Im looking for some advise from some of you Polaris fans out there that have been wrenching these bikes for a while. Ive made a few posts in the past asking for advise on my old school three year build on a 1985 Polaris Trail Boss 250. Ive finally been able to get her running and with the exception of a few minor tweeks, she sounds great, besides the sound of a minor "ping" that I just can not place. The entire motor has been re-built. Crank shaft bearings, wrist pin, piston and rings....done done done. However, when she is sitting at idle, it sounds like a knock from the top end... when reved, even slightly over idle, it goes away. Ive checked the exhaust, the motor mounts, even the damn rear end. Everything is tight. The "squirrel cage" that is used for the manual pull cord that is bolted to the end of the crank, tight. I hate to say it, and I doubt that it is possible, but the sound resembles that of the connecting rod slapping the cylinder... (I think the motor wouldve blown by now if that was the case). I dont get it, and was hoping that this is a "normal" rattle that this bike produces, and others have came across the same problem? Any advise or stories of similar phenominons would be greatly appreciated. Thanks once again in advance for anything you might think of! (by the way, the right size spark plug is in there....come on give me some credit! lol)

 

If makes a slapping noise at idle and goes away or quietens at speed,could be primary clutch button/counter weights wear or pressed in bearing wear in primary making the slapping noise and not the engine.I've had a bunch over the years that just pecked ,all the way up to sounding like a diesel running because of clutch wear! Just remove clutch,check if slap is still present,if is then you have a may have a problem with an "over bore" (cyl too big for piston) or rod bearing problem(upper or lower) If noise is gone check out clutch! If engine is rebuilt right,MY moneys on the clutch! Keep us updated. OPT

 

Old Polaris Tech, Thanks for the quick response. My biggest problem that made me start this build was that someone had taken apart the crank before I owned it, probably to replace the center bearing or the push rod. They put it back together (without alot of experience) because they only eyeballed the lobes on the crank. They were infact .055 out of alignment, and with that said, blew all three crank case bearings, and piston skirt. When I took her apart, the cases were in tact believe it or not, and the cylinder was only worn .002 in 10 places that I had measured with my bore mics So I replaced the blown piston with another stock piston. The clutch is something that I had never thought of, Im just really weary about running it. Do you think that it would harm it? And another thing, and this may just be my mind going, but it seems that the longer she runs as far as idling, the sound seems progresively worse? Its not really really bad, but something out of the norm.... thoughts? And thank you again!

 

If you just replaced with a stock piston I bet there is more than .004-.005 clearance in the cylinder somewhere! I've never been able to get by with a stock bore after skirt breakage!Usually egg shaped,out of round! Clearance on new piston should be between .0015-.002 between bottom of piston skirt to bottom of cylinder! If close to .004-005 will usually make a slapping noise and break the skirt with as little as .006 ! Plus was inside of cases where crank bearings ride ok? No distortion or worn spots in case?,plus crank bearings MUST be a tight fit! I always put the two clutch side bearings on the crank and heated the cases so the crank would just drop in,on the stator side same thing ,heat case and drive in bearing. Just remove clutch first to eliminate if engine is the problem! If engine slap still there, you might have to tear back down and have cylinder bored for over size piston and rings OR reinspect cases and crank. Did you just have the crank retrued or all new parts?,did you check for up and down play on lower rod bearing? 2-strokes a lot more demanding than 4-strokes. OPT

 

The cases where the bearings ride are in imaculate condition believe it or not! Had to use heat gun to place the bearings back in the casings. I did take the crank to a good friend who owns an automotive machine shop and had a crank case alignment jig. He got it to run true within .001. For a 26 year old machine, I was kind of stunned to see that there was only .002 play in the cylinder....you might be on to something, but I have been a macinist all of my life, how could I have missed that? Uggh....I really hope that I didnt spend all that money on a stock piston, only to turn around and have to buy another one.... It sucks, building for three years, and having mass disaponitment. Well....the one thing I can say is that there are alot of good folks like yourself out there willing to offer advise and stories of past experiences. Cant thank all of you enough. If anything else might come to mind, it would be greatly appreciated. Thanks to all once again!

 

If it was the original piston after all these years, I would absolutely go to at least first over size just to clean up bore! Good luck OPT

 

Im not sure if it is the origional piston or not, I dont think it is, not for 26 years of use, but it is something that I am definitely considering. I just hope that if an oversized bore is in order, that I only have to buy a new set of rings, and not another brand new piston.... that would suck. Guess Ill just have to call the manufacturer. Once again, thank you for all of your help. I hope there are some more people out there with possible sugestions? If there are, please, I am open to any type....never afraid to ask for a little help. Thanks again!

 

I hate to be the bearer of bad news, but as far as I know there's no way around a new piston with a bore job... if you think you might be having problems with piston wobble now, you would have piston SLAP after boring it out and trying to use the piston you have now.

 

Had the same machine...bored one time over and installed a WISECO piston. Not as expensive as you may think.

 

Over size pistons and rings are available from .010 (25 mm) all the way past one mm oversize. You cant just get by using oversize rings and reusing stock piston,must have a piston and ring assy(People used to do this in cars but not in these engines.) Use Wiseco as suggested,best piston,forged not cast. OPT