I need some general advice regarding this bike motor (( Internal Combustion Engine ? )) I am rebuilding. Picked up this 94 YZ250 for 400 bucks. Cosmetically the bike looks great. The guy had a number of project bikes and was moving out of the country, so I got some others too for cheap. He said that he put a fresh top end in this bike, let it idle for 15 min and it cut off. Then the kickstart was very difficult to turn. He said the rod had a little play, but he thought it would be ok. He already had a rebuilt crankshaft and included that with the bike. I pulled it apart this evening the crank did fail and there is tons of play in the rod now. I found bits of metal everywhere.

I have read that these bike jugs are plated and often do not require boring, is there any truth to that? The jug seems pretty good, minus a few small nics on a couple of the ports. A small part near the base of the cylinder skirt appears to be missing, not sure if this will effect its operation or not. there is a hole there already, but with the missing part it just becomes a bit bigger. The head got tore up pretty bad. Would it be acceptable to remove any stuck bits of metal and maybe lightly sand it a bit to smooth it back out? I couldnt get any decent pics of the cylinder walls in the garage, but here are some of what I'm talking about.

hard to tell but the cylinder looks like it already has been bored so the nikasil would be gone and if it has chunks missing its toast, it should be easy to find a decent used cylinder some where ebay or cycle junkyard, the head is salvagabe but if you find a deal on a complete top end go for it.

to check it out you need some type of measuring device to check specs or take to a machine shop

are there numbers on top of piston?



we need much better pictures, lots more light.


on a good note they were great engines with lots of power and torque and no real issues parts should be easy to come by

You need to find out where the metal came from.

Crank needs rebuilt?

I believe that the metal came from the rod bearing in the crank. It has lots of play all around. I am most concerned about the missing part on the cylinder. It doesn't look like a fresh break, its perfectly symmetrical and it is right behind the boost port. I am wondering if someone did that on purpose in some mod attempt. Not sure of the bore, I should have known to check the piston. The cylinder walls look good and fresh, but there is on small score on it that I can feel with my finger. I did alot of reading on the bikes and bike motors in general last night on Eric Gorrs site. I think I will send him some pics and get some advice. His big bore total top end job on these bikes is 350. including piston, boring, plating and porting. Sounds fair to me and people seem to think highly of his work. I wont know if its worth it or not until I get the case split though and make sure nothing else is going on. The crank shaft is very hard to turn so some metal must be jammed in there somewhere.

after reading some of that eric goor stuff, i guess that year wasn't quite the torquer the older years were, and it looks like that missing part of the cylinder is a mod for reed stop clearance, but again that thing looks bored out big time, get any ruler with mm and see what it is. did you see the tip for the 97 yz250s? they say the pros used the 94 jug, that might suck cause use 94 jugs might hard to find, but i bet other years fit 94 as well

i think 90 was my last YZ 250 before switching to KTM.
Used to be,a set of reeds, flywheel weight, some bs lights, pipe, suspension, o ring chain, handlebars and gaurds all around and the YZs were very worthy woods machines, when they got it right in early 90s KTMs rode the same right out of the box, set of bark busters and go race.

that's pretty much what I had read too, and I was worried about finding a replacement cylinder. I am not sure of the bore size but the piston has 4SR00 made in japan stamped on it. I am having trouble locating the part number. That year is supposed to have a good mid range powerband, for 400 bucks as long as it can hang with my 96 cr250 I will be happy. I sent pictures out to Eric and I am hoping to hear back from him on Monday. I'll post here what I find out.