400ex motor locked up

[82CSMiata]

Well, I was out riding my 400ex today, when my trip got cut short. First it started making strange noise, then started losing power, then lost compression, then died. Now it is seized up. This all happened in a matter of, hear noise, stopped to check it out, restarted, when I shifted into second, it started losing power and then died.
Now I believe in if you have to repair something, you may as well upgrade. Right now I have a stage 2 cam, high comp piston, port and polish head, k&n, FMF powerbomb header & muffler, Modified airbox lid, no choke, and a jetted carb. I know that I am going to have to replace the piston and have the cylinder bored some. My question, I don't race my toy, but I do want it to keep up with a 450R, Y-Raptor 700, and some other things. I am not looking to spend a bunch of money, but I know that I am going to have to spend some. I have been thinking about 416, 426, 440, and 460. I want reliability and trail ride ability. I don't mind keeping it in the power band like the stage 2 cam requires. What is the most cost effective and reliable way to repair my bike. What other parts are needed with each piston size. I know nothing about this, but I know that I can change the piston and jug. Please help and links and prices are very helpful too. Or phone numbers to call vendors.

[1fst400]

coming from a guy that had a 400 that tired everything to get it to keep with the 450's. It cant be done. It just doesent work that way.


but if your looking for a little more power I would recomend a 416 kit with somthing like a JE 10.8:1 piston, cosmetic thin gaskets, hotcams stage 2, HD head studs.


That would run ya like 700? I havent a clue. call up c&d racing, they can hook ya up with what you need.

[Chino886]

Originally posted by 1fst400
coming from a guy that had a 400 that tired everything to get it to keep with the 450's. It cant be done. It just doesent work that way.


but if your looking for a little more power I would recomend a 416 kit with somthing like a JE 10.8:1 piston, cosmetic thin gaskets, hotcams stage 2, HD head studs.


That would run ya like 700? I havent a clue. call up c&d racing, they can hook ya up with what you need.

I have a Sparks 12.5:1 JE Piston and the HD head studs for sale, the piston is very slightly used, needs rings and is good to go....

PM me if interested.....I will throw in a OEM head gasket (new) if you decide to take them.....$200 shipped to your door.

[Chris S.]

If you want to keep up with 450's, you will have to go with a stroker motor, and that will only keep you next them through 4th gear. When you both shift to 5th the 450 will pull away. I had a 400ex with a high compression 440 and a 500 stroker motor. Both were well built and I am embarrassed to say how much money I dumped in them to try to keep up with 450's. The 500 was close but still not what I was hoping it would be. How ever both were huge improvements over stock and were a blast to ride. For the money and reliability from what I have read and expierenced. The 416 with a good port job and 11:1 to piston with FCR carb and full exhaust and open or no airbox properly jetted is very close to running with the 440's and is a whole lot more reliable. Your sacrificing a little low end torque but your engine won't run as hot and will last alot longer.

[GPracer2500]

Originally posted by Chris S.
If you want to keep up with 450's, you will have to go with a stroker motor, and that will only keep you next them through 4th gear. When you both shift to 5th the 450 will pull away. I had a 400ex with a high compression 440 and a 500 stroker motor. Both were well built and I am embarrassed to say how much money I dumped in them to try to keep up with 450's. The 500 was close but still not what I was hoping it would be. How ever both were huge improvements over stock and were a blast to ride. For the money and reliability from what I have read and expierenced. The 416 with a good port job and 11:1 to piston with FCR carb and full exhaust and open or no airbox properly jetted is very close to running with the 440's and is a whole lot more reliable. Your sacrificing a little low end torque but your engine won't run as hot and will last alot longer.

Great post!

[82CSMiata]

Okay, I have sorda gotten the understanding that going 440 or bigger is not cost effective. What do you have to do to safely make a 416, and a 426. I am interested in these sizes, but I don't know what is required to get there. I defenitely don't want to have to machine the case, so I just want to have to modify the jug, piston, and maybe the head. It is allready ported, but I don't know how hurt it is since I haven't torn it down yet. I also want to be able to run 93 octane. I here that 11:1 is as much compression that is safe. Also on ebay they have pistons, and sleeves listed. What do you guys think about these pistons and there quality. There is one that is only $80 and talks great about a special coating, then there are some that are $130. I just need help with this decision. For those that think I can't keep up with the 450 and raptor, I am a better rider than they are, so my locked up engine would hang with them as long as we didn't drag race, but in the trails I normally lead because I have better rider control which makes me faster.
What is the largest piston I can put in with out boring the case, and what is required to do this? Bigger sleeve? Heavy Duty head studs? Special head gaskets?
Is there a company that I can send my top end to and have them set me up to just put everything back together, then jet, then ride?
What dan I do to make my motor run cooler after I get it back running again?

[Chris S.]

To make a 416 you just have the stock cylinder bored. Anything larger requires a bore job and a new cylinder sleeve pressed in. That is another bonus of the 416 is that you can go larger when it is time for a rebuild. Stroker motors are the only ones that require having the case machined. Some guys recommend a heavy duty rod when building a 440 which requires spliting the case. Both the 416 and the 426 are reasonably inexpensive to build. J.E. pistons are probably one of the best and what most guys recommend and as long as your compression ration is 11:1 or less 93 octane will be all that is necessary. I haven't heard a whole lot of feedback reguarding the kits on ebay. Price out a top end gasket kit, piston and rings, and a local shop to bore your cylinder and your pretty much there on cost. Also, with your motor locking up there is always the possiablity of need some valve work needed especially if the rings blew apart. As far as extra cooling, oversized oil tanks help and auxillary oil coolers is about all you can do. Perfect jetting is the most important thing for proper cooling in my opinion. There are several good shops that will hook you up any way you want but if your capable get the cylinder bored and the correct piston and get ur done. 400ex motors are easy to work especially if buy yourself a clymer manual. With a good rebuild you will leave the 450's behind in the trails, my ex went through the trails alot better than my 450r. Less shifting around and I still think the ex handles better in tight trails. I mostly ride in sand and that it why I was always in search for more horsepower.