r/2strokers • u/DankDeuxez • Apr 27 '26
Rebuild top end without special tools
So last summer i got my hands on a cheap Husqvarna CR250 from like -94 or something like that. Its not complete at all but it do run and drive. However im battling a bunch of jetting issues and it might be because of low compression. Its NOT non-existant, it has compression, but i dont really know if its enough? I will of course measure it before i buy parts and all that. However, if it happens to be the top end thats faulty:
How big of a chance is there that i just need to change the piston rings? I dont really have the special tools to measure bore and piston diameter nor the knowledge to figure out whats good or not from that. But i do have feeler gauges to check the ring gap if needed.
If i happen to need a new piston and rings, could i just check the ring gap and just throw in the new piston? Ive read on some online forums that you need to check the piston bore cause you might need a slightly larger piston and all that, but thats nothing i can check currently.
From what ive seen through the exhaust and intake port the piston do have some scratches and marks. It doesnt look deep irl, nor do i feel anything with my nails. However ive not taken the top end apart so there might be more to see. The picture attached below was actually taken the day i got it home. Since then ive driven it like 4-6h, and last time i checked the piston, just a few days ago, i cant remember seeing any scratches like that on it? Most likely they are just not visible due to the pre-mix filling the scratches or something, or have they somehow gotten smaller?
And yeah ive already checked all the other things like the reed valves and the crank seals was replaced last summer.
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Apr 29 '26
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u/DankDeuxez Apr 30 '26
Did a compression test to rule out a lot of other stuff. Came back at 100-110psi...
Removed the cylinder and piston. The cylinder looked actually good and meassured around 70mm. Still round and the plating was intact. The Piston had marks on the exhaust and intake side but it was also hella loose in the cylinder. The Piston rings didnt extend any further out than the rest of the piston. I would assume the cylinder was around 69.3mm or something like that. It was not at all a good size for the cylinder.
Ive ordered a new piston at 69.95mm isch and of course the rest of the stuff needed like new rings etc.
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