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Discussion Starter · #1 ·
I just bought this 400ex a couple days ago and when I measured my bore I got .8624mm so I was thinking its going to need a 86mm piston is this right? This would make it a 416 correct? The piston that came with is .8618mm it looks brand new but a ring was broke.

Can you just buy the rings for it? looks like a wiesco but not sure this is the part number:4154A4628M08700

This is the first 400ex I have ever owned and only rode one other. however I have owned a blaster 200x yfz grizzly and a outlander 800r so I have some experience with quads. My other question is when tearing down the bottom end I found lots of shavings. So a crank is on order.

My problem is should I replace the tranny bearings too?

Also if anyone has any plastic, or parts they want to sell I would be interested. Thanks for any help
 

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I'm not sure where you got the piston number, but 4628M08700 is the number for a Wiseco piston to fit an 87mm bore and a compression ratio of 11:1 - as for the measurement...... something is wrong. The bore has to be 87mm for the piston you have to fit.

Standard bore is 85mm for a displacement of 397cc and a compression ratio of 9.1:1

Standard piston size is 84.960 to 84.965mm measured 15mm from the bottom of the piston from front to rear (not side to side) - pistons are slightly tapered and slightly oval.

Standard piston to cylinder clearance is .0016"

The 87mm piston you have gives you 416cc displacement with the standard stroke of 70mm - for larger pistons the clearance should be increased - for the 416 piston, a clearance of .0020 is adequate for normal break-in. For racing and quick break-in, set the clearance to about .0025.

Replacement ring set is Wiseco part number 3425XC

You don't have to replace the tranny bearings, but since you have the motor apart and the bearings are less than $100, it is cheaper to replace them now instead of tearing the motor down later to do it.
 

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I'm not saying you did it wrong - I'm saying there is something wrong with your numbers.

Calipers are not accurate enough for measuring the piston and cylinder - for the piston you need a good c-frame micrometer and for the bore, a bore gauge. My micrometers cost over $100 for the 0-1 inch (digital accurate to .00001) and over $400 for the 3-4 inch (analog accurate to .0001). My bore gauge (1.4 to 6 inches accurate to .0009) cost about $160. Calipers are good for a reference (get you in the ball park), but there is no way a Wiseco 87mm piston will fit into a cylinder more than 2 thousandths of an inch less than 87mm. The cylinder needs to be 87.0000mm and then the M08700 piston will fit. For a proper fit, the piston needs to be measured (correctly) and the cylinder honed to the exact size for desired clearance. It's technical.

Using your measurements - you have a piston to cylinder clearance of .06mm (.00236 inches) - standard clearance is .015 to .050mm (.0006 to .0020 inch) - which is acceptable, Honda specs a wear limit of .10mm (.004 inch) before re-boring is required. However, you did not say at which end of the cylinder you took the measurement. The top of the cylinder usually has more wear than the bottom of the cylinder. It's a good idea to check the taper of the cylinder. The taper should not exceed .05mm (.002").

Whenever I bore a cylinder, I measure the piston and zero the bore gauge at the size of the piston less the desired clearance. An engine will break-in faster with a greater than specified clearance, but will break if the clearance is excessive.
 

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Discussion Starter · #5 ·
ok great I measured the bore from the top and the piston without rings. Holy cow you know a lot! haha I think I will measure it again or take it to a couple shops and see if they can give me a better measurement. I will also call some other piston companies to see where my piston came from so I can get rings. Im guessing I will need the original rings that I need for that piston right? I mean if its a cp piston I need cp rings?
 
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