Cometic EST Gaskets - Extreme Seal Technology

My EG 2009 Yamaha YZf 290 has had a constant problem with bad head gaskets. The bike gets about 10-15 hours of run time before coolant loss begins. It starts out slow at first and gradually looses more coolant over time. I've checked head and cylinder warp and even sprayed the gaskets with copper gasket sealant but still have coolant leaks over time.

I've recently noticed on the Cometic website that they now offer a new line of gaskets called EST (extreme seal technology).

http://www.cometic.com/est.aspx

Same part number as before but with an -EST suffix added. The price is also more for the EST line. (almost twice the cost)

Has anyone had better luck with coolant leaks using the EST gaskets on a 4 stroke?

On another note, the bike runs great and power is perfect. Raced for decades, owned dozens of bikes and brands, the 290 YZF is hands down my favorite bike.

Rich, Thanks for the reply.


1. I install new stubs each time the top of the cylinder gets hand sanded / emery cloth (which is almost every time now). I use a granite slab (purchased from Rockler.com for a perfectly flat surface). Figure eight motion over the wet emery cloth to make sure there isn't any warp. Still using the original nuts (without locktite).
2. Craftsman torque wrench. I establish a calibration ratio before every use. Clamping the wrench end in the vise and using a 20lb weight (accurate within 0.002 lbs) string method. Wrench is within 1% of the setting and it's always turned back to zero when not is use to prevent spring drift.
3. Torqued in an alternating pattern (1, 3, 4, 2) and incrementally. 1 is front left and direction is clockwise.
4. Doesn't always leak in the same place but sometimes it's not possibly to find the leak location with the copper spray coating.

My theory is the gasket isn't wide enough at the location next to the water jacket. Not sure at this point.

The next rebuild I'll reinstall new nuts and studs but don't think it's the problem. When the bike was a stock 250, I rebuilt the top end a few times using the same hardware and the problem wasn't present. I don't remember the exact torque increments used during installation. Any recommendation on an increment amount?

The leak increasing over time. The first ten hours there isn't any coolant loss. Then it slowly starts to loss coolant at a rate of 1/4 capacity per hour. It's starts to escalate from the 10 hour point to the 20 hour mark. At the end it's half empty after 10 minutes.

My first thought was overheating but further investigation concluded head gasket leak. I attached a small plastic bag to the overflow hose to contain the antifreeze. Once you let it settle, you can see a fine skim layer of black on top. I'm assuming it's the exhaust gas / carbon. Also, if you remove the radiator cap and start the bike, you'll see very very small bubbles on top during idle. If you rev the bike, it's difficult to see an increase since the whole bike vibrates. Pressure test doesn't work well since the leak starts out so small. It needs to be track tested / hard riding to create coolant loss. Running on the stand didn't show any increase in pressure on the gauge. I also did a combustion leak test which test for exhaust gases in the antifreeze. The color turned from blue to yellow which confirms the presence of exhaust.

I still believe it to be an inadequate seal by the gasket. I find it ironic that Cometic now makes the same gasket set but with "Extreme Seal Technology." Shouldn't the standard gasket be sufficient?

Rich, Thanks for the help. I'm going to call Cometic and get more information. I'll call EG too and pick his mind. He's always steered me in the right direction.

Unfortunetly, I won't be able to test/ride the bike for a couple months. It just snowed again here in the northeast. I'll post back in the spring once the bikes gets some hours running.