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Discussion Starter · #1 · (Edited)
Well after a couple days of fun at the dunes this weekend i blew the head gasket on my 400ex. Just a small oil leak coming from the left side of the head. So now im looking at options.

When i bought the bike i was told it had a 440 kit in it as well as cams. Would the cams be marked and be obvious when i start the tear down? And then i assume i can just measure the cylinder bore to find out if it actually has a 440 kit in it? What measurements am i looking at?

And then, I have no idea what im expecting to find once i start to tear this engine down. Would you guys recommend to do a full top end rebuild with the head gasket? Should i do timing chain and tensioner while its apart? And if it truly is a 440 kit, from my understanding, HG's are a fairly common issue?

I want the engine to be fairly reliable. Im looking for the best balance of power and reliability and im not afraid to spend some more cash if it means i can get a few years of trouble free riding. Would you recommend I go back to a stock bore and just put in a high compression piston and just do full bolt ons? Should i look into a 426 kit? Or will the 440 kit last awhile if its properly gone over? I want to stay with being able to use 92 octane pump gas as well.

Also, what sites to you all recommend for parts online?
 

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The cam will be marked and it will be missing the auto decompression mechanism between the cam gear and the lobes of the cam. It will either be a skinny smooth piece of metal or a bumpy larger piece of metal that feels loose and spins. As for your bore measurement a 440 kit would be 89mm. 426 88mm. 416 87mm. Food for thought. But you will most definetly want to get rid of that 440 if you have the means. That size bore is at the ragged edge of what the 400ex platform can run reliably. The gt thunder hd head studs will make it better but still wont fix the entire problem. Its almost impossible to keep the 440 build running cool enough to not do damage. The 416 or sleeved 426 is the way to go for reliability. You will need to buy a different cylinder because yours if it is a 440 will be sleeved already and not able to be put back down to a 416. Make sure you use a good mls (mulit layer steel) headgasket. Im partial to cometic. Also when it comes to pistons i really like wiseco. They arent an arm and a leg and they hold up well. My .02 on the matter.
 

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Discussion Starter · #3 ·
So is the 440 kit really that unreliable? I see such mixed reviews all over the internet, some saying to expect HG issues and over heating constantly, and some saying the GT thunder head studs solve the HG issues? Wondering if i actually have a 440 kit, if i can get away with just doing a cometic top end gasket kit, rering the piston and install the HD head studs and get the rest of the summer fairly problem free, then redo the engine come fall/winter in the rainy season..
 

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You have to pull the cylinder to get the head studs installed anyway so you may aswell build it right the first time and be done. Thats my opinion. The only way to make a 440 reliable is alot of aftermarket cooling mods, getting the cylinder sleeved and o-ringed if possible (very very hard to find a machine shop capable) and gt thunder hd studs. Then the only thing you have to worry about is the crank bearings going out at an accelerated rate because that big of a high compression piston was never designed to be in there. When people spent crazy money and had ct racing, or a high end racing team build them they went full aftermarket everything. Crank, bearings, rod, all of it was purpose built and they still rebuilt every season lol. Anyone who says they built a 440 kit reliable is 1. Lying, or 2. Dosnt know it but they dont have a 440. They are just going off what someone said they did. (90% of atv owners)
 

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Discussion Starter · #5 ·
Okay, so on that note, is there any piston/cylinder kits that you recommend? Really do not want to deal with machine work if i dont have to.

Realistically i just want the best balance of power and reliability. Im a heavy guy (280lbs) so i need to be able to haul my fatass around no issue while still having fun. Im not looking for a rocket or to win any races. I simply just want decent power and not worry about the engine every time i take it out. And itll be a 50/50 mix of dunes and trails.

And i also have a completely stock exhaust and carb right now as well (Which makes me wonder about a 440 in it) Only has a K&N air pod on it with a open air box.

Would just going back to a stock 400 with a cam, intake and exhaust be my best option or am i going to be disappointed with that? (assuming i have a 440 now)
 

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Piston kits i always suggest wiseco. They are priced reasonable and i have had zero problems with any of them. As far as buying a cylinder kit i dont recommend doing that at all. Because every piston is sligtly different and a competent machine shop will taylor make that piston work with that cylinder. So machine shop is the way to do it. Only piston and clyinder kit i ever see is made by niche. And i strongly suggest against going that route. Honestly 416 or 426 is going to do plenty moving you around and having fun. My 416 with the list of mods below will put a smile on my face everytime i ride it. More than enough power a torque and my buddy who is about your same weight cant get enough of the power over his stock bore high compression 400. The cheapest route is 416 becuase you can just bore the stock sleeve and add gt thunder studs and be done. The 426 to be reliable needs the jug bored to fit a bigger sleeve.
 
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