Neil, remember way back when some car engines had a sheet metal nut acting as a locknut to the big end nuts? Usually left them off. Same as cars - eg Aston - that used split pins on the big end nuts. Simply left them out. I don't know of any engine since the 60's that used locking devices of any sort on engine bolts / nuts save plain washers under head bolts and maybe spring washers on ancillaries.
On my motorcycles I use plain washers on the engine mounting bolts, and avoid stainless for two reasons; they're not as strong as HT bolts and, as this thread highlights, are difficult to get tight due to thread binding. Also, whilst I spray frames and tinware with two pack, it's just a thin coat of primer and another thin coat of black cellulose on engine plates, otherwise one struggles to get anything tight (or stay tight) due to thickness of finish - be that powder coat or several coats of primer / two pack. I hardly ever use locktite for anything. When fitting helicoils (to ensure they don't come unwound with the stud / bolt) and maybe as insurance a bush doesn't move in a casting are about the only instances I can think of.