Deck Height Uneven

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7/10/2018 6:17 PM

Im rebuilding the top end in my 2018 ktm 150sx after noticing pitting on the piston which some on here think was due to a coolant leak by the photos. Anyways ive got the new piston in and checked deck height with the nihlio concepts deck tool and the rear break side of the piston sits slightly lower compared to the shifter side. The jug is torqued to spec and tightened in a cross pattern. Should i be concerned with the uneven height?

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7/10/2018 7:11 PM

How much different? They should really be even.

Paw Paw

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7/10/2018 9:13 PM

Maybe something got warped from heat?

Is the gasket surface clean?

I’d check to make sure all the other surface like the top of the cases and the bottom side of the head are flat.

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7/11/2018 3:13 AM

When i slide a feeler gauge under the deck tool i can get only my smallest one under the rear brake side of the tool and that is .102mm. I can slide the feeler about 3/4 of the way to the shifter side before it stops. Both the case and jug are clean from any old gasket material.
Photo

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7/11/2018 6:57 AM

That is strange! I would hope that the gauge you are using is uneven and giving that result. The only other thing I would think would cause that is the dowels between the case and cylinder are too long or the dowel holes are too short and won't let the cylinder sit flat. maybe try pilling the dowels out of the case and retry the measurement. Worst case would be a bent rod (ouch) or improperly machined cases. I highly doubt the piston was machined wrong. good luck and let us know what you find!

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7/11/2018 7:51 AM

How did the old piston fit?

Paw Paw

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7/11/2018 8:43 AM

I never checked the deck height on the old one and didnt notice any issues with the old piston other than the pitting the top. I also ran a metal ruler over the cylinder and got the same reading. I also have the same problem with the deck front to back the front of the piston sits lower then the back. Ill pull the dowels and see how everything fits then. I may put old piston back in to see what readings i get.

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7/11/2018 9:55 AM

the piston will rock back and forth which is to be expected as it rotates around the wristpin. Checking it like you did in the pictures is correct (parallel to wristpin) is correct. Let us know what you find, I'm curious to see what the culprit is.

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7/11/2018 12:34 PM

It's not ideal, but if its 0.1mm less I wouldn't worry too much.

A lot.of rm250s had uneven case halfs, so if you didn't tighten the cylinder in the right order, it would air leak, but the squish was slightly off and the bike ran ok

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7/11/2018 3:01 PM

How about taking a squish test and measure the squish?

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7/11/2018 6:50 PM

Okay so i ended up putting the old piston and base gasket back in and it lined up perfect. I went back to my new piston and new gasket which was a .50mm that lined up perfect however wasnt thick enough because the piston would hit the deck tool. So i added a 2nd gasket a .15mm which gave me .65mm that was okay height but i had that gap issue again well after 3 hours of mixing and matching gaskets i found the perfect combination between oem and cometic gaskets. None of this makes sense to me why going over .65mm of base gasket would tilt the cylinder. Yes i removed the dowels with the same results

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7/12/2018 7:10 AM

Wow. That's scary that the cylinder would tilt like that with different gaskets. Are they OEM gaskets or aftermarket?

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7/12/2018 10:29 AM
Edited Date/Time: 7/12/2018 10:31 AM

Sure that wasn't just the piston rocking in the cylinder?

Edit: Just saw you are measuring across the wrist pin. Weird!

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7/13/2018 8:47 AM

Sounds like the gasket might be defective, thicker on one side.

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