O ring on KZ1000J cyl. block or silicone?

I thought about it but being there is a gap in between the head and bore, I wouldn't try it. A one piece gasket covers the whole area. The silicon just fills in the grove. I fill it and put the gasket before it cures. No leaks.
You might check with the seller for a one piece meant for the J model
I had a choice, 70mm or 72. I bought the 72. That little space will fill up with carbon and have no effect on compression. I plan on a copper gasket in the future but the fiber gasket was a known item I could be sure of no leaks. It's in my good guys thread.
Steve
kzrider.com/forum/13-bike-related/599607-the-good-guys#689665

barney wrote: Zed. Is there really enough crankcase pressure to blow out a properly applied top quality silicon in the timing chain tunnel o ring groove ?

Yes! and in addition to that the combined heat of the engine and the combustion by products that are held in the oil will also soften any exposed silicon which will cause it to fail.
If you have ever pulled blobs of silicon from the bottom of the sump where a PO has used excessive amounts you will notice that it is usually very soft, elastic and degraded and this is what will happen to even the best quality silicon .
The original 'O' ring material is designed to resist the effects of heat and oil contaminants and this is what should be used if using the 2 piece gasket.

That's right, just not 30 year NOS rubber.
Steve

Thank you for all of the responses I've decided to use an OEM O-ring and do away with the idea of the silicone

Good decision Barney the price is minimal in 29 years of J ownership i have never had a leak from the o ring. I do however "replace it" every time the motor is down