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Head bolts, torque, and lack of compression - KTM 150 Xc

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Today I blew/ lost a head screw on my new-to-me KTM 150 XC two stroke. The bike runs, no odd noises, but lacks compression. My friend and I ran a compression test after the ride. It read around 100-95 pounds. However, when my friend and I did this test, we did not replace to screws or torque them to spec, as one head bolt was lost. Could our reading be off? The P/O claimed a new top end... 

 

Thanks! 

Edited by frozenpepper70

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If the engine ran hot missing a cylinder head nut,

not being torqued evenly there's a chance the mating surfaces are now warped.

 

Make sure you take your compression readings holding the throttle wide open as you crank.

Edited by mlatour

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11 minutes ago, mlatour said:

If the engine ran hot missing a cylinder head nut,

not being torqued evenly there's a chance the mating surfaces are now warped.

 

Make sure you take your compression readings holding the throttle wide open as you crank.

...probably more than just a 'chance'........

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IMG_20170430_185145.jpg
I pulled the head off today... This is what I found. The cylinder looks rough​ and in need of replacement. However, the seating of the head seamed okay. The red circle shows the area where the bolt was lost and not torqued properly.

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It has to be measured for flatness, you won't see any warpage of the head or cylinder by eye.

As an example, the specs for a late model YZ125 is: less than 0.03mm / 0.0012" allowed

 

"seemed ok" and "looked fine" is the usual words used when people write about their just rebuild engine

that blows up / failed soon after because they cut corners on checking things.

Edited by mlatour

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It has to be measured for flatness, you won't see any warpage of the head or cylinder by eye.
As an example, the specs for a late model YZ125 is: less than 0.03mm / 0.0012" allowed
 
"seemed ok" and "looked fine" is the usual words used when people write about their just rebuild engine
that blows up / failed soon after because they cut corners on checking things.

Thanks mlatour, I am very new to 2 stroke engines. I appreciate your patience and response.

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Everything is back together, and the bike runs well. I checked the cylinder for flatness and found no warpage. The bike burns true (no coolant burned) and tears.

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