into the cylinder and a negative is the piston sticking out of the cylinder. This is because KTM intend for you to measure the dimension from a straight edge laid across the top of the cylinder down to the top of the piston with a feeler gauge or in on some models using the depth gauge end of a pair of calipers from the top edge of the cylinder down to the top of the piston (see picture below), or in the case of a domed piston from the special stepped tool laid across the top of the cylinder down to the top of the piston.

It is evidenced by the spec in the manual stated various ways on different models saying something like X can be from "plus something to minus 0" and a statement to the effect of "the top of the piston must not stick out of the top of the cylinder" or in the 125/200 manual it has various clues like the caution note: about too large and too small an X, and the one near the bottom about how to adjust the X with base gaskets.

Byte, I am guessing you don't have a shop manual. When you read it in the manual, even in Germenglish, it is quite clear like the example above. Notes and pictures help clarify what they are trying to get across. Read the following excerpts from the manual and see if you understand.

Posted Image

Edited by Gary jp4, December 04, 2010 - 07:35 AM.


  • Gary jp4

Posted December 04, 2010 - 06:58 AM

#6

could be. I am not concerned about it though, as I set my X as close to 0 as I can. if you find out different for sure, post up. but, does it really matter as long as you know if it's above or below and what the effects are?


That's right.

Byte, based on all your questions about X dimension, decking, squish, etc. I appears you are considering some mods. Some advice on X dimension and head mods:

  • Weather the terms plus and minus X dimension are used correctly only matters when you are first trying to understand squish, port timing and reading posts on same.
  • All that doesn't matter if you are just shooting for a 0.0 X.
  • You probably wouldn't be asking about positive and negative X if you had a manual because KTM clears it up with side notes and pictures.
  • Stay with an X of 0 (or the stock spec if other then 0) until you have a thorough understanding of all the variables or follow your tuners advice. This will apply even with most head mods.
  • You can experiment and modify the X to a negative value (piston sticking out of the cylinder) to get more compression without a head mod and get, better squish, and more low end power due to retarded port timing. If you do this then pay carful attention to the squish dimension to avoid serious problems. This means check the squish dimension. You will also loose some top end (high rpm) power by modifying this way but you get a chance to see if you like it.
  • You are better off getting a head mod and leaving the X stock (0.0 in most cases). You can gain low end without the loss of high end this way.

Edited by Gary jp4, December 04, 2010 - 07:50 AM.


  • gmoss357

Posted December 04, 2010 - 07:43 AM

#7

Gmoss has it backward and so does MikeS on the thread in KTM Talk you were involved in, at least the way my KTM shop manual and the others I have seen state it. I can't speak for all models maybe they turned it around on newer ones. On my 2003 (2004-2006 manual) and the 125/200 manual below it is the other way. It is stated both ways all the time in forums, but I usually understand what the poster meant by context. It doesn't really matter and when MikeS mentions a positive X with a meaning contrary to the manaul he usually adds a note to the effect "the piston sticking out of the top of the cylinder" or something like that.

A plus dimension is having the piston recessed down into the cylinder and a negative is the piston sticking out of the cylinder. This is because KTM intend for you to measure the dimension from a straight edge laid across the top of the cylinder down to the top of the piston with a feeler gauge or in on some models using the depth gauge end of a pair of calipers from the top edge of the cylinder down to the top of the piston (see picture below), or in the case of a domed piston from the special stepped tool laid across the top of the cylinder down to the top of the piston.

It is evidenced by the spec in the manual stated various ways on different models saying something like X can be from "plus something to minus 0" and a statement to the effect of "the top of the piston must not stick out of the top of the cylinder" or in the 125/200 manual it has various clues like the caution note: about too large and too small an X, and the one near the bottom about how to adjust the X with base gaskets.

Byte, I am guessing you don't have a shop manual. When you read it in the manual, even in Germenglish, it is quite clear like the example above. Notes and pictures help clarify what they are trying to get across. Read the following excerpts from the manual and see if you understand.

Posted Image


Thanks for the clarification Gary...we know the manual is never wrong. :ride: :blah: not saying it's wrong here, more likely that I am. :thumbsup:

  • Gary jp4

Posted December 04, 2010 - 08:15 AM

#8

Like we already implied it is only academic.

I don't know this for sure, but in this case I don't think the manual can be wrong because when we use the term "X dimension" it is a term KTM made up. They defined it, set the rules, the dimension, and called it dimension "X" so it is what ever they want it to be.

I don't think anyone else uses this term and it is not generic or industry standard like "squish" or "deck height". If we didn't have the term we would be trying to describe it through cylinder decking terms or just plain saying the piston stick out of, or is recessed into, the cylinder by this X amount.

  • Gary jp4

Posted December 04, 2010 - 08:18 AM

#9

Gmoss,

BTW, I believe your original post answered the OP's question well. This was all for Byte.

  • Byte

Posted December 04, 2010 - 05:26 PM

#10

thanks guys.

no offence was intended in any way and all i want to do is learn and understand...

byte




 
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