due to the large frequency of social media posts over this topic that pop up, i figured i'd do a blog article specifically about it, even though i touch base on everything i'm about to talk about in other articles.
it's really common that people suggest to other people that they run solvents in the crank case of their vintage unrestored engine after they purchase it to clean it out. or the same advice gets recommended when someone pulls the valve cover on their unrestored engine and they ask what to do about all the grit that they see right off the bat on the top end, coating the head and rockers. there's always a bunch of guys that suggest filling the crank case with some ATF, marvel mystery oil, or even kerosene, prior to running it with the said mixture, then changing the oil, to clean it out. i cannot stress enough that this is literally no different than dumping sand in your engine!
all of these vintage engines from the era of leaded gas, in unrestored condition, have varying amounts of grit, coating the whole inside of the engine, on top of the oil pan being coated with an inch thick of sludge thats likely already partially plugging the oil pump pickup. this is not specific to nailheads. this contamination is a compiled mixture of lead contamination, road draft contamination, and deposits from crappy vintage oils from that respective era. aside from the oil pan being pulled to remove this sludge, NOTHING should be done to attempt to clean this grime within the entire inside of the engine without disassembly of the engine. any part you clean, should be done off the engine. you want to clean the insides of the valve covers and valley pan? pull them off and clean them on the work bench. want to clean the inside of the block and the oil galleries? completely disassemble the engine and get all of it tanked. quite frankly, this stationary/dormant grit that's been undisturbed throughout the years is essentially for the sake of argument, "harmless" until/unless it gets broken loose and introduced as a contaminate. in other words, aside from pulling the oil pan and cleaning the sludge out, leave the rest of the engine alone until you're ready to rebuild it.
you may still be wondering where the "sand dumping" comparison comes from? adding solvents to the crank case breaks loose all this grit contamination on the top end, where it then gets thrown off the crank on its way down to the pan. when it gets thrown off the crank, it goes right up into your cylinder walls and everywhere else. deposits in the oil gallery which may have caused some restriction prior, are now introduced as grit that gets fed through your bearings. then the sludge that's in the pan thats never been cleaned out, starts to dissolve and plugs the pickup instantly if it wasn't plugging it already (which can happen from the crank case contamination that's present even if the pan was already pulled and cleaned out.) what gets past the pickup screen gets fed through the oil pump and then plugs the filter, bypasses it and then again, gets fed through the bearings.. (take note 53-58 oil filter housings have extremely soft bypass valve springs on top of the fact the spring is usually damaged and most the oil doesn't get filtered as it is.) so how is that any different than a result of dumping sand in your crank case?
for those that don't know, marvel mystery oil is just mineral oil and solvent. transmission fluid is a solvent. diesel oil is not recommended also because of its excessive added solvents and cleaning detergents. there is NOT ONE single advantage to putting any solvents in your crank case. instead, you are just self sabotaging your investment. if you have rings that are stuck, dumping solvents in the crank case are not going to free them up.