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1053
Joined
11/9/2014
Location
Costa Mesa, CA
US
Edited Date/Time
3/10/2020 2:10pm
2017 KTM 250SX first piston and rings at 100hrs, checked compression at sea-level before tear-down at 195psi with stock cylinder head. Bike has been running well at the time of the tear-down.
I replaced the piston and rings and base gasket with OEM parts, checked compression after rebuild and was at only 180psi. That was with the bike sitting on the stand, no run time on new parts.
When selecting my new base gasket from the KTM optional set, I measured the stock and replacement new gasket with Vernier caliper at about 0.4mm. Bit hard to tell if I had an exact match, but pretty close.
I am concerned that the new compression is lower than the old compression; should I check the squish or X-dim before I fire it up? Or maybe I should just put an hour on it before testing comp again and allow the rings to seat?
TIA.
I replaced the piston and rings and base gasket with OEM parts, checked compression after rebuild and was at only 180psi. That was with the bike sitting on the stand, no run time on new parts.
When selecting my new base gasket from the KTM optional set, I measured the stock and replacement new gasket with Vernier caliper at about 0.4mm. Bit hard to tell if I had an exact match, but pretty close.
I am concerned that the new compression is lower than the old compression; should I check the squish or X-dim before I fire it up? Or maybe I should just put an hour on it before testing comp again and allow the rings to seat?
TIA.
While I'm at it, does anyone have the control flap height measurement (aka z-dim)?
The Shop
Bike ran great for the first 15 min warm up on the street last night. I'm going to recheck compression once I pass the first hour.
Owners manual says 3hrs <75% and 5hrs <100%, oh boy! Don't think I'll survive for that long without getting excited 1 hour of heat cycles and she'll be good to go riding.
I see trail riders on ktmtalk go 200-400hrs on a piston, and I see the Vitalmx crew replacing top end at 25-70 hrs so its all subjective, IMHO. Not many take the time to measure compression, bore and piston diameter accurately anyway, and that's an art IN itself as the clearance is not the same the entire length of the bore.
I didn't want to run it past 100 hrs, but don't have a solid reason for replacing it apart from the fact that it was in the middle of the road for hours and cost is not that much. I ride mostly in the meat of the pwr, but about 10% of the time, I'm just poking along technical trails, and about 10% of the time, I'm holding it WOT on a long straightaway (much longer than regular MX).
I found that my ring gap was out to 0.85 mm on the top ring and that's a little wide. I did have some slight intake scuffs on the front side, but no detonation or marks on the exhaust side. Ring wear was even around the circumference. Coating was gone/worn on the skirt tips due to some piston rocking in the bore. I'll grab some pics of the skirt and under the crown. Black spot under crown was a bit too big/lean IMHO. I never used to see that much black under a YZ250 piston. I might bump the main one or two sizes 440 to 450 to keep it a little cooler. Wrist pin and bearing were all in really nice shape.
My nikasil was a more little worn than I would have liked; I could clearly see crosshatching below the piston landing, but not as clearly in the stroke. I run 50:1 pump.
Original spark plug electrode was visibly worn and due to be replaced IMHO.
- leaner jetting
- longer WOT
- and 30+ more hours
1. Double-check compression.
2. Reset deck height to 0 x-dim.
When I tested YZ250 in the past with retarded ignition timing and thicker base gasket, the bike became much more linear and revved a lot longer. It was more effective in the desert.
This wasn't the case at all on my KTM; I lost boost/bark and the topend is kinda anemic. Just goes to show how critical this measurement really is.
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