I just picked up a decent little trailboss 250r/es for $250 from a kid. Plan is to get it up and running for my daughter. Pulled the plastics off to make for easy access to everything. I know I need to clean the carb up on it. My question is, I did a compression test on it and its right around 100 lbs. Should I put a set of rings in it? Everything looks good as far as piston and jug. If so then what brand of rings will fit in the stock 72mm piston? or does that not matter? I've never just replaced rings. I've always had to replace pistons as well.

 

100psi is bare minimum..Very rare that you can just throw in a set of stock rings on a piston and make it last.Cylinders may look good but most of them will be egg shaped over time. Have the cylinder measured for the over size Wiseco piston that will clean up the bore.Usually first over will clean it up. Wiseco Powersports Forged Pistons - Wiseco Piston Inc. Plenty of gasket kits on ebay such as Cometic and Vesrah for the 250.Cranks seals are a given on a machine this old also. They're cheap parts on ebay or amazon but important. Shindy carb kit #03-401 also to rebuild the carb. Also make sure there is NO up and down play at all on the rod.Side to side play is ok.If there is play,engine has to be split then a rod kit and crank bearings installed. Some of these old ones may look ok,but when you start taking one apart the money can add up.If you don't have a manual here's one that will help you on engine specs and clearances along with other repair and services.Well worth the 5 bucks.First R/ES I can remember was an 88 model,but here's both..1986 Polaris Trail Boss Service Manual PDF Download 1988 Polaris Trail Boss 250 R/ES Service Manual PDF Download

 

Thank you! I've seen top end kits made by WSM that include everything. Are these any good in your opinion?

 

The machine is for my 11 yr old daughter. It wont be having too many WOT times unless I'm taking it for a little rip.

 

Choice is yours but I've see the Wsm and Namura kits for around 100 bucks or a little less. For 20 bucks more you can have a quality forged Wiseco kit that will out last either one of them.https://www.ebay.com/itm/Polaris-Tra...1aQrHC&vxp=mtr

 

Here are a couple of choice bits from Wiseco's website. "Compared to cast pistons, forgings are tougher and more durable...", which I think most people will agree with. "Most castings are brittle, have no grain structure and tend to fail catastrophically when the material yield strength is exceeded. Forgings have condensed grain that is aligned to the features of the part, which produces higher mechanical properties, and resistance to impact and fatigue."

 

I always liked the hammer test. You can tear up anything with a big enough hammer,but it does take a little more effort on the forged piston. Plus if you can understand these crazy Austrians here's a Ktm 4 stroke forged piston versus a cast two stroke piston under a press.

 

I like the hammer test that shows how the forged piston cracks first before it breaks, instead of just shattering into pieces. I skipped through the second video just to see the action.

 

 

If it is a 250 R/ES it's an 88 model. Look on the right frame down tube. Should show a W888528 if it's a R/Es model. 86 models wil either show a W867527 or W867627. Either way there should be a reverse limiter module close to the voltage regulator. Pull the black wire from the limiter module at the terminal board and see if it runs better. 112 psi is still low and some machines as they heat up may lose compression when the cylinder expands and rings lose their seal. Some may be hard to start or some will start and run good for a short while and then start bogging because of the above reasons. Defeating the limiter may help if it's faulted as most do over time and some can cause limiting in forward range also. On the 88 model the limiter shows to be Item #32.