Need 400 motor advice......

[RIDER11X]

Well, my 2001 400ex is finally starting to show some signs of needing a top end rebuild. I need some input from anyone knowledgable on this.

1. Will I need to pull the motor out of the frame to do it right?

2. What is the best way to maintain maximum reliability in the motor, while gaining reasonable performance. I am a woods rider, and don't want to have overheating problems. I am thinking about using the HRC cam so I don't need to go with hardened rockers. Who has the HRC cam, and is it for a XR400 or a 400ex?

3. Does a 416 or 426 sound like a good option? Will I need to resleeve for either one? Is a 406 better for me?

4. What piston sizes are 406, 416, and 426? What brand and compression ratio is best? I don't want to have cylinder stud problems or head gasket problems. Reliability and durability is key.

5. Will I need to pay attention to anything special in the head? Any common wear areas? Should cam chain be swapped out? Will that be possible without splitting the cases?

Thanks in advance for any help!:cool:

[bradley300]

i'm no 400ex expert, but i think this is how it goes, 80 over=416cc, the 400ex can be bored 100 over=425/426 cc, but this motor has very thin walls, wich tend to overheat. not sure what stock compression is, but going to an 11:1 piston seems like a reasonable jump. more power, not too much stress on the motor, and you can still run pump gas (i do in my 11:1 mtor) but, ive heard if you have a little extra money, that you can feel the difference of a good race gas

i dont see why the whole motor would have to be taken out, the cyl. and head should be all you need. i would just make sure all your sufaces and not warped, and nothing looks out of the ordinary.
not sure if thats much help or not, but thats a few things ive gathered

[cals400ex]

1. engine can stay in

2. i would do a drop in cam. you can do the hrc or a hotcam. neither one will use hardened rockers. the hotcam will be cheaper. the stage 2 hotcam and the hrc will be similar cams. the stage 1 hotcam and xr400 cam will be similar. you should be able to get the hrc from service honda.

3. if reliability is the key, i would do a stock bore, 406 or 416. you will have good reliability with any of these. the 426 is the largest you can go in the stock sleeve but at that point it is getting thin. i have a 406 right now that i am taking apart and going with a 426. i was pleased with the 406 but summer is not here and i was bored and needed something to do. :D

4. like i mentioned, a stock bore to 416 will give best reliability. for the compression you will probably want 10.8:1 at most. a 10.5:1 should work fine too. in my 406 was a ross 10.8:1 and i didn't have any problems with the studs or head gaskets and it ran good on 92 and 93 octane.

5. your head should be fine. many people will go ahead and replace the valve seals since those do go bad over time. i left mine along and haven't had a problem. if you notice you bike wanting to smoke a tad bit on start up, i would go ahead and get new ones. for more power you can get the head ported but it won't be a necessity. with the crf 450 timing chain only being $35, it wouldn't be a bad idea to replace it. i have one in my bike now but i just put it in when i took my 406 out. splitting the case won't be needed. all you do is remove the right side case cover and remove the clutch and basket. i also removed the 3 bolts holding on the oil pump and i was able to slide the timing chain right on in. its not that difficult. the hardest thing for me was removing and retorquing the clutch basket nut to the specified torque.

[cals400ex]

Originally posted by bradley300
i'm no 400ex expert, but i think this is how it goes, 80 over=416cc, the 400ex can be bored 100 over=425/426 cc, but this motor has very thin walls, wich tend to overheat. not sure what stock compression is, but going to an 11:1 piston seems like a reasonable jump. more power, not too much stress on the motor, and you can still run pump gas (i do in my 11:1 mtor) but, ive heard if you have a little extra money, that you can feel the difference of a good race gas

i dont see why the whole motor would have to be taken out, the cyl. and head should be all you need. i would just make sure all your sufaces and not warped, and nothing looks out of the ordinary.
not sure if thats much help or not, but thats a few things ive gathered



406 = .040 over
416 = .080 over
426 = .120 over

[RIDER11X]

What about if I stick with the stock bore and go with a 11:1 compression piston? Will I need to change anything other than the piston? Will that give more low end or top end power?

