I'm recovering from a stupid crash, and couldn't resist to take the shee apart. I took the cylinders off, and am playing to match the ports correctly, then I'll probabaly polish the exhaust. But then, I thought about the 370.

I was wondering what is involved if I do that.
I'll need a set of +2 oversize pistons (from Wiseco or Prox?).

Now the boring. Can I rip off 1mm from the current sleeve (is it thick enough) or do I need to take off the current one and drop in a new one, bigger?

Now 2mm oversize...how many inches is that? Exactly? I talk mm, and the guys with the machines talk inches...sometimes things get goofy.

I know there is supposed to be a little slack between the piston and the cylinder...but where is that slack located? I mean,do the piston measure EXACTLY 66mm nd I have to bore the cylinders slightly bigger, or do I bore the cyliner 66mm and the piston come standardly slightly less? Or is it a defined stuff that the cylinder is "that much" bigger than 66mm and the piston "that other much" smaller than 66mm, or is it a custom thing, when I get my pistons, I measure them, and only then,I'm able to say the cylinde should be bored to "this particular figure that fits with the piston"?

Thanks for any info about this.

 

If Im remembering correctly the cylinder is recommended by yammy to be 0.25mm larger than the piston, so thats like 1/4th of a mm.. almost nothing..

Make sure you dont polish the ports, get them both the same but dont polish them, the bumps and stuff make the gas splatter around and fires better, if they were polished the gas would just roll into the cylinder instead of "spraying".. if that makes any sense..

Anyways....

 

A 370cc banshee is a 66mm bore, It would be an 0.080" overbore, Your stock cylinders should take upto a 66.5mm bore (or 0.100" overbore) When you bore your cyclinders you have to match them to the new pistons that are going in (when you get them bored the machinist will ask for your new piston(s) ) The sleeve will be slightly bigger than the piston, Not sure what the clearance should be though. The big thing with going to a 370cc displacement is that your pistons will be considerably weaker than when they were at the stock size, You will also have to find a piston that is large enough for that bore, Sorry dont know who makes them. A little rule of thumb is that for every 0.010" (1 thousanth of an inch) it is 0.25mm (1/4 of a millimeter) hope this clarifies some things for you.

 

Can any machine shop do this or do I have to find a shop "specialized" in boring with a special boring machine?
I checked the wear, and everything is almost ot of spec (.5mm gap between the piston and cylinder -max is .6mm-, rings are out...everything works, but it's close to the end...
I have seen Wiseco up to .100 overbore, looks like ProX doesn't go bigger than .060, and Vitos are unbelievably overpriced (although they do go to .080 overbore).

 

No, not any machine shop can bore your cylinders, find an experienced engine shop and have them do it. They will take your piston, measure and bore the cylinder accordingly. I think most wiseco pistons run about a .004" clearance, although i may be wrong... and yes a 1/4 of a millimeter does make a difference on high rpm, high HP motor!! Trinity makes a oversized piston, in both long rod and standard rod. I am running their SuperLite piston and i am pretty happy with it. They make it all the way up to 72 mm. I am using the long rod version, and i am pretty sure that it comes in a few more increments than the standard rod piston, starting at 67 mm. You really need to resleeve the cylinders to do it right. Boring it .080 over only leaves you one more bore if you burn down, and the sleeves are getting pretty thin (weak) at that point. And as far as the pistons being weaker if they are larger?? not sure where that info is coming from, but i surely don't believe it!!

Give trintiy a call and they can set you up with the pistons, and probably sleeves too.. just don't let them do any of the work. If your going to do the 370, you really should ahve someone port it also (someone who knows what they are doing) just punching it out to 370 won't give you that much.

 

Finding a reputable shop is always a good idea. I think the only real "big" thing with boring your motor is that the person uses a rigid hone to clean up the cylinders, a flex hone is only good in a 4-stroke motor. As for the pistons being weaker as they get bigger, From what I have been told, it is due to the fact that they use the same amount of material to build the pistons (same weight). Thus causing thinner sidewalls on the larger pistons. I may be wrong as to why, But I do know that the when you go to an oversized piston they do not last as long as the stock sized ones.

 

2 summers ago i went to a 67 mm piston, last year i went up one more size to 67.5 to clean up the bore, and have not seen any problems with pistons. What you say may be true that they use less material, but i think they are still plent strong. If you are making alot of power anyway, you should be rebuilding your top end every season anyway... or at least giving it a good look through. I know the pistons I got from trinity are not just an oversized stock piston, they were designed by trinity and built by wiesco, and they are alot light than a standard wiesco piston... so they must be weaker right?? I doubt it...

 

weight will not determine strength, but the way they make pistons will. I think the 2 different methods for making pistons are to mill them or cast them. Anyone know where to find any info on this? It would seem that a cast piston would be stronger than a milled one, but I would like to be able to read up on that before I followed through on that theory.

 

ok when i want to bore it to 370 and installing the 66mm pistons in it do i have to add an after marken head like trinity or noss or i can use it on the stock head with shaved one ?

 

ok when i want to bore it to 370 and installing the 66mm pistons in it do i have to add an after marken head like trinity or noss or i can use it on the stock head with shaved one ?