You guys are just lucky, and there IS an ignition sourch in the crankcase, especially in older engines---it's called blowby

In my lifetime I've been directly privy to at least 3 cases of crankcase explosions--admittedly these were all caused by fuel pumps leaking fuel into the crankcase

CASE 1 About 1973, Sand Diego, CA. I was on TAD from Miramar, at the "downtown" Navy shore patrol, the "old" San Diego jail. A habitually late sailor had a weird excuse for being late--and I demanded proof. He said his kid started up his 57 Chev to warm it up and BOOM. It drove the oil filler cap off hard enough to dent the hood, blew one valve cover off the engine, and ruined the other cover and pan

CASE II Aftermath, a friend had a 68 Ford 390, also in San Diego. His engine just warped the front/ rear pan seals and blew out the dipstick and filler cap. We weren't smart enough, then, to realize what happened. When he took the car in to get it fixed, the mechanic said "probably the fuel pump", and it was

CASE III After the navy, we were leaving a local cafe, someone had jumped into their 78 Ford Granda/ whatever and cranked the ignition. This was a cold winter night, and BOOM came sparks, oil and junk from under the engine. We didn't stick around, only to take a quick look under the car. It obviously had tried to remove the pan.

These were all caused by fuel in the oil, but it doesn't matter. Anything you do to set up a combustable air/fuel mixture in there, and a little "flash" by the rings is all you need.

Where did "we" learn this propane / acet "trick?" Is this being taught in some mechanics school? Is it in some manual / book you bought? I've never heard of this before, but as a guy who used to service HVAC, including nat gas/ LP/ oil heat, I'll guarantee you that if you do this enough, you'll regret it.