I would not put stronger bolts in. The factory ones lasted 600 hours plus, and the last thing you want is a busted bell housing. Its easy to check the bolts or just add replacement bolts to your 500 hour service interval. A busted bolt is nothing for a guy like Rob to get out. The dreaded busted bell housing would require splitting the tractor and making repairs, which I am sure Rob can do and actually make it better while he is at it.
Parry,
The OEM bolts were 8.8 like an SAE grade 5, I think.
I replaced them with the same grade for exactly the reason Chris pointed out. I had them tensioned really good and I'm glad they snapped rather than stripping out the bell housing or even cracking it. Chris gives me more credit than I deserve, but I sure don't need to add a repair like that to my workload right now. My Koyker FEL has been great and seen a lot of work. I'm very happy with it.
Richard, yes, I changed out the throw out bearing and had to split the tractor.
You may have seen the thread? If not, go here to read all about it. Clutch Repair Kama 554
It was quite an interesting thread.
Rob-