Hey guys, I am trying to get my 400ex rebuilt to a 426.. does anybody know the piston size and if I need to get a new cylinder or just bore out the stock one? The reason I'm asking is because my local parts dealer just has the piston listed in several different sizes, unlike the kits on ebay that clearly state it is a 426 kit.
CGriff

 

i think you need the sleeve for the426 big bore. if you bore it one or two over then they could just machine your old sleeve to fit the piston, but nut much hp gain.

 

You shouldn't need a new sleeve for the 426, just the 440.
Pick up a piston and have it spec'd and they will bore your jug to that size.

 

OK but what size piston is the 426? I probably sound like and idiot here but I was hopin for like a size in mm? I'm pretty sure thats how the pistons are listed

 

O nevermind I just found it... 88mm [img]i/expressions/face-icon-small-smile.gif[/img]

 

Determining that a piston set needs to be changed out and a bore job is needed.

First examine the skirt and ring area of the piston perpendicular to the wrist pin.
See if deep groves are embossed in the walls of the cylinder and piston.

Normally the piston to cylinder clearance is between 0.005 to 0.007 inch this is measured at the front or back of the piston. This is measured with a simple flat feeler gauge. (Gauge must extend past the length of the piston, and measured at the full length of the cylinder where the piston travels)

If the groves are not deep and if the clearance is at the low end, a good hone job can get it back in shape.
Test the honed cylinder with the same piston to see if it is in tolerance. A good 3 or 4 flat stone honing tool will remove the elliptical shape or worn area the piston had made over the duration of the engines use.

This is the least expensive way to determine what size piston to go by.

If the gap is over 0.007 inch then it would be wise to go with the next size over from stock size to 1 over, which I believe it would be 0.010 inch, 2 over would be 0.020 inch, etc., etc., etc? convert these numbers to mm multiply 25.4 mm = 1 inch.

If the groves can not be honed out then the machine shop will have to determine the next size piston.
When they bore it out to clean up the grooves, they would test gauge mic. it to see what size will be the closest piston size taking into account the hone job needed after the boring.