99 YZ400 refresh


7 replies to this topic
  • rtgincnm

Posted February 23, 2008 - 07:09 PM

#1

Pulled of the cylinder head on a buddies 99 YZ400 today. Had very low compression and would not start. Plan to do a complete valve job. Have not pulled off the cylinder yet to look at the piston but the bore still looks nice and you can still see the marks from the hone. There was a lot of carbon on top of the piston but it looks good after the carbon was removed.

Wondering if we should go ahead and pull off the cylinder and do rings and maybe piston. If the piston measures within spec would you replace it just for protection?:smirk:

Also have a new timing chain ready to go.

  • grayracer513

Posted February 23, 2008 - 09:04 PM

#2

Remove the piston and replace the rings. Carefully inspect the small end of the rod and replace the wrist pin. Reuse the piston IF it is in excellent condition, but do not cut corners.

  • rtgincnm

Posted February 24, 2008 - 06:30 AM

#3

Sounds good.

What's your opinion on prep of the cylinder with new rings. I've seen some talking about using a "scotchbrite" pad instead of a soft hone.

Also can you tell how the big end bearing is doing by checking the play of the rod while it's down to this stage?

  • 2grimjim

Posted February 24, 2008 - 09:30 AM

#4

Don't use anything but a ball hone to deglaze the cylinder. One that is 1/4" to 1/2" bigger in diameter will work.

Most available are a 180 grit but that is a little too coarse; you want to remove the glaze, not re-size the cylinder. If you aren't VERY CAREFUL with a #180 you will remove too much material from the cylinder wall.

A 320 grit hone is a much better choice and will allow the rings to take seat much quicker. The abrasive type is unimportant, it only affects the price of the hone and how long it will last.

If the big end bearing has obvious up-down play you probably need to pay some attantion to the crank. The old 400's bottom end weren't as tough as the 426/450's. Aside from tearing the crank out of the engine and checking it on centers there's no easy way to check for radial clearance in the big end bearing.

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  • grayracer513

Posted February 24, 2008 - 10:11 AM

#5

Don't use anything but a ball hone to deglaze the cylinder. One that is 1/4" to 1/2" bigger in diameter will work.

Most available are a 180 grit but that is a little too coarse; you want to remove the glaze, not re-size the cylinder. If you aren't VERY CAREFUL with a #180 you will remove too much material from the cylinder wall.

A 320 grit hone is a much better choice and will allow the rings to take seat much quicker. The abrasive type is unimportant, it only affects the price of the hone and how long it will last.

Your point is generally well taken, except that the Nikasil plating of the cylinder is extraordinarily hard. If done right, even with 180 grit, almost nothing will actually be removed from the bore.

rtgincnm, check this post. The honing process is described in detail.

http://www.thumperta...142#post5671142

  • 2grimjim

Posted February 24, 2008 - 10:37 AM

#6

Your point is generally well taken, except that the Nikasil plating of the cylinder is extraordinarily hard. If done right, even with 180 grit, almost nothing will actually be removed from the bore.

rtgincnm, check this post. The honing process is described in detail.

http://www.thumperta...142#post5671142


Yup, composite nickel bores are tough but the reason I PREFER a 320 hone is the finish. Bucause they are so hard it takes a long time to re-season a composite cylinder bore after honing if the surface finish is too rough. A 320 hone will allow you to get a more uniform finish, its easier on the piston skirt, and the break-in time is very quick.

Btw, the factory finish and many shops like the 180 because it cuts much faster and saves time.

A 180 is ok if the user keeps a very close eye on what's going on. The biggest problem I've found with a 180 hone has nothing to do with this thread though. A 180 hone in the hands of the inexperienced user on a 2-sroke cylinder can be a disaster (especially if the cylinder has a lot of hours on it or Kawasakis Electrofusion cylinder).

The final clean up on the cylinder should be with hot water and dish soap. Cleaning a freshly honed cylinder in a filthy parts washer isn't going to do any good.

  • grayracer513

Posted February 24, 2008 - 10:50 AM

#7

In the Nikasil cylinders I have refinished with a 180, the cylinder was completely deglazed in 3-4 passes, and the surface was too smooth to be felt with a fingernail. Even in an iron bore, a 180 grit ball hone removes an immeasurably small amount of metal when used as recommended, which is the whole idea of them in the first place. Break in/ring seating is nearly immediate, and there is no effect on the piston skirt.

Soap and water is better than solvent, even clean solvent, but what should be done after using either cleaning media is to wipe the bore out with a lint free cloth wet with ATF (auto trans fluid). Motor oil is better than nothing, but ATF works the best. If you use a white T-shirt, you'll be amazed by the stuff that wipes off of your "clean" cylinder.

  • rtgincnm

Posted February 24, 2008 - 06:41 PM

#8

Everyone, thanks for all of the help. I'll check out the thread grayracer refers to above.

Sure is great to have TT to help with these projects. Really makes the learning curve much faster.





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