1997 Honda Foreman 400 Cylinder/Piston Dilemma

[UgXvibe 0]

So we purchased a 1997 Honda Foreman 400 from a guy over a year ago for farm work. It was running well for awhile and then started billowing blue smoke about a year later (oil burning) when ever it was running, even under acceleration. So I decided to take off the cylinder and sent it to a machine shop. I am an auto mechanic by trade so was pretty easy.. Anyways, They are telling me the bore measures 91.5mm. This was sold to us as a 400. The fenders say Foreman 400. I cross referenced the VIN and it matches to a 396cc bike.

What could have been done to this? From what I have read a 400 has an 86mm bore, 450 90mm and 500 92mm. Is this correct? And how is this cylinder measuring 91.5mm? They want to bore it to the next size which would be 92mm (91.96 piston). It seems like they upgraded to a 450 cylinder and that was bored somewhere down the line. The odometer shows 10,000 miles (which I though was crazy for an ATV). I am so confused right now. I have no idea how to tell. This is my first ATV. I never had any idling/performance issues so I assume it's jetted correctly for the 91.5mm bore? How do I check the jet size and what is recommended? If this is a 450 cylinder and it's now at 91.5mm and need to be bored to 92mm is the sleeve now going to be too thin?

Sorry for this long rant but I thought I had it all figured out until they told me the bore size. Any help would be appreciated. Pretty nuts for a first post huh? Thanks!

[jeepwm69 7,750]

A 450 cylinder will bolt right on so someone might have swapped a 450 cylinder on a some point in the past.

 

Any markings on the cylinder?  The stock cylinders are marked with cc’s on the outside on a flat spot

[Fishfiles 20,147]

Sounds like you have a Highlifter Big Bore kit on a 450 , which makes it a 500 , there are no oversized piston or rings available  , that jug has been sleeved  , you need to purchase  a 450 jug , piston and rings and start over , I would rejet the carb back to 450 stock