piston & Cylinder

[red400ryder]

ok guys this is going to sound stupid but i'm just curious and i would like to know if this is right......I was looking earlier at my 400's cylinder and i put my old ring on the piston..the very top one only...i slided it into the cylinder to see how it looked and fit....well this is my question...the piston does not fit perfectly into the cylinder...does the piston suppose to touch the cylinder wall all around perectly or can there be alittle space like where maybe it has wore it in so only about 80% of the piston touches the wall and the other 20% ain't against the wall...the ring took up the space cause you could see it but i'm thinking because of that its going to have to be bored..I can also wiggle it alittle bit not much but i can....what do you all think about that...please guys give me your answers..thanks for any help on this..later madmike

P.S. sorry for asking probably a stupid question but i know this place is for information gathering so i'm trying my best to learn everything.

[beerock]

you have to measure the bore with a dial bore gauge and then measure the piston with a micrometer.

Im not sure but there should be a .002-.004 gap between the piston and cyl wall(around the whole piston). When you measure the cyl wall and subtract it from the piston size it whould only b in between the gap(.002-.004), other wise its out of spec.

so yes you should be able to see the ring even when still in specs.

correct me if im wrong on the gap numbers..

[red400ryder]

thanks for your post beerock
What i will do is take my cylinder and piston to the shop and get them to measure everything for me and see what they think....appreciate your help..later madmike

[Predator36]

A good piston is both tapered (smaller at the top) and egg shapped at the bottom. The piston is round and straight at operating temp since the top is hotter. To check piston to wall clearance correctly it needs to be measured at the bottom of the piston across the front and back - not across the sides.