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eprovenzano
09-13-2011, 07:53 AM
Rebuilt the engine from the crank up. Its been running great, until last weekend. The bike developed a small head gasket leak between the cylinder and the head on the side opposite of the timing chain. As a testament to these engines, its still runs... I've ordered a new head gasket, and won't be able to touch it until the weekend, but I'm curious if maybe the nut tightening the head down loosened a bit allowing the leak. Has anyone out there had this issue?
AtvKid4Eva
09-13-2011, 01:46 PM
usually the stock head studs pull out of the cylinder..its VERY common with 400ex's once they have been rebuilt. Contact GT thunder and send your cylinder there and they will install HD studs and its only like $150 with labor. and you'll never have another problem. Chances are if you don't get the HD studs it will just keep happening. Ive seen is so many times.
eprovenzano
09-13-2011, 02:49 PM
I too was thinking a head stub may have come loose. I won't know until the weekend when I have some time to work on it.
CJM
09-13-2011, 02:54 PM
Agreed, probably a head stud. First time I rebuilt I did them and wouldnt have it any other way.

Also are you using quality gaskets? Either OEM or cometic MLS?
eprovenzano
09-13-2011, 03:15 PM
cosmetic gaskets. Its a very small leak, so I'll betting a stud. I'll know more this weekend.
AtvKid4Eva
09-14-2011, 08:13 AM
get ALL 4 studs replaced with HD studs. Dont just put a new OEM stud in.
400exshop
09-14-2011, 09:02 AM
I have cut the top off the nut to get mine tight. I also have had my cyl. O-ring that helps with the 12 to 1 and up builds. my bike has 13.1 comp with nos on it and we do not have HD head studs.
AtvKid4Eva
09-14-2011, 01:22 PM
Cool :rolleyes: Sounds like a hackjob.
eprovenzano
09-24-2011, 07:21 AM
Well I finally had a chance to tear the engine down. I found 3 of the 4 nuts holding down the head loose. I couldn't believe the engine would still run. I pulled the head to check the studs, no issues there. There was nothing wrong with the gasket, but since I had a new one, I put it on. I've reassembled 90% of the engine and should be ready to refire today. I need to change the oil and do a few other maintenance items.