Engine Hours Modified or reset

No

not to hijack .....but...I seem to remember hour meters that adjusted the hours to the amount of RPMs the engine turned..true???

in other words ; if the engine idled for 1 hr, the meter would show 1/4 hr of use..

No, not that i have ever heard of. CAT records fuel burned and has a fuel burn average and recommends maintanence based on fuel burned or hours. Hours are hours. Aside from CAT, most all of the manufacturers recommend maintanence based on running hours, whether it be at idle or cruise.

i believe they were on a Stevens i ran....meters never were up to my log records...also about the time DD was sold, conversation at a major truck sales and supply place were i got my oil/filters, was about the trucks depending more on the cycle count as opposed to hrs.

I can see trucks using another method because most OTR truckers just leave them idling all night long at the truck stop while they sleep.

The old CATS that had a mechanical one off the tacho drive might have used this method.

Most mechanical tach/hour meters use a magnetic drive and are calibrated so that they read hours accurately at a specific rpm. Lower rpm will record less time, higher rpm will record more time. The difference exists because the drive consists of a spinning magnet that drags a disk connected to the indicator.

Recip aircraft typically use this type of tachometer plus a separate electric hour meter that starts recording at engine start. Maintenance is performed by "tach time" and revenue is charged by "Hobbs time" - so named after the company that makes many of the hour meters.

However, 99.9% of the tachs I've seen are simply electric and record the hours the engine has been running. They just have a feed off of the oil sender and as long as the engine has oil pressure the hour meter is recording.

Increasingly trucks are fitting APU's as the per hour cost is significantly less than idling. And, 6-7000 hours on an APU is not unheard of.

I'll stick my neck out a bit and say that most of the (pre-electronic at least) CATs and DDs had a mechanical drive fitting. An adapter was available to fit a tach generator to drive an electric tach.

Since mechanical drive cables were problematic when the distance between engine and tach got a bit long (depending on bends) most boats that placed the engine instrument panel some distance away installed the electric tach generator or used a mag pickup to read the ring gear.

I ran a 1979 58' Hatteras YF for years that had mechanical driven tachs (no hour meters in them), it had pretty darn long runs to the flybridge with the speedometer type cables and seemed to work ok.