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2000 Honda Foreman 400 Cylinder/Piston Dilemma

ugxvibe

By ugxvibe
in Honda ATV Forum

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ugxvibe Newbie

ugxvibe

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So we purchased a 2000 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.

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). Could a 400 be bored that much? Or was this actually a Foreman 450 engine? I am so confused right now. Maybe an aftermarket jug or engine swap? 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?

Also, they are telling me the valve seals I ordered for the Foreman 400 are too small for my valves? Are the valve seals for all 3 engine sizes different sizes as well?

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. Thanks!

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spock58 Collaborator

spock58

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I believe that the top ends for those Honda 400/450's were interchangeable & people were doing the swap for more power. Not sure how to tell the difference in cyl. heads but using the 400 head would provide more compression. You can go to an online Honda parts place to determine what jetting came stock and compare to what is in yours. Another thing to check is the VIN (serial)# to see what model the frame is - maybe the fenders were switched? There are lots of aftermarket parts out there these days so finding out what you have gets tricky.

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