what's your thoughts on each?

 

may i suggest doin a search??

 

I have a 416 kit in mine. It made a world of difference. I would start with a 416. If something happens to the cylinder, you still have the 426 bore left on it. From what I hear, the 426 leaves you with very thin cylinder walls, and gets hot really quick. The 440 piston is heavier than the 416, and doesn't rev as fast, but you can't beat a 440 that is DONE RIGHT.

 

Don't forget 465.

 

Hey MX, I have a dyno sheet with 2 425's a 416 and a 440 on "my page"
The top green run is a 416cc with all our trail stuff (stk carb) Biggun Exhaust
The red run is a 425cc same as above but with R4 exhaust
The yellow is a 425cc with 41mm FCR (hurts bottom power but still races well)
The high reving green run is a 440cc low compression10:1, others are 11:-1-12.6:1 all on pump gas
Yes all are above 40RWHP best being 44+RWHP
Hope this helps,
Ray

 

thanks you guys. I did a search and got tons of responses, but I got way too many mixed messages. Most of the guys would say something then completely contradict themselves, with totally false facts. I'm trying to get some straight out answers from people who have first hand experience, and weren't talking out of their asses like most of the guys on the forums do. But I got some pretty straight out answers here. thanks ray that helps alot. who did those bore jobs?

 

Quadfather is right. But so is Bartlett.
I built a 14:1, 426, p&p, stage II Hot Cam, FCR39, carillo rod, LRD full sys., and it was way fast, but it was all top end. You had to really on it to go fast. Kind of 2 stroke-ish. Alot of that had to do w/ carb and cam selection. it was cool for the dunes, not for the track, IMO.
I like 416 (you have 426 left to bore), 13:1, XR400 cam, and FCR39 for MX. 440 is ideal, but make sure it gets done right, use a Ross or JE piston, make sure CR is what they say it is. Check deck height. Use a company that uses a cyl., jig to bore install the sleve.
For any 400ex, I highly recommend the 4 stroke tech Oil Cooler.

 

your right KGB, the 39 (or41) FCR does hurt bottom end, as seen on my dyno sheets. If the head porting is not done correctly it will to. You can see on my dyno sheets, with the Hotcam stg II, stk carb proper porting with Biggun exhaust (top green run) it makes 28 rwhp at 4600 rpm. That's more then a stk ex at peak (25rwhp) and 44rwhp peak. These ex's beat piped (FMF) Banshee's in 300ft drags.
Ray

 

No problem mx, We built and dynoed these engines at my shop.
Ray