YZ400 Head gasket


10 replies to this topic
  • edhjr9

Posted June 12, 2009 - 08:19 PM

#1

I picked up a 99 YZ400 a few weeks back. To try and keep this short thanks to you guys I was lead towards a blown head gasket because the water was being blown out. I took it apart today (my first time going deeper than just adjusting valves) and with my knowledge of automobile engines I could see where the headgasket was blown. I would have never believed it could have blown head gasket and run as well as it did. My question for you guys is what should I be looking for while I'm in there? I will probably replace the cam chain because it feels like some of the links are tight. Also what causes it to blow like that. The blown part was on the bottom of the gasket against the cylinder and not against the head.

Does the cam chain tensioner (if that what the long black plasticlike piece is) normally have a ridge on both side or has the chain wore into the tensioner. It looks like it is made like that but I've never seen them before.

Thanks in advance for any advice. I took a couple pictures of it.

http://i435.photobuc...r9/DSCI0745.jpg
http://i435.photobuc...r9/DSCI0746.jpg

  • Birdy426

Posted June 13, 2009 - 10:40 AM

#2

Make sure that the sealing surfaces are true and flat. I blew a head gasket on my 444 big bore, and, even though I knew better, I put it back together without trueing up the surfaces, and it blew again after just a couple of hours. The next time, I dressed it using some 400 grit wet/dry paper taped to a 1/4 in thick piece of glass. Blued it up and "figure eighted it" till the blueing was gone. About 50 hours on it now with no issue.

Cam chain is a good idea. I would measure the piston and bore, and at least do rings while I have it that far down. I would also measure the small rod end bearing. If the 400s have a weak point, that's it. Obviously, check the big end bearing as well. With the SS valves, you should be in good shape, but I would also do a "solvent leak test" as well, just to be sure.

  • edhjr9

Posted June 14, 2009 - 08:09 AM

#3

but I would also do a "solvent leak test" as well, just to be sure.


What's a solvent leak test?

  • grayracer513

Posted June 14, 2009 - 08:32 AM

#4

What's a solvent leak test?

Start by gently tapping each lifter to bounce the valves against their seats, then fill the port with solvent, mineral spirits, etc. There should ideally be no seepage, but from a practical standpoint, some very slow seepage is tolerable. More than that indicates a poor seal at the valve.

  • edhjr9

Posted June 15, 2009 - 08:45 AM

#5

Thanks guys......I just picked up a piece of glass and the sandpaper and will do my best "wax on wax off" figure 8 on the head tonight. Should I do the same with the cylinder?

I'll do the solvent test too.......

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  • grayracer513

Posted June 15, 2009 - 09:17 AM

#6

...(I) will do my best "wax on wax off" figure 8 on the head tonight. Should I do the same with the cylinder?

Yes, and remember, "Focus".

  • 642MX

Posted June 15, 2009 - 04:50 PM

#7

Also what causes it to blow like that. The blown part was on the bottom of the gasket against the cylinder and not against the head.


Lots of things can cause them to blow. Like Birdy said, a surface thats not perfect is a sure bet, but I've seen them blow and both surfaces be perfect too. I never put a head gasket on without using Copper Coat spray on gasket sealer. Right or wrong, its worked for me on many top-ends. I put 3 light layers on each side, let it tack up and install.

Does the cam chain tensioner (if that what the long black plasticlike piece is) normally have a ridge on both side or has the chain wore into the tensioner. It looks like it is made like that but I've never seen them before.


Thats the chain guide. It has ridges on both sides to help keep the chain centered. Its probably fine, I've never seen one on a YZF thats wore to the point of replacement. And replacing the timing chain is a great idea... it should be replaced yearly.

  • 642MX

Posted June 15, 2009 - 04:51 PM

#8

and remember, "Focus".

:thumbsup: :lol: :lol:

  • edhjr9

Posted June 16, 2009 - 07:32 AM

#9

I got it nice and smooth and flat. Never heard of the copper spray but this site hasn't let me down yet with the advice.........anyone strongly advise against the copper spray? Otherwise I'm gonna give it a shot if it can't hurt anything. I don't want it blowing again especially in the middle of the HS I plan on racing in on Sunday! :thumbsup:

  • grayracer513

Posted June 16, 2009 - 08:03 AM

#10

I advise against using it on any gasket that already has a factory applied coating of sealer, such as the OEM gaskets for the 450. Copper Coat is an excellent product for use with any uncoated steel or copper head gaskets, though, and if your gasket is bare or plated metal, I would use it. If it has a factory coating, the best bet is to simply clean all the surfaces.

  • cf3232

Posted June 17, 2009 - 10:56 PM

#11

I would say no on the copper sealer on the head gaskets. The heat and pressure applied when torquing the head bolts down will seal the head and cylinder into place. Just clean the surfaces and make sure to torque the head bolts down properly. That will create the seal between the head and the cylinder for the head gasket. At that point let heat do the rest.





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