95 ATK 250 Rebuild/freshen up

Got the frame stripped, but before the engine came out, I did a compression test. I thought, cool, a healthy 160psi. But, I pulled the head and to my dismay, found the piston burned out at the ring locating pin.(probably over heated cyl, pin takes a lot of heat and cant get rid of it to the cyl so the aluminum piston melts) But, there is a boost port at the back of the cyl., right where the piston is pined. So, for this ATK/Rotax engine a few questions.
1 - is the boost port stock or is it a modification, I have seen older CanAm 250 air cooled and no boost port
2 - has single ring piston pined in center of boost port??
3 - compression was 160, that seems pretty healthy
compared to most Rotax engines that I see. Sleds are
generally 140 - 150 in a healthy engine, Pwcs range from
130 to 175 in healthy engines.
The piston is as pictured/super clean and single ring Elko, was single base gasket(didnt measure yet) and no head gasket/shim

I took the top end of the engine apart the other day and found the piston burned out at the locating pin. So, time to order a piston. Oddly enough, the compression was 165psi like this.


The piston I ordered is for a SkiDoo/Rotax 503 & 521 engine. I new it was a 2 ring piston & I liked that 2 ring design for an air cooled engine. Of course the big difference besides being 2 ring is the sled piston has no hole for the boster port in the rear of the cylinder, so I'll have to measure and drill a hole in the new piston.


Here's the new piston next to the old one after I cut the hole in. Pretty happy with the result, the hole in the new piston is a little smaller, but fitts the boost port really well.

A little more done with the bike, finished stripping the frame and cleaned/degrease it and went over it with Scotch-Brite & degreased again before paint. I'm very happy with the color. I was looking for a brighter red than what was original. I think this is more of a Honda, bright Red. I used Rust-Oleum 2X Gloss Apple Red. So, this picture is after the first coat. I'll let it sit the weekend in a nice warm room before the next coat, I figure 2 days is long enough for flash-off(I read a while ago on a different project that is what they recommend)

Well, I'm not 100% familiar with this bike, but, I am learning. It is supposed to come with Talon billet hubs front & rear, but looks like mine just has a Talon rear. So, theres a few things going on right now. I'm sorting through the incorrect front hub(may be a spacing issue) and along with that I guess my front brake is supposed to have a fixed caliper and floating disc. I know my rotor is not floating & I have to check my caliper.

Some more work done, though its been slow going. Benn doing a lot of on-line searching for info on different things. The White Power 4054 USD forks are different than any Ive ever worked on before. After what seems like 10hrs of searching, I think I've found enough good info, and manuals from ATK & Ktm on-line sources. The engine is in the frame, the swing arm is cleaned and serviced. What a great system/setup on the swingarm bearings. A pair of 9r10 sealed ball bearings on each side with aluminum spacers and a primary seal of a beefy oring. When I disassembled the spacers off the swingarm, they were perfectly clean. I'm very impressed with some of the engineering I'm seeing.

I've been spending some time researching sprockets for the ATK. CanAm used to make a ATV, the Ds650, that used the same spline(22tooth & 22mm/24mm cross distance), but the Ds650 used a 530 chain. Well, Rotax also sold that engine to BMW for their f650 in the mid to late 90's. So, as I checked, it was for a 520 chain and was a basic match for my bike. The only difference was the width was correct but there was a difference in side to side spacing. The ATK & the BMW sprockets were both 10mm wide but the BMW had different offset......no major issue for me. So, I found what I was looking for in a front sprocket.
On the same BMW, I looked at rear sprocket dimensions. My 95ATK uses a Talon hub. The rear sprocket bolt cross distance is 160mm and the sprocket hub bore is 135mm. So, oddly enough, the BMW's were 160mm cross distance and 136mm center bore..... so the sprocket will fit the slightly bore of my hub and should center on the 6 mount bolts. If I were running a 2hr Hare Scramble, I might worry a little about the 1mm distance......But I'm not a racer, the bikes hardest might be if I were to ride it at Unadilla Rewind, Other than that, trail riding. Shouldnt be much stress on things.
When I bought the bike, it gearing was 15/46. It is supposed to be 15/50. The Jt sprockets I got were 15/49 as a 50tooth was not available.




Just as a side note, I believe the BMW sprockets will fit other older Rotax & CanAm bikes......I'll have to double check that on my 400 & 125 CanAm's

It's been a few weeks since I posted, so thought I'd take a few photos & post. Have been doing a bunch of little things. I had to get a new front axle for it as the axle that was on it was not the right one. The axle i had was correct for my front wheel/hub, but was incorrect for my forks. The wheel/hub was off an older model and that axle was not threaded to screw into my forks. I was able to contact Frank White at ATK Usa and he had one so I bought it. Thankfully, the axle that came with the bike didnt destroy my forks and with a thread chaser, everything was good.
I was looking for some different plastics, as I'm not a fan of black. I was going to go with a red, white, and blue scheme. but, finding the plastics that I want is proving to be a challenge. The side plates and fenders are mid 90's KTM, but finding the right colors is probably not going to happen. Was going to do blue/white front fender, white/red front # plate, white side plates and a red rear fender.......The rear fender is the big problem, and I havent been able to locate a red rear fender.....so, Im going to use the black plastics I got the bike with and have designed some red, white, & blue graphics at motocal.com.

Its been a while since I posted an update, I have been pretty busy with other things. The discolored white gas tank has been annoying me and I wasnt sure what to do about it.. I did a lot of research and some people reported good results using salon strength hydrogen peroxide. I have to say, I am very impressed. I'll let the pictures tell the story.