Hey guys, I have a stock 96 trailblazer 250 and I am rebuilding the top end just to be on the safe side since I piped, jetted, and did a clutch kit on the quad. Here is my question, what size do I bore it over to? 20mm over, 40, 60, or 80mm? I dont really plan on reboring the bike ever again, if it gets to that I will buy a new bike since this one is somewhat old. I assume the larger I bore it the more power gain I might get..Is this assumption correct? What size should I bore it to? I was thinking of boring it over 60mm from stock. Any suggestions? This is my first rebuild so I really dont know what the hell I am doing. Any input would be helpful! Thanks in advance.

 

never bore out any further than needed to clean the cylinder up.Pull the cylinder and take it to the shop that is going to do the boring,have them mic the piston and cylinder and get the piston from them too.you get a better job this way.

 

Am I safe by buying a wiseco piston that is 20mm bigger than stock and bringing that piston and my cylinder to get bored? I found a online vendor selling wiseco pistons and top end gaskets for a unbeatable price. My stock cylinder should be in good condition, I only want to bore out and change piston because I have read polaris pistons are the weakest link in the polaris 250 motors. Again, any input is very helpful. Why dont I want to bore out the cylinder to the biggest possible size? Would this not give you more power?

 

The weak pistons are on the 400s 250 pistons are good.Dont do a topend until you need to for low compression

 

my compression is 90 to 95psi at best. Do I need to rebuild my top end?

 

I would say that's borderline. Anything less than 90, I'd rebuild.

And yes, take the piston to the shop that bores the cylinder.

And you don't want to bore out to the largest size just in case you have a problem later, or with the new rebuild. Always leave room for error. The boring itself won't give you that big of a power boost. Now if you were to bore and stroke it, then you would make a bigger difference.

You could also lighten the flywheel and bore the stock carb. The quick rev you get from the lighter flywheel will help get those wheels up.

 

Thanks for the info. Well my compression is not lower than 90 right now so should I do the rebuild or just wait another year?

I am already in the process of changing my starter, so the clutches are pulled and tearing the head off would not be hard in its current state. What should I do.

 

Well, if you halfway there, and it's something you think you'd do in a year anyway, I'd have it apart all at once. You can at least look at see what the cylinder looks like. Who knows, you may be lucky and just need new gaskets.

 

I pulled the head last night. The cylinder looked good, not scratches, nothing. I did notice that I had what looked to be a small leak from the head, through the gasket.

Should I replace the gasket then do another compression test. I dont want to spend the money on this rebuild if its not necessary. Thanks again for all the replies.

 

yhea I would just replace the gasket and do another compression test if you donw want to keep the quad for a lot longer.