2018 250sx overhaul

Related:
Create New Tag

10/19/2020 5:19 PM

The bike has 90 hours on it now. This past weekend the pipe cracked on me so I decided since I had to replace it I might as well tear it down for a rebuild. That’s 90 hours on the stock top end for those of you who haven’t seen me mention it before in other threads that I haven’t cracked it open yet.
Photo

Photo

Photo
Photo

After an hour this is where I’m at.
Photo

I did a compression test before pulling the engine. I’m still at 200psi which is exactly the same as I tested at 50 hours.
Photo
Photo

Then I pulled the cylinder off to see what the piston and cylinder looked like. I’d say running amsoil dominator at 55:1 has been the best oil for this engine for sure. No scoring, scuffs, wear marks or heavy build up. Inside the pipe is also very clean and no scaling or build up. The ring gap measured .53mm for the top ring and .45mm for the bottom ring and spec. is .40mm, but that’s at 20mm down and I measured about 7mm down so I’ll have to recheck it tomorrow. For being out of spec. the bike sure did run amazing.

Photo

Photo
Photo
Photo
Photo
Photo


Plans for the rebuild; new grapics, plastics, pipe, repack the silencer, Boyesen rad valve, top end rebuild, suspension rebuild, grips, grease, replace wear parts.

Done so far; replaced the wheels with DID dirt stars and stainless steel spokes, front and rear SKF wheel bearings.

|

2018 KTM 250sx
Instagram CamaroAJ

10/19/2020 6:31 PM

Have any interest in making yours a 300?

|

10/20/2020 6:49 AM

That bike looks very good inside and out for 90 hours. After 100 hours of XC racing mine looked like hot garbage.

|

Unleaded tastes a little tangy. Supreme is kinda sour, and diesel tastes pretty good.

11/6/2020 1:20 PM

Might be worth repacking the silencer, definitely check the linkage and swingarm pivots for grease/wear.
Very impressed with how it's running! Seems like you've got it right in the design window (A:F, track conditions, maintenance, fuel). Nice job.

|

KTM250SX YZ125 YZ85 PW80 ZX6 FZX700

11/6/2020 4:20 PM

I just rode mine today ... it has about 90 hrs but I replaced the piston at 60 hrs. I run Dominator also at 40:1 with 25% leaded race gas / non E pump gas. My top end looked great as well. I checked stock reeds they were fine, still running them. Bought the carb jet block oring and replaced it but the stock oring was fine. My powervalves were functioning fine but had some minor carbon build up. I bought the Slavens complete top end kit with Vertex B piston. His piston ez circlips make life much easier, the standard Vertex clips are a Pita. My bike is used as a backup or do it all bike. I have raced MX on it, GNCC”s and woods... once the jetting is sorted, the bike has an answer for everything. Great bikes...

|

11/9/2020 4:36 PM

Are you going to split the cases and check tranny gears and crank? If you do, I'd like to see what you find. Mine has 56 hrs on it. I'm curious what mine will look like at 90 hrs and what type of rebuild it will need. Your top end looks great though!

|

11/9/2020 7:30 PM

erik_94COBRA wrote:

Have any interest in making yours a 300?

I didn’t want to change it. I’m happy with the 250 for now. Buddy of mine is doing a 300 conversion so I’ll ride his and decide.

|

2018 KTM 250sx
Instagram CamaroAJ

11/9/2020 7:31 PM

YZed250 wrote:

Might be worth repacking the silencer, definitely check the linkage and swingarm pivots for grease/wear.
Very impressed with ...more

Way ahead of you. And thank you.

|

2018 KTM 250sx
Instagram CamaroAJ

11/9/2020 7:32 PM
Edited Date/Time: 11/9/2020 7:34 PM

sandman768 wrote:

I just rode mine today ... it has about 90 hrs but I replaced the piston at 60 hrs. I run Dominator also at 40:1 with 25% ...more

Agreed they are great bikes. My reeds looked brand new and had the gap everyone makes a big deal about, but runs great. I did upgrade though while it was apart. I bought the same kit and slapped it together. I had zero carbon build up on the power valves lucky.

|

2018 KTM 250sx
Instagram CamaroAJ

11/9/2020 7:33 PM

rollin64caddy wrote:

Are you going to split the cases and check tranny gears and crank? If you do, I'd like to see what you find. Mine has 56 hrs ...more

No, I checked crank play and had zero play so I only did a top end rebuild.

|

2018 KTM 250sx
Instagram CamaroAJ

11/9/2020 8:13 PM
Edited Date/Time: 11/9/2020 8:33 PM

Ok so the bike is done. Let’s run through all that went into the rebuild.

