2019 KTM 250 SX Base Gasket Need Help, What Size?

rob27ma
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rob27ma 1/4/2020 8:14am
1/4/2020 8:14am
Getting ready to put a new OEM top end into my 2019 KTM 250 SX and received a vertex gasket kit with 4 gaskets. Unfortunately I already removed the original gasket that came in the bike and threw is away a couple weeks ago so I'm unable to measure the original one to see which should be used. Does anyone know what gasket(s) size is used direct from KTM on this bike?

Also, if I increase the gasket size, how would that impact power? More low end or top end?
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tzmike
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tzmike 1/4/2020 8:40am
1/4/2020 8:40am
Thicker is less compression more up top as it moves the ports higher. The Athena kit also comes with a tissue thin plastic gasket for the slightest adjust you need.. you will need a caliper and solder
Put the middle of the road gasket on with no ring on the piston. Use a dab of grease and lay the solder along the wrist pin location on top of the piston. Throw the head on and kick at least 2 rotations. Pull the head and measure the "squish" . The flat spots on the ends of the solder. Take the caliper and measure the flat spots. You really want to be in the .028 to .032 squish. I raced TZ250s for 15yrs and the squish never set above .030
I also had the luxury of 3 different head cc configurations. I dont want confuse with all the head cc stuff.
Go with the. 030 and you'll be fine.
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DynoDan22
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DynoDan22 1/4/2020 9:21am
1/4/2020 9:21am
When you disassemble, measure exactly what thickness base gaskets came from the factory and duplicate with new gaskets. I would not recommend deviating from the factory cylinder height. They are pretty optimized from the factory.
CarlinoJoeVideo
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CarlinoJoeVideo 1/4/2020 9:40am
1/4/2020 9:40am
DynoDan22 wrote:
When you disassemble, measure exactly what thickness base gaskets came from the factory and duplicate with new gaskets. I would not recommend deviating from the factory...
When you disassemble, measure exactly what thickness base gaskets came from the factory and duplicate with new gaskets. I would not recommend deviating from the factory cylinder height. They are pretty optimized from the factory.
He threw away the original gasket.

I +1 the comment about measuring the squish to make sure you’re in the correct range. It’s very easy and tons of videos on YouTube to help.
rob27ma
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rob27ma 1/4/2020 9:49am
1/4/2020 9:49am
I tried the squish test with solder, but my numbers don't seem to make sense. I'm in the 1.7mm range according to my caliper tool with the .5 gasket installed.

I was hoping someone would know which of the gaskets came stock on this year bike so I could just use the same exact one. So far I have seen conflicting feedback as to what size comes on KTMs, other than .5 or larger seems safe.

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sandman768
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sandman768 1/4/2020 2:32pm
1/4/2020 2:32pm
Sometimes you have to stack base gaskets to get the desired measurement, look up Jeff Slavens on utube, he has good videos on how to set your base gaskets. I believe you want the piston to just kiss a flat edge across the top of the cylinder.
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seth505
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seth505 1/4/2020 3:30pm
1/4/2020 3:30pm
Same motor but on my 2017 it was .030”.
BobPA
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BobPA 1/4/2020 4:50pm
1/4/2020 4:50pm
rob27ma wrote:
I tried the squish test with solder, but my numbers don't seem to make sense. I'm in the 1.7mm range according to my caliper tool with...
I tried the squish test with solder, but my numbers don't seem to make sense. I'm in the 1.7mm range according to my caliper tool with the .5 gasket installed.

I was hoping someone would know which of the gaskets came stock on this year bike so I could just use the same exact one. So far I have seen conflicting feedback as to what size comes on KTMs, other than .5 or larger seems safe.

So you are at 0.067" or so.....So a rather large squish band, which is not uncommon for stock bikes. A few thou either way is not going to hurt anything....
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FGR01
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FGR01 1/4/2020 5:39pm
1/4/2020 5:39pm
Correct me if I am wrong, but I thought each KTM 2-stroke was unique in that they put on whatever thickness base gasket is needed to obtain the desired squish at the factory. There is no "stock thickness" across every bike.
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LumpDog841
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LumpDog841 1/4/2020 8:07pm
1/4/2020 8:07pm
FGR01 wrote:
Correct me if I am wrong, but I thought each KTM 2-stroke was unique in that they put on whatever thickness base gasket is needed to...
Correct me if I am wrong, but I thought each KTM 2-stroke was unique in that they put on whatever thickness base gasket is needed to obtain the desired squish at the factory. There is no "stock thickness" across every bike.
I think you're correct. Seth above said his 17 was .030" stock, mine was .060".

OP, for my modified engine I was running 1.2 mm of squish. keep in mind though I'm running T2 fuel (105 octane).
Layton
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Layton 1/4/2020 9:35pm
1/4/2020 9:35pm
sandman768 wrote:
Sometimes you have to stack base gaskets to get the desired measurement, look up Jeff Slavens on utube, he has good videos on how to set...
Sometimes you have to stack base gaskets to get the desired measurement, look up Jeff Slavens on utube, he has good videos on how to set your base gaskets. I believe you want the piston to just kiss a flat edge across the top of the cylinder.
That is correct.
project racer 1/5/2020 3:58am
1/5/2020 3:58am
I had one of those little gauges for the KTM 50 from Nihilo that you lay across the cylinder deck. Don't they make one for the big bikes? It was $20 or so.
tzmike
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tzmike 1/5/2020 11:12am
1/5/2020 11:12am
Solder comes in different diameters
I sent my TZ250 cyl out and tried to them to the porting specs I was given. Nothing made sense as the readings came out all F$#%d up
I then realized I was using the larger diameter solder. .
Use the 32 thou solder and your reading nightmares are gone.
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tzmike
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tzmike 1/5/2020 11:15am
1/5/2020 11:15am
Fyi, the larger solder will actually move the rod slightly to accommodate the added mass in the head
Tomok
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Tomok 1/5/2020 6:13pm
1/5/2020 6:13pm
Google KTM "X" dimension.
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rob27ma
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rob27ma 1/6/2020 4:57am
1/6/2020 4:57am
tzmike wrote:
Solder comes in different diameters I sent my TZ250 cyl out and tried to them to the porting specs I was given. Nothing made sense as...
Solder comes in different diameters
I sent my TZ250 cyl out and tried to them to the porting specs I was given. Nothing made sense as the readings came out all F$#%d up
I then realized I was using the larger diameter solder. .
Use the 32 thou solder and your reading nightmares are gone.
Thanks, I'll give that a try.
euffeh
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euffeh 1/8/2020 7:37am
1/8/2020 7:37am
FGR01 wrote:
Correct me if I am wrong, but I thought each KTM 2-stroke was unique in that they put on whatever thickness base gasket is needed to...
Correct me if I am wrong, but I thought each KTM 2-stroke was unique in that they put on whatever thickness base gasket is needed to obtain the desired squish at the factory. There is no "stock thickness" across every bike.
Exactly.

Which is why they include several gaskets with various thicknesses in the OEM piston kits, and advice you to measure the "X-distance" and add/remove gaskets to get it right.

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