Why is it important to understand top dead center vs. overlap on a 4-stroke?

We're going to show you the difference between top dead center compression and overlap, the intake valves opening pistons going down okay as I so we're  going to fill the cylinder okay, as the  piston comes back up you'll see the exhaust valve that's not moving at all  okay so now we've got this tight enough  combustion area our both .

Our valves are  closed there's a nice close-up showing  both valves closed against their seats but this is where we check valve  clearances when we do maintenance okay I should be able to go in here if this has the clearance and I should be able to rotate that bucket with no effort if I can't rotate the bucket what do I know I either have no clearance or possibly on overlap, so it should be a clue right away make sense okay so let me keep  going here now.

We've sealed the chamber  in there we've power this combustion as  well so we're going to shove that piston  down now the piston is still moving down  okay and the exhaust valve is starting  open what's the intake valve doing field  it's closed right so now we're going to  come up and we're just going to Foley  over the exhaust valve when we're going  to shoot those gases out as we're done  with them watch what we have.

We have a  thing called overlap so now of our  second time up okay piston is all the  way up look at the cam lobes on this  trigger smoker here you see their faces  each other watch no valve clearance okay  doesn't have to be wholly open just soak  a little bit and you see where there's a  gap in there here's a nice close-up of  the valves slightly open in that overlap  position just off their seats okay it's  just barely off it's seat but both of  them are watch what happens.

When I continue on start the process over again  the exhaust valve I was on its way closing if you remember and the intake  valve was opening okay so we'll start  over there's intake compression go ahead  power hey I want to make a video really  spend some time in here because this is  an area that every year teaching or training text or whatnot thigh sees.

A problem where people seem to have this lack of understanding of top dead center compression because overlap is not  talked about in a service manual because  it's not anything needed for us as  technicians or maintenance for assembly  so that's why it's not discussed.

It's a actual fact that happens but you're just  not going to see it so people find top  dead center and then take for granted  that on a four-stroke engine you need 720 degrees or two full revolutions to  have a full complete cycle the big trick  here is really if you're turning the  motor over on the right direction travel  which when I say right I don't mean  right I mean correct you're going to  find that when the intake valve fully opens and fully closes and the piston works.

Its way back up the next time  around you will always be on top dead  center compression just from intake compression power and exhaust so if you  keep that clean mind you should be safe and and be good to always set valve  clearance do cam timing and do the needed tasks that we need to do on these  engines I cannot recommend enough if  you're unfamiliar unsure use the service  manual follow it like a checklist and  everything you need is in there you  build to do this well..