Setting valve lash at TDC on combustion stroke?

[LeakySeals]

My 8kw Generac propane generator manual says to set valve lash between 002 and 004 at TDC. Ok. Is that during the combustion stroke? Or at the end after the exhaust valve opens? I'm assuming at the combustion stroke because that's what online videos recommend. But when I watched them do it I wonder if they really are at TDC in the combustion stroke. Will try to explain.

Watching online videos I see people using a screwdriver in the spark plug hole to see the piston come up to TDC. Ok. But then I see something I wouldn't expect. When the piston is at the top they move the engine back and forth to get both lifters relaxed at the same time. Hmmm.. if the engine is in the combustion stroke the only lifter that should be moving is the intake closing. The exhaust should remain closed and not move as the piston goes down drawing air, then back up to compress. If I'm seeing both moving together when the piston is at the top that's the end of the exhaust stroke when things start over?

Can somebody help me understand this? It matters because the lifters are only fully loose for measurement in the combustion stroke they are tight at the end of the exhaust stroke even when the piston is at the top. Hope this makes sense

 

[skyactiv]

 

[FowVay]

Set the valve lash at top-dead-center of the compression stroke.

 

[atikovi]

I always adjust them when the pointy end of the cam lobe is pointing away from the lifter. What difference does it make what stroke it's on or where?

 

[JimPghPA]

I always adjust them when the pointy end of the cam lobe is pointing away from the lifter. What difference does it make what stroke it's on or where?

+1, The vast majority of the cam is round ( the part without the lobe ) and has the same diameter. So, as long as the lobe for the valve is no where near activating the valve you are adjusting your adjustment should be spot on.

By TDC actually what you are looking for is the point between the compression stroke and the power ( combustion ) stroke. So, this is when the lobes of the intake and exhaust valves of that cylinder are far away from activating their valves.

 

[Jimmy_Russells]

It's not technically correct but it works. As long as you're on the base circle of the cam it doesn't matter when you do it.

 

[vw7674]

top of combustion/ignition stroke. just crank the engine manually, watch the intake valve move to CLOSED, you are now near or at true TDC, not the exhaust TDC.
And why the broad clearance range? Usually the intake & exhaust have different clearance values, due to the exhaust running hotter than intake.
Meaning the exhaust has a bigger broader clearance, intake tighter/smaller.
edit: the exhaust stroke, there is 'overlap' when both valves are open, helps scavenge the exhaust gasses from the chamber.
another reason to stay well clear of Exhaust cycle TDC when setting valves.

 

[wpod]

I guess I have been adjusting the valves correctly for the past 55 years.

 

[FowVay]

I guess I have been adjusting the valves correctly for the past 55 years.

I learned about this compression stroke importance when I installed a distributor 180º out of time. There's a lot of importance to that particular alignment.

 

[dwendt44]

Adjust each valve when the other valve is open.

My 2¢

 

[LeakySeals]

top of combustion/ignition stroke. just crank the engine manually, watch the intake valve move to CLOSED, you are now near or at true TDC, not the exhaust TDC.
And why the broad clearance range? Usually the intake & exhaust have different clearance values, due to the exhaust running hotter than intake.
Meaning the exhaust has a bigger broader clearance, intake tighter/smaller.
edit: the exhaust stroke, there is 'overlap' when both valves are open, helps scavenge the exhaust gasses from the chamber.
another reason to stay well clear of Exhaust cycle TDC when setting valves.

This is from the Generac maintenance page. I actually used the 003 feeler gauge. When I checked it the first time I noticed the exhaust valve was too tight.

 

[wag123]

On a small 1 or 2 cylinder 4-cycle engine, the best place to adjust the valve lash is just past TDC of the compression stroke, slightly into the combustion stroke, so that you are past the point where the compression release can affect the adjustment.

 

[Dave Sherman]

I've seen procedures two ways; my 2008 CR-V the procedure was at TDC on the combustion stroke, and all the valves for that cylinder are adjusted at the same time. The recommended procedure on my MG has you adjust one valve at a time, observing when opposing valves are fully open, like adjusting the #1 exhaust valve when the #4 exhaust valve is fully open, and not when the piston is at TDC. I suppose someday when I'm bored I could see if the valve lash is the same at TDC. Since I was warned not to try it the other (simpler) way, I can only assume it's slightly different at TDC.

I think it would be possible the lash may be slightly off if you adjust them at the end of the exhaust stroke when the exhaust is closing and intake opening.

 

[Jimmy_Russells]

The technically correct way to do it is to set the intake valve when the exhaust valve just starts to open, and the exhaust valve when the intake valve just starts to close. But again it simply does not matter as long as the lifter is on the base circle of the cam.