The engine was reassembled with a vertex B piston kit. I’ll do a compression test on it tomorrow as I wanted to break it in and get a broken in reading.
Photo

With the engine back together I upgraded to a Boyesen Rad Valve. The stock reeds looked to be in great shape other then the gap everyone complains about and obviously when the engine is running doesn’t affect anything.
Photo
Photo

The parts stock pile in all its goodness.
Photo

So here’s a rebuild list; repack the silencer, clean and grease bearings, suspension was sent out to be rebuilt, new Pro Circuit Works pipe, chain sliders and blocks replaced due to wear, frame guards, new grips, brake pads replaced, oil change in the steering damper switched to 15w40, Boyesen Rad Valve, new plastics, BNG with a 2004 Factory KTM vibe, and a new seat cover to finish things off. I’ve already replaced the rims, spokes, and wheel bearings a few weeks prior.

While the bike was apart I decided to test some sprockets. Ping mentioned something in his Ask Ping post about chain angles and freeing up the rear suspension. After a lot of research I found that really is a thing and it really does help. I also found Keefer talking about it some too. I was running 13/49 vs. the stock 13/50 for a little more pull in each gear. That put me at a 3.77 ratio. I wanted to pull 2nd and 3rd a few feet longer and I also wanted to run a 14t because of the research I did on the suspension and sprocket sizes so I went to a 14/52 and a 3.714 ratio. .056 ratio change isn’t very much, but maybe it will be enough to get over this triple at 3 palms cleaner in 3rd. 14/53 would have given me 3.79 so I would have lost some pull in each gear.

Anyways, here’s the finished product ready for a ride this weekend.

Photo
Photo
Photo

|

2018 KTM 250sx
Instagram CamaroAJ

11/9/2020 11:34 PM
Edited Date/Time: 11/9/2020 11:36 PM

I see a lot of bikes that run Amsoil Dominator around here and most the top ends look like that after a lot of hours. Some well over a 100. Like yours, the pistons look almost new and the rings are the only thing out of spec. I was impressed enough to switch to it, myself.

Bike looks good, BTW.

|

11/10/2020 6:21 AM

Mr. Robinson wrote:

I see a lot of bikes that run Amsoil Dominator around here and most the top ends look like that after a lot of hours. Some ...more

Agreed on the Amsoil, very impressed with it as well. I measured the new rings when I put them in and had about the same gap, so I'm not sure what the deal is with that. I will do a compression test later today to get solid new piston numbers to track what I lose over time though.

|

2018 KTM 250sx
Instagram CamaroAJ

11/10/2020 11:08 AM

Nice ride!

|

Instagram @factoryfarley25

11/10/2020 4:04 PM

How do you keep the bolts rust free? My biggest fret with my KTMs is that the hardware always looks trashed after the season.

|

Unleaded tastes a little tangy. Supreme is kinda sour, and diesel tastes pretty good.

11/10/2020 4:14 PM

Super clean. Love the graphics and works pipe!

|

11/10/2020 5:29 PM

Rickyisms wrote:

How do you keep the bolts rust free? My biggest fret with my KTMs is that the hardware always looks trashed after the season.

