We're working on rennovating a very old flat, from the late 1800's. The old cast iron sewer drain is still in tact & is what we're currently using. I've sealed every visible crack or hole I can find in the pipe, but I still occasionally smell residual sewer gasses. After inspecting it down in our crawlspace, I'm not surprised. It's mostly rust, with some very questionable work having been done to it in the past. We need to have it replaced in the next few years, it's just not in the cards for the immediate future.
When we started this project a few months ago, I never smelled sewer gasses at all. The clean-out cap was also completely missing, and had been for years. By the way, the clean-out cap is in the middle of our sidewalk outside that sees very, very little foot traffic. As one of the final items our plumber fixed, he installed a cap & sealed off the clean out access. Shortly after, I started getting the strong sewer smell in the building, which led me to hunt down as many leaks as I could find.
My main question, then, is this: Would it have any negative impact whatsoever if I simply drilled out a few ventilation holes in the cap? This would allow gasses to escape, or at least drop pressure, before going up our vent stack. It would also serve to prevent large debris from falling down our clean-out pipe. If there's really no down-side, I'd love to drill a bunch of small holes in it for ventilation until we can get the drain & stack properly replaced with modern ABS.