400ex piston clearance out of spec

[mattl31]

Hey guys,

I have my 00' 400ex torn down for what I thought was going to be a simple valve seal replacement, however I have a few questions that have popped up.

This all started with it burning a touch of oil upon startup, but only after it sits for a while. Valve seals seemed like the logical choice being that the motor was rebuilt just a year ago, however I measured the piston to cylinder clearance and its seems out of spec. The motor is just a 11:1 Wiseco piston, stock bore (once over i believe). Top of piston looks clean, cylinder walls arent scored, but for some reason seem too shiny to me as I can hardly see any honing marks.

The largest measurements I'm getting are .005" and .008" (on opposite sides of the piston. This means .00625" clearance on each side if centered. My service manual states that the service limit is .004"

Is my logic correct that this thing needs a rebuild or am I missing something?

[hocman123]

well i think in order to know u need to bring the cylinder down to a machine shop and have them measure the cylinder and see if it's in spec. u might just need a honeing and a new set of rings might not have to bore. even though it might be worth to do it anyway b/c u can get that thing really kicking

[CJM]

A machine shop needs to accurately measure it. They have several tools to do this that the standard person does. Have it checked, ya never know.

Something like this is what they use: http://www.eastwood.com/dial-type-cylinder-bore-gauge-kit-2-6in-range.html?srccode=ga150120

[killer454]

you should see a mark on top of the piston saying what size the piston is.clean it off and look.

a buddy of mine payed for a new engine around 2 years ago at a shop and when they got the thing done they charged him for a bore and oversized piston.come to find out no bore and std piston,all they did was hone the cylinder out and slap a new std piston in it.

he is haveing his bored out right now as we speak.

sounds to me you got a hone and go to

i do all my own work so i know what i got in my toys for myself,i even found a guy that has the boreing equipment and does it on his spare time after work for 50.00 a cylinder.

your problem does sound like valve seals though i would replace them and see what happens.
you are forgeting that when that cylinder heatsup to operating tempeture that gap will get smaller.

put your valve stem seals in and ride it for awhile.

this is what i would do top end gaskets are cheap if you need to bore later,in other words all you will be out of is the gaskets if you have to take it apart again to bore and go.