Lite coat of wd40 after I wash it unless I’m at work and then I blow the whole bike off with air.

|

2018 KTM 250sx
Instagram CamaroAJ

11/10/2020 5:37 PM

Bike looks like it turned out great. Can't complain about 90 hours on a top end. My last 250 smoker was a 16 husky tc250 and I ran Dominator with similar long lasting results.

|

11/10/2020 6:27 PM

Bike looks good👍 Please report back on your opinions with the 14-52 gearing, I too run the 13-49. I have heard the 14-52 works really good on the 450sfx,

|

11/10/2020 8:35 PM

sandman768 wrote:

Bike looks good Please report back on your opinions with the 14-52 gearing, I too run the 13-49. I have heard the 14-52 works ...more

Will do. I’m going riding on Sunday so I should have an idea then of how it feels.

|

2018 KTM 250sx
Instagram CamaroAJ

11/11/2020 8:31 PM

So the difference between a new piston, 50 hour piston, and 90 hour piston is 25 psi. That’s actually not as much of a loss as I thought it would be, but it is noticeable when kicking the bike over.
Photo

|

2018 KTM 250sx
Instagram CamaroAJ

11/12/2020 9:02 AM
Edited Date/Time: 11/13/2020 12:16 PM

Bike looks sick! Another 250sx here running Amsoil. I put in a Wossner piston/top end rebuild at 50 hours and your internals and piston crown look very similar to mine when I opened it up.

ps- my top ring end gap was right at the limit and my bottom ring was still in spec.

|

11/13/2020 10:24 AM
Edited Date/Time: 11/13/2020 10:27 AM

As I said, you are right in the design window, and taking good care to see such a small drop in comp @ ~100hrs interval.
This piston replacement interval should be adjusted to compensate for dirty intake/poor fuel/lesser oil/detonation etc. But for you, keep on doing what you're doing!

|

KTM250SX YZ125 YZ85 PW80 ZX6 FZX700

11/15/2020 4:54 PM

sandman768 wrote:

Bike looks good Please report back on your opinions with the 14-52 gearing, I too run the 13-49. I have heard the 14-52 works ...more

Order the 14/52 and a chain, you’ll like it. I don’t really know how to describe the feeling other then it feels like it’s under sprung because the rear is free of binding or sticking. The fender shows it’s not under sprung though. Obviously you don’t notice it when off the power going into a turn, but out of a turn the rear felt more stable and better connected to the ground. It also felt that way on the 4th gear high speed sweeper and had no hopping or sideways bucking like it has before off bumps. It’s literally like the bike is floating under you.

|

2018 KTM 250sx
Instagram CamaroAJ

11/18/2020 11:47 AM

AJ565 wrote:

The bike has 90 hours on it now. This past weekend the pipe cracked on me so I decided since I had to replace it I might as ...more

hi, sorry in advance for my poor english I’m French, i have a 250 tc 2018, i only drive in sand, at sea level (enduropale du touquet on youtube) what are your carb tuning suggestions for these conditions. thanks in advance

|

11/18/2020 6:13 PM

AJ565 wrote:

The bike has 90 hours on it now. This past weekend the pipe cracked on me so I decided since I had to replace it I might as ...more

barret1997achille wrote:

hi, sorry in advance for my poor english I’m French, i have a 250 tc 2018, i only drive in sand, at sea level (enduropale du ...more

here is a huge thread for jetting these bikes. they seem to have it tuned very well.

https://www.vitalmx.com/forums/Tech-Help-Race-Shop,42/17-TC250-Jetting-Help,1329212

|

11/18/2020 7:26 PM
Edited Date/Time: 11/18/2020 7:27 PM

AJ565 wrote:

The bike has 90 hours on it now. This past weekend the pipe cracked on me so I decided since I had to replace it I might as ...more

barret1997achille wrote:

hi, sorry in advance for my poor english I’m French, i have a 250 tc 2018, i only drive in sand, at sea level (enduropale du ...more

rollin64caddy wrote:

here is a huge thread for jetting these bikes. they seem to have it tuned very well.

...more

This. I’ve been riding in sand the last few months and it’s still been great with the jetting in that thread.

|

2018 KTM 250sx
Instagram CamaroAJ

11/19/2020 11:51 AM

That one of the nicest home garages I've ever seen. Tiled floors woh.

|

11/20/2020 6:10 AM

rjg wrote:

That one of the nicest home garages I've ever seen. Tiled floors woh.

Lol I wish. I live in an apartment so all my tools are at work and they let me do cool shit here.

|

2018 KTM 250sx
Instagram CamaroAJ