Cometic cylinder base shims

5 replies to this topic
  • jimmydoorlocks

Posted March 29, 2009 - 06:26 PM


I posted this in the general forum yesterday and didn't get a single response. I know I should have posted here first, but for some reason, I felt like trying something different.
Any help would be greatly appreciated. I just want to make the monster run so I can ride :thumbsup: .

I'm rebuilding a 2000 YZ426 that dropped a valve and destroyed the entire top end. Because I'm cheap, I picked up a head, cylinder/piston, and gasket kit off ebay for a steal (all in great shape, I might add). The cylinder and piston are the 444 big bore kit and the gaskets are specifically for the big bore. Along with the gaskets came another Cometic package that contained 2, .032 aluminum cylinder base shims.
My question is about the shims. What is their purpose? Do I need them? Do I use one or both? Do I need gaskets to go between them, or would copper coat alone work? Do I even need the copper coat? This is the first time I've seen these and I don't want to do something wrong when putting my bike back together.

Thanks in advance!!

  • Birdy426

Posted March 29, 2009 - 10:09 PM


HHHmmmmm...I didn't get any shims with my 444 kit, and it came with Cometic gaskets as well. Do you know who'se big bore kit it is? Gorr? Luke's Racing? RPM? If you can find that out, I would suggest calling them or e-mailing them asn asking about the shims. If you find out anything, post back and let us know...

  • grayracer513

Posted March 30, 2009 - 08:06 AM


The '06 and up uses a pre-coated steel shim base gasket, but none of the earlier ones do as far as I know.

If the head gasket is bare steel, Copper Kote is a good idea. If, however, the gasket has any kind of coating on it as the OEM gaskets do, you should only apply what the manual calls for (usually nothing).

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • jimmydoorlocks

Posted March 30, 2009 - 10:03 AM


So, I decided to just call Cometic and see what they had to say about it.
I kinda stumped them too.
I was on the phone with the guy (very friendly and helpful guy, by the way) for about 10 minutes and he couldn't even find the last time he sold those shims. It's been at lest 5 years.
He did answer most of my questions though: The shims are to reduce compression. They extend the length of the cylinder, therefore creating a larger combustion chamber and less compression. The reason for doing this might be because of someone had put a stroker crank in the bike or had a high compression piston set up (wrist pin in a lower location) and needed the extra clearance. No, I should not need them with a mostly stock set up, but I have several things to check before I'll know for sure. If I do run the shim, there would be no reason to run more than one, and yes, it should be copper coated because it is just bare aluminum.

If you're wondering, I just don't like to do things the easy way.:thumbsup:
Guess I'm heading to the shop. I'll let you know what I figure out.

  • jimmydoorlocks

Posted March 30, 2009 - 08:40 PM


I checked everything I could figure to check and for the life of me can't come up with a reason that I would need anything other than the regular base gasket. I guess I'll put it back together and see how she runs.
As for the big bore kit, I still don't know who produced it. I did a little rough measuring of the piston and it's thicker than stock (from the top of the piston to the wrist pin) by about .027". Using simple logic (and nothing based upon actual fact) it would seem that installing a .032" shim would increase the size of the combustion chamber by .005", therefore reducing compression to a level lower that factory. I can't really see the point in that.:usa:

If you've got any other words of advice before I put it all back together, I'd love to hear them. I've got a few days before assembly because I'm still waiting on the new cams. :thumbsup:

  • Bus

Posted April 01, 2009 - 05:04 AM


I'm 95% sure it's the wiseco 13.5:1 97mm kit. I've got an email in to a previous owner to see if he knows for sure.


Related Content


Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

The "going back to the other darkside" adventure... by Monk

Dirt Bike   Make / Model Specific   KTM   250/350/450/505 SX-F & XC-F (4-Strokes)
  • Hot  113 replies

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F

Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies
WR Camshaft Swap Info - last post by jamesm113

WR Camshaft Swap Info

  • 0 replies

Yamaha YZ450F 2017 by Chris.GVS

Yamaha YZ450F 2017
  • * * * * *
  • 1 review
  • Last review by Tommyk_55
  • On Yesterday, 06:20 PM

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.