Multiple Base Gaskets?

[YZThumper]

I'm replacing the top end on my 91 KTM 500 and I just disassembled the top end this evening. When I pulled the cylinder off the head, I discovered 3 different gaskets stacked up. When I look in my manual, the diagram only shows one base gasket. The parts listing on the same page shows 3 different width gaskets, but I assumed this was done to provide the user a choice, kind of like the different size pilot jets listed on a page. Does this bike require 3 base gaskets stacked on top of each other, or did the previous owner misunderstand the manual and use all 3 different width gaskets. What affect does stacking or broader width base gaskets have on the performance of the bike?

By the way, I could kick myself in the ass because the piston and cylinder looked nearly new when I pulled it apart. I probably could have gone another whole season without replacing the top end !!!

 

[cr25096er]

i wonder because its a 500 if they were put on their so it had less compression. would make it easyer to tune and run cooler wouldnt it? also it could rev further if the compression was lower.

 

[Eggnchips]

I would guess that it would be to tame tha beast a little. Putting in three base gaskets would reduce the compression ratio and hopefully mellow out the power a little.

If you go back to one gasket be prepared for a monster when you fire it back up!

 

[Husky Texan]

The base gaskets shown in your manual are different thicknesses. Some of the European manufacturers offered different thicknesses so you could set the deck height correctly to make up for manufacturing tolerances in the cylinder and piston. Maico did the same thing. Seting the height correctly will keep the port timing right and make sure you don't have piston to head contact. :scream: Sounds like you need one thick gasket. Measure the thickness of the three gaskets together with a micrometer and give Barry Higgins a call at H & H KTM. I bet he can set you up with what you need. Of course you could always make your own base gasket if you have time and patience. :D

 

[YZThumper]

I wish I could have measured the thickness of these 3 gaskets, but when I pulled the head off, they fell apart like wet tissue paper. I'm the 3rd owner of this bike and I only know the 2nd ownder put a new set of rings in the bike. I decided to put a new top end in to be safe, but now I have no idea how thick the base gaskets should really be. I bought the middle size base gasket from the manual. Can I tell if the piston will contact the head by placing the cylinder over the new piston and runing the crank to TDC?

 

[Husky Texan]

You're probably OK with the middle thickness gasket. If I remember correctly that should be .5mm thick and the next size up is 1mm. The thin gasket is either .2 or .3mm I believe. If you want a definitive answer call Barry at H & H.

 

[David Trustrum]

Here’s my guess & I thought it was a pretty obvious one. Owner no.1 found out it was damn hard to kick over & decided to lower the compression somewhat.

Reassemble it with the thick base gasket & put some solder through the spark plug hole to the end of the bore & turn the engine over. Measure it with some vernier callipers (cheap ones can be bought for bgr.all). Then again in 3 other positions. You should have at least say 1.5mm on such a big engine.

Try starting it & decide whether you can live with this when you are tired. Fit extra gaskets when you are honest with yourself (unless you are Arnie?). Extra base gaskets can be a liability for leaking if you are using more than 2, but stock gaskets are probably good quality.

Do check that KTM weren’t fitting extra base gaskets due to a rash of detonation problems.

 

[MrLuckey]

Here is a pretty good thread that discusses the KTM deck height/X/Z dimension fairly well. Don't know if this will help but its good reading nonetheless!

http://www.dirtrider.net/forums3/showthread.php?threadid=5502&highlight=deck