I am the original owner of this 1998 Scrambler 400. When it was about 3 years old I sank it and the top end was rebuilt. Something wasn't quite right so they rebuilt the top and bottom around 2002. Since then it has been absolutely the best running Scrambler out there, but honestly only sees a few hours of use per year. Last year it sounded loud to me and I determined that there was an exhaust leak so I replaced the donut. A few months later I took it back out and it was still really loud and not quite feeling right, but it did run. I looked it over more closely and saw that at least 2 of the studs that connect the cylinder to the crank case were sheared off. I have not yet gotten the time to take it apart and see what kind of damage has been done, but now that I've got all of my kids Predator 50s rebuilt, I'm feeling bold and ready to take this on. So what might have caused the studs to shear off and what kind of damage may have been done? Photos attached. Other than the studs and the nuts, what do I need to replace when I tear into this thing? I'm assuming Cylinder Gasket and Head Gasket at a minimum.

 

I would think replace Rings, Gaskets and new studs all 4.
Why they broke is a guess. Over tightened when last rebuilt or not tightened enough or simply fatigue. Probably never know the real reason.

 

Got it opened up. Cylinder looks good, so maybe there痴 no internal damage. I ordered the gaskets, studs and nuts. I just need to help making sure the rings are standard. Like I said it痴 been rebuilt and I知 not 100% sure it痴 not been bored out or if they put a new jug on it all those years ago.



piston says 4280D on it. I can稚 read the other markings. How can I identify if it痴 a factory sized piston or oversized?

 

Clean off the top of the piston. Sometimes you will find a mark that will say something like +10 or +20 and so on.
Only other way is match the numbers on it or take the cylinder to a machine shop and have them mic it then check it against the numbers in the manual for cylinder bore.

 

4280 could be the bore but i really have no clue. See if it measures 4.280

 

Any idea what the bore diameter should be on a stock 400cc scrambler? I have a fairly good caliper and it should tell me if it痴 stock sized I would think. At least close enough that i would know it痴 not bored. I値l clean the top and see what those other numbers say as well.

 

No i don't sorry but if you look around on line you should be able to find a manual in PDF.

 

Slim chance this is correct but i did see this. Correct or not i dunno . 3.270

 

Yeah I知 seeing numbers closer to that as well (83mm) as standard. If this thing as been bored out it痴 not a full inch bigger so those numbers must mean something else. I know the piston is supposed to be marked if it痴 bored over, but I知 not sure a .4280 bore makes a lot of sense either. I値l get the full numbers in the morning, maybe I値l get lucky and it値l match a Polaris part number or something. I also saw that the rings could possibly have a stamp on them so I値l check them too.

 

Got it all apart this morning. The numbers on the top read "675M08500". There are no numbers on the rings. Google tells me this is a wiseco 85mm piston, which means this thing is bored 2mm over (and judging by the top of the cylinder, possibly sleeved unless they all look like that). I will order rings to match it, but there's another problem. I found some minor scoring on the jug and it matches some scratches on the back of the piston. I'm guessing when the front bolts broke off of the cylinder, it was able to move just a bit and the piston was starting to scratch the jug. I've never been this far into this type of project. What's my next step? Is there a certain amount of scoring that would be considered OK or can minor scoring be removed (honing?)? No way I can put the money into for a new factory cylinder and the aftermarket ones are junk (from my experience on the little 50 predators I just rebuilt).