Ok so received a free 97 trail boss 250 from my dad. It had sat for about 2 years, although, back in the day I rode the thing pretty hard. He got a new sportsman, so he gave it to me. I drained the crankcase, and a few tablespoons of gas/oil came out. I got it to run and finally got it to idle. It was running pretty good, but was pouring white.smoke. I was thinking it had something to do with the oil gas. The oil tank was full of polaris 2 cycle oil. I'm unsure if its working or not. Well I was riding and it gradually slowed, made a pop, and when I tried to pull start, it felt siezed. When it cooled there was very little resistance during pull start. I pulled the tank off, drained it, and pulled the plug. I found a dime size hole in the piston. What would have caused this? Obviously I need a new piston, will use wiseco. But should I get a full rebuild kit? Is there anything else I should do while I'm in there?:I need input on what caused this, and if I should be looking for other problems? Thanks guy

 

This happens a lot on the older machines that have been moth balled up and then put into service. Usually blowing a hole in the piston can be from several causes,but most of the time it's from lean running conditions,getting hot,sucking air through crank seals or head/base gasket leaks,etc. Pumps are usually dependable. Once you check it out and IF you repair it always mix the first tank of fuel at 50-1 ratio. Polaris oil, or other brands are fine. To check the pump always bleed any air from the 10mm screw at the top of the pump(another cause of holing pistons) Have the small line clean that goes from the pump to the cylinder to where at an idle you can pull the oil pump lever all the way up(this pumps oil at maximum rate) and see if the line to the cylinder base is pumping oil to cylinder. I wouldn't guarantee just a top end on an old machine at the shop!! Best to do it right if it's never had a new rod and bearing along with crank bearings and seals.An old bottom end can take out a new high $ top end if the crank or rod bearings goes out. Seen this happen many times when people brought cylinder in for me to bore and came back later with a seized up engine. A new top end just puts more stress on an old bottom end,even if you can't feel any up and down play in the rod or the crank bearings may feel okay.

 

Can you provide a list or links to what I should replace? I was thinking of.the wiseco piston kit with gaskets, and top end bearings, full gasket set, and rod bearings. I would like to.keep this around 300-400 bucks including machine work, but with.me.putting it.all together.

 

https://www.denniskirk.com/wiseco/pk...prd/PK1084.sku
https://www.denniskirk.com/vesrah/co...prd/262947.sku

That's the list I've compiled so far

 

You need a rod kit and crank bearings! The new rod will have to be pressed on and the crank shaft has to be trued. Hot Rods or Vesrah are the main ones we used at the shop. The crank bearings have a universal number on them and you can get them anywhere other than paying the dealers prices. Either gasket kit you have is fine. We also used Cometic (sometimes cheaper) have the cylinder measured as to what oversize piston you need! Don't just order a stock size!!!

 

I'm really trying to not have to mess with the crank bearings. My machine shop should be able to do it right? I was thinking of just the top end kit, but I'm wanting some reliability.Would.I be compromising that by just doing a top end build?

 

On a 16 year old year old machine,Yes you would be compromising reliability with just a top end! Most machine shops shouldn't have a problem on pressing a new rod and lower rod bearing on and truing up the crank shaft. The oil pump drive gear has to be pulled off ,but no bigger with a bearing separator and a little heat.Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse