Cylinder base gasket

Hi all kp members… i have noticed this small gap in the cylinder gasket. Its leaking a little bit from it. Should i be worried or is this a idication of something. The engine had some performance loss on high rpm. For example the straight i had 99km h my best on the straight with the same setup is 113kmh. We did the jetting right but no improve. The engine has run around 8hr since full rebuild. Should i do some thing or just wait until i do top end rebuild to fix it.

And a second question is what 04mm head gasket vs 02… gasket plus and minus and what i want to choose for rebuild

I think in the world of x30 you are well past the prime rebuild interval for maximum performance. Piston / Ring age, leaking base gasket, fatiguing carb gaskets could all contribute to the falloff in performance you are seeing.

It’s pretty easy to replace the base gaskets if you wish to remedy that immediate issue but I’d pull it apart and measure what gasket stack is in there now and make sure you very accurately measure and duplicate that stack when you put it back together. The base gasket thickness determines the end squish value for the engine which is a tech item.

I would suggest sending it in or replacing in a DIY Top end. I’ve had jetting issues and melted a piston due to base gasket sealing issues. If fluids can leak out, also possible for the gasket to leak air and cause issues.

Base gasket thicknesses are used to dial in port timing, squish, and ultimately legality. If unsure, send it in.

I read in the manual that top end is from 7 ro 10 hr i want to do it at 9hr so i can run the whole season on it before bottomend and crankshaft

Not even close. Replacing the gasket is easy, you can go ahead and do that even without replacing the piston, which you can do as planned - or just do it now if you’re planning at 9 hrs anyhow. I run mine to 15hrs top end (but open the head at 10h, clean the piston face and do a detailed inspection with a magnifier) and 30h full rebuild, don’t notice any performance drop.

The gasket size (thickness) sets your squish gap.

The base gasket doesn’t look ideal, but not super concerning either. Sometimes you’ll get a little seepage over time.

It’s probably worth inspecting things. Lift the barrel, examine the piston skirt, crown and underside (post pics).

Replace the base gasket with same thickness if you know what was in there.

Make sure you measure squish (gap between piston and head) before running again. In fact, maybe not where it’s at currently

If you really want to, you could throw a new ring in it if there’s signs of excessive blowby on the piston skirt.

Can you explain how to deterine excessive blow by? Do you have sample pics of this.

While you wait for his answer…there are 3 ways I’d look at blowby. Visually, you can see burn marks below the piston ring, they get progressively worse as the ring wears off. You can then measure ring gap compared to what it was at new and what iame prescribes as a limit, which will confirm if it’s an anomaly. Third way is to also look at transfer ports in the cylinder. If you have a lot of blowby, you’ll see some of the cooking or burnt color in the ports, which means the bottom is ingesting burnt gasses that come from above which could be one of the reasons why the gasket is leaking and looks black-ish (too much pressure/too hot)from blowby, that’s I think where he was going with his suggestion). My personal opinion is that, even though you set yourself up for let’s say a 10 hr interval, it’s a good idea to inspect it visually (inspection light from top and back) every time you are out and tear down/measure at 5hrs or 2.5 if you are picky. If everything looks good, you close it back up. You’ll just need a micrometer and a feeler gauge, that way you can be sure everything is wearing out as it should

Little to no blowby
https://youtu.be/h0eIpriUUdw?t=15

Really bad (note transfer ports are brown too)
https://www.youtube.com/watch?v=WXymRF3wYfc

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Another beginners question.
Should you always replace the gasket when lifting the barrel, or can you re-use the gasket (if it’s okay)?

Always replace it. It will never seat the same.

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For what little it’s worth and to address your original question, it looks like the staining is caused by either a gasket that is not of uniform thickness or that if you rebuilt the engine there was some bits of an old gasket left on the sealing surface. Above the left most stain the gasket looks like it is not of uniform thickness. The solution would be to get a magnifying glass and get a closer look. I can only judge by what is in the picture. A closer look is warranted. Good luck!

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