I had to refurbish the top end of my year 2004 350 iron engine because of a failed exhaust valve seat and burned valve. The bike had just over 3,100 miles from new. It came fitted with a composite gasket but as I’d read varying reports on their longevity I decided to change to a copper one, which I was more familiar with. I used a little sealant around the pushrod aperture as recommended.
All went well for a few thousand miles then one night when out with a couple of other RE riders, it suddenly sprang an oil leak - from around the pushrod area. I checked everything over and re-torqued the head fixing nuts but couldn’t find any particular reason for it.
I changed back to a composite gasket and later did the same again when I fitted a domed Meteor Minor domed piston, which increased the CR by quite a margin.
Despite the extra stress on the gasket, it’s been fine ever since. The bikes done another 20,000 miles and no sign of any leaks.