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Discussion Starter #1 (Edited)
Hey everyone, im craving for more power again. I want to put a Big bore in my 400ex. I have a full exhaust, 450 carb, and K&N filter with outerwears and no airbox lid. Im pretty sure those are the main mods beside a cam you want for a big bore.

I dont want to put the 440 kit in, i want to do the 426. I have done some research, its the last reliable bore before i need to Re-sleeve, with 11:1 compression and Heavy Duty head studs its just as reliable as stock. Head studs are $60! are they worth it? I have seen a few brands out their. JE, Wiesco, Namura, and Wossner. What brands have good rep, and product. What do you guys have experience with?

I plan on putting a Stage 2 Hot-cam in, some light port and polish, Heavy duty head studs?? and What else should i do while i have the engine ripped apart? Valves? Timing chain? Clutch?

I have been looking at these kits, why are they so different in price? I know the cheaper JE one has no gaskets, where would i go to get correct gaskets? The Wossner also has no gaskets. Is the price $125 in difference because of Gaskets?

TRX400EX XR400 TRX 400EX 400x 88mm 426cc 3mm Bore 11 1 Je Piston Kit | eBay

XR400 TRX400EX 400EX 88mm 11 1 Wossner Piston Kit 426cc | eBay

426 CC 11 1 Big Bore Kit Honda TRX 400EX TRX400EX XR400 | eBay


I plan on doing all the work myself with some help from my grandpa. He has tons of experience with engines and such. The only thing i wont do is the boring of the sleeve.

I feel like i got most of it figured out. Just a few questions.
 

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Discussion Starter #2
Priced it out, looks like its going to cost $380 + cylinder sleeve bore. This is including the cheaper JE piston, Full top end 89MM gasket set, GTthunder HD Studs, and Stage 2 Hot cam.
 

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Discussion Starter #3

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I did the 416 kit with a stage 2 cam and it had tions of power. U did the 416 so I will have the option of doing another bore before I went to 440 to keep some life on my cylinder. And I never did the hd head studs and never had any problems but it never hurts
 

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Discussion Starter #5
I dont think i will do the studs, people with the same setup didnt need them
 

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Discussion Starter #7
Im probably going to keep my jug because im just going to bore it. I would sell the piston, what's your price?
 

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i think only time you need big studs,,is when you put 440 kit,,their's not much left to hold cyl in..cutting to much just weaken the cyl....
 

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Discussion Starter #9
Yeah im only going to run 11:1 compression too, so i doubt i need them. I will probably have the build started in mid November, im not sure if i should do any sort of write up. I may just take pictures and keep everyone posted.
 

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I've used Namura on the last 4 or 5 builds with no issues. The only.thing I didn't use was the timing tension gasket. It did leak.the head studs are are your choice? Im still trying to understand the whole reason behind the heavy duty requirement when the thread diameter is the same. If the studs are re-installed with the correct lock tight and torqued properly you should be fine? Since the.whole issue is with them backing out in the first place
 

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Discussion Starter #11
Im not sure i have read that you will blow head gaskets without them. Mainly for the 440 kit, i just thought i would mention them to see others opinion. Im not going to get them after reading about the more and more people that have not needed them.

Well i have come to the end of the road on kits and pricing. Its basically the same amount for a 416 and 426 both 11:1 compression and both made by weisco.

I dont know if i should go with a 426 or 416. Im leaning for the 426, because it will rev faster than a 440 and it is is almost a full 30cc larger than the stock 397cc engine.
 

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Don't listen to all the idiots that complain about the 440 kit being unreliable.... I'm running a 12:1 440 kit, with stage 2 hot cam, mild port and polish, full exhaust.... all this on stock head studs, and a modified stock gaskets.... I've never blown a head gasket, and I blow all the other 400's out of the water
 

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I am on the second rebuild on my 440. I have stock headbolts, and am reusing them, again. I have had no head bolt issues....ever.
 

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I've used Namura on the last 4 or 5 builds with no issues. The only.thing I didn't use was the timing tension gasket. It did leak.the head studs are are your choice? Im still trying to understand the whole reason behind the heavy duty requirement when the thread diameter is the same. If the studs are re-installed with the correct lock tight and torqued properly you should be fine? Since the.whole issue is with them backing out in the first place
The big issue with the 400EX head studs isn't them backing out, it's when you go with a high compression piston. The extra compression, combined with extra heat makes the studs pull out of the cylinder. I refuse to build a high-comp 400 without them, and try to discourage high comp pistons altogether, but if that's what the customer wants, then so be it. I think the issue is more common as you get into the oversize bores, such as the 416, 426, and 440. Stock bore may not yield as much if a problem, but I have seen one stock bore, high comp rip the stock studs out.
 

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I hate to bring back an old topic but I just bought a 400ex (with 440 big bore) from a buddy. He told me the head gasket needed to be replaced when I bought it. Turns out one stud was pulled lose. So re threaded that and put new gasket in. 10 minutes in to riding it after fixed the head gasket blew again. This time 2 more studs were pulling lose. So re thread those and another gasket in. I just got it back last night so I will find out tonight after work if they are going to hold up.

Any ideas why this is happening?
 

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440 kits require larger head studs . . . GT Thunder sells the stud kit
 

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It was the threads in the cylinder that were the issue. Does that link look like a good kit though? If not can you recommend one?
 
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