[stng_96]

Hey guys, new guy here, bought a 2006 400ex today, and I have a few questions. First a little background though.... I bought this bike not running, the previous owner basically rebuilt the motor, and ran it for what he said was less then 5 hours total, and the headgasket went.

When he rebuilt it, he put in a 426 big bore, performance cam, jets, and a lexx pipe. So here we go with my questions....

How common is it for the headgasket to blow that quick? I figure he might now have torqued the headbolts incorrectly, and that may have caused it, or he may have just ran the piss outta it as soon as he got it running, but im not sure. Anyone think I could be looking at bigger issues than just replacing that gasket? Also, I have been reading about the headbolts needing to be changed when a Big Bore kit is installed, whats the deal here?

Any other tips are much appreciated, I wont have time this week, so next weekend Ill be tearing it down, and would like to be at least half way educated about it before I dive in.

Thanks in advance!!

 

[shadetree]

Hey guys, new guy here, bought a 2006 400ex today, and I have a few questions. First a little background though.... I bought this bike not running, the previous owner basically rebuilt the motor, and ran it for what he said was less then 5 hours total, and the headgasket went.

When he rebuilt it, he put in a 426 big bore, performance cam, jets, and a lexx pipe. So here we go with my questions....

How common is it for the headgasket to blow that quick? I figure he might now have torqued the headbolts incorrectly, and that may have caused it, or he may have just ran the piss outta it as soon as he got it running, but im not sure. Anyone think I could be looking at bigger issues than just replacing that gasket? Also, I have been reading about the headbolts needing to be changed when a Big Bore kit is installed, whats the deal here?

Any other tips are much appreciated, I wont have time this week, so next weekend Ill be tearing it down, and would like to be at least half way educated about it before I dive in.

Thanks in advance!!

i've read on here..that ppl change out the cylinder bolts.because they won't hold up to high compression..just what i've read on here anyway..as for what the other owner did..like you said..he probally ran the crap outta it..before it had time to seat the piston..and all that..but..i would change out the cylinder bolts to harder bolts..i'm not 100% sure on this..just what i've read on here..and welcome to the forums.

 

[Redracer]

I'm thinking he either did not set the gasket correctly or did not tighten the bolts to spec in the correct pattern.

It is EXTREMELY hard to blow a head gasket in 5 hours from just running it hard. My break-in method for my motors is, ride the piss out of it. The harder you ride it the better power you will get from the motor in the long run. This method has been proved on Dyno machines and I have been doing it for years.

 

[mud99]

If you have to tear it back down go ahead and get a set of harden head bolts. We have a free manual you can down load on this forum and follow the torque sequence. Welcome to the forums.

 

[stng_96]

Ok, any reccomendations on where to get the hardened bolts from? And he said it runs on 93, so I assume that its just an 11:1 Comp piston, but he didn't know for sure.

 

[Helmut]

Welcome to the forums.

 

[stng_96]

Forgot to mention in the last post....Thanks for the welcome guys, I have been wanting to get on a sport bike for a little while, I already have a 99 TRX300, bought it almost new, mods are as follows....

FO-4 Pipe
Jetted
Snorkeled with K&N Filter
2K pound Winch
Grill Guard
27 and 28" Mudzillas
2" Lift
Warn 424
Floorboards, skitplates, etc

So that being said, I am not new to the honda atv scene in general, but the sport section is a learning experience currently. I have decided that since I have about a week until I get the gasket, Im gonna completely tear down the 400 and clean up the whole thing. Ill take some pics of the process.....Hello new project!!

 

[TRX2fiveO]

welcome to the forums... Another thing make sure that there are dowl cylinders around 2 of the head studs opposite corner from eachother. they help keep the head gasket alligned properly, and torquing is a must. I usually torque in a sequence example: if the touque is 20 ft lbs. I set my wrench to atleast half of that. I snug them down with a short 3/8 ratchet so I don't pull to many ft lbs. then I torque them to 10 ft lbs in a cross pattern then I go to 15 ect ect in 5 lb increments until I reach the desired specification in the manual.

 

[jgar]

Blown head gasket for a built 400ex is not a uncommon problem. Heavy duty head studs should be used. You will have to drill and tapp. Might want to check the head to make sure it is not warped.
Link for studs.
Critical Note