BTW, thanks for all of the help so far......This is my first engine rebuild so please excuse the lack of knowledge. :huh

[Hotdog]

if your taking the cylinder off. your better off going with a 416. that where there the cylinder has all the wear out of it. the piston is not anymore money and to bore it out only costs around 50 bucks.

[K_Fulk]

Originally posted by Hotdog
if your taking the cylinder off. your better off going with a 416. that where there the cylinder has all the wear out of it. the piston is not anymore money and to bore it out only costs around 50 bucks.

Thats what I would do if your looking for reliability. I ran a 11:1 in my for 440 for for over 2 years and never had to run race gas or had any head stud problems. You may want to get your valves looked at and make sure they seal correctly. And if you need new valves I would go with stainless there only a few dollars more than stock and you can regrind them.

[IL400EX]

I prefer to not to skip overbore sizes, That way I can get 3 bores on one sleeve. .040 - .080 - .120

My first rebuild I went with a .040 bore and 11:1 piston

Second rebuild was an .080 bore and a 10.5:1 piston a valve job and a head port

Hopefully my motor will last this summer and then go to a .120 bore along with a bottom end rebuild.

The easiest power upgrade to most of my friends ATV's is to correct their jetting for them.

That's my thoughts.

[RIDER11X]

Originally posted by IL400EX
I prefer to not to skip overbore sizes, That way I can get 3 bores on one sleeve. .040 - .080 - .120

My first rebuild I went with a .040 bore and 11:1 piston


What kind of gains did you get on that first rebuild? Were they noticable?

[IL400EX]

I felt that the gains were not huge but definately noticable, The gain probably came more from the 11:1 piston than the bore size.

[redroost85]

This is good stuff guys.:) I too am looking for some more ponies under the plastic!:devil:

[RIDER11X]

So do most of you do the work yourself, or do you send it out? I was thinking of disassembling and sending out the head and cylinder for inspection and needed work. Seems like it reqires alot of specialty tools that I don't have.......Is this true?
Times like this is when 250r's are awesome........no many parts to worry about.:(

[cals400ex]

yes, do most of the work yourself. just get a service manual and the tools needed. you will want to send the cylinder somewhere and the head too. do not mess with this yourself. a local atv shop in your area should be able to do all of the honing or boring. however, i would go to a builder for the head porting if your having that done. i, as of right now, am still running my stock head.

[JRDrider22]

guys take my advice.....my gaskets blew...put new ones in ...everything is perfect ..just wont start...yea its weird...when you do stuff to your motor expect problems. and high gas prices

[Doibugu2]

Originally posted by RIDER11X
So do most of you do the work yourself, or do you send it out? I was thinking of disassembling and sending out the head and cylinder for inspection and needed work. Seems like it reqires alot of specialty tools that I don't have.......Is this true?
Times like this is when 250r's are awesome........no many parts to worry about.:(

It takes longer to get your plastic off and all the other stuff off around the engine then it does to actually dissasemble the engine to do the rebuild.

The only thing that you really need is a torque wrench that goes down to around 12 ft lbs. and a service manual.

Take your time, read the manual before you start, then jump in.

It's not hard.

[cals400ex]

Originally posted by JRDrider22
guys take my advice.....my gaskets blew...put new ones in ...everything is perfect ..just wont start...yea its weird...when you do stuff to your motor expect problems. and high gas prices


i would disagree. i have done a few bikes as of now and have had zero problems yet.

things to check:
--oil level
--timing
--new spark plug and make sure boot is in place
--gas in tank and turned on
--correct jetting
--run switch on

[JRDrider22]

oh really..maybe you should do mine...no one can figure it out dude its pretty freaky. gets sparks and everything...just cant figure it out

[cals400ex]

well changes did you do to the bike when building it?

[RIDER11X]

What is better.....10:1 or 11:1?????

[cals400ex]

Originally posted by RIDER11X
What is better.....10:1 or 11:1?????



well, i would try to get inbetween these numbers. however, your gasket choice can do that. with a true 11:1, a mix of race gas might be needed. however, a rated 11:1 many of times isn't a true 11:1. i am running 10.8:1 right now on premium fuel without any problems.

[RIDER11X]

Decided to take the entire motor to the builder to get the full 416 or 426 kit, as he does other "special" mods which seem to make them really haul. Can't wait to get the money together to get it done.:macho