Anyone ever blow a head gasket on a 400?

[BossmanEX]

I have a 416 with a stageII hotcam and some head work done. It has a Full HMF, K&N without the airbox lid and the carb was worked over to be compatible with the rest of it. I dont know what the advertised compression ratio is for the piston that I have but this weekend I went riding and notced a sound that resembled an exhaust leak at higher rpms. I noticed where the oil started to leak and on closer inspection realized what it was. My quwstion is, is this something that happens often with theses kinds of mods or is it maybe a head bolt that rattled loose or should I take my head to a shop and have it checked to be sure that its not warped? Should I worry or should I just fix it and go on?

[firefighterjosh]

my friend had a 440 kit and couldn't keep head gaskets on the thing. I think with his there was something wrong with it. Like the head was misshaped or something. I don't think its a common thing.

[BossmanEX]

So if I dont find a loose headbolt or something it would be a good idea to have the jug and head checked for warpage?

[firefighterjosh]

Well this is the story.

My friend had a 400ex he bought for cheap. It was burning oil and needed rebuilt. Instead of rebuilding it he kept riding it. the thing blew up domolishing the piston. He decided to put a 440 kit in it. well he did it all wrong. he bored it only replaced the valves that needed replaced and had his neighbor shave the head. o yea he also put a new cam in it. Then another valve went out. He replaced it. Then head gaskets started blowing.

He was retared to do this he should have put all brand new stuff in at once and had a proffesional do the work.

maybe your is just one of those once in a life time happenings

Yes if there isn't anything loose I would check the head for warpage. Have you ever over heated it? Or blew the motor up?

[GPracer2500]

It is common on higher compression, larger bore 400EX engines that have not been properly reinforced. The problem is the studs like to pull out of their threads in the cylinder. The solution is HD studs installed in new threads formed with a roll tap. For extra insurance you can have a small groove machined into the top of the sleeve. A thin stainless steel wire (think O-ring) fits into the groove and provides extra compressive force on the gasket/head mating surface.

[underpowered]

it is a very common problem. even with larger studs i have still gone through 4 head gaskets on my 426.

[BossmanEX]

Thanks for the replies. Yall are awesome. I didnt build mine personally but I know the guy that did and he doesnt halfway anything. I knew the bike needed rings when I got it so it didnt scare me that it smoked a bit. It still had all kinds of power and only smoked if you let it idle for a while then revved it.

I think I may have gotten it a bit hot the last time I rode it and Im sure it didnt help the situation any. It didnt get so hot that it acted up or died or anything like that but I could tell it was starting to labor knock a bit so I putted it down a dirt road at about 1/4 throttle in 5th to get some air and it cooled down some then I parked at the truck to let it cool some more (about 45 minutes) When I started it back up there was no problems then about halfway thru the next trail I started to notice what I thought was an exhaust leak. I found where the leak was coming from (front right corner of the jug) So I told my partner and we headed back to the truck. By the time we got there it was leakiing all the way around 3 out of the 4 sides of the jug. It still runs but Ive got it parked untill I get the parts to fix it as I dont want it to get any worse.

[F-16Guy]

Send the head and cylinder to GT Thunder and have the gasket surfaces checked (machined if necessary) and a heavy duty stud kit installed. I had the same problem, and it's held tight since I got that stuff done. The key is not the oversized studs, necessarily, but the roll form tap. It creates threads by compressing the metal instead of cutting it away, which gives you a much stronger hold for the threads. When aluminum reaches a certain temperature, it breaks down and loses a lot of it's strength, which is why they usually leak near the exhaust side. My advice is to do it right the first time, because you will be doing it again soon if you don't.