I'm throwing this question out to everyone but I'm really looking for a response from Ron Black. I know that most all bronze cannon tubes had copper inserts in the vent hole. A large hole was drilled and threaded and this insert was placed. Then when the vent hole started to wear excessively it could be replaced. Did they use these inserts on cast iron tubes? Or did they just weld up the worn hole and re-drill it? I've only heard of the weld and re-drill option. Is there a spec dimension where the vent hole is considered too large to be servicable? I can't find this answer in any manual of equipment, drill, or maintainence. I know the original size of the hole was specified to be .21 inches. But it says nothing about when the hole was too big to be safe and dependable. I have a serious stake in the answer to this question with The Virginia State Park Service. Thanks

Keith