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Does the engine HAVE to be at top dead center to adjust the valve/rockers? Or can the engine be anywhere in the cycle as long as its a point where both valves are fully closed?

TDC on the compression stroke is the only place both valves are fully closed at the same time. Briggs says piston 1/4 inch past TDC.

But for ease of operation you can adjust the valves having one valve open and adjust the closed one, and then turn the engine over until the adjusted valve is open and then adjust the newly closed one. Which is basically how multi cylinder engines are adjusted.

I always adjust Phillips when the one valve is fully open end of the other is closed and loose. Adjust the loose one, then turn over and adjust the others. When you're all done, crank the engine a few times, and check the gap again, this "breaks in the gap". Adjust again if needed. Pro tip: I gap my valves a little bit tighter for power gains .

The way I found to adjust clearance itself on OHV engines is take the gauge that minimum spec adjust it just slips pas and the next size gauge wont pass. This be condensate for each of each having a different what a good thru is. Most valve clearance are usually across a couple thousandths. So say you are doing a valve that specd at .003-.005 you be very close .003 but not under.

I do this because OHV tends to open the gap as the engine wears and would a longer ime between needed valve clearance adjustments.

hlw49 said:

TDC on the compression stroke is the only place both valves are fully closed at the same time. Briggs says piston 1/4 inch past TDC.

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I don't understand this. Aren't both valves fully closed for the entire compression stroke and then the entire power stroke? Or at least almost all of both of those strokes?

l008com said:

I don't understand this. Aren't both valves fully closed for the entire compression stroke and then the entire power stroke? Or at least almost all of both of those strokes?

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Correct, but you have to be careful because the ACR at cranking speeds bumps the intake on most engines sligntly BTDC compression stroke. So the piston to be in some position that avoids the ACR action when setting the valves.

l008com said:

I don't understand this. Aren't both valves fully closed for the entire compression stroke and then the entire power stroke? Or at least almost all of both of those strokes?

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Depends if the engine is starting or running. The problem is because they use a weaker starter than one that can over the full compression. This is why you can't depend on the compression numbers while cranking as they will be nearly half of the actually engine compression. The leak down test is better at determining cylinder condition.

TDC compression is at the very end of compression stroke just as it enters the power stroke. What Briggs and others are doing is slightly opening either the intake or exhaust valve (depending on design) to lower cranking pressure slightly before TDC compression stroke. Once the engine starts the ACR swings out and the engine goes to full compression. Some older engines actually a camshaft grind that did this via a bump or ramp on one the camshaft lobes but that always had some compression bleeding off.

When the mechanical compression release is used you must be off it in order to adjust the valves; hence, the 1/4" down pass TDC compression method but many of us find it just easier to do it the old way instead finding a measuring stick every time we need to adjust the valves.

ACR = Automatic Compression Release
TDC = Top Dead Center

Craftsman Garage said:

Pro tip: I gap my valves a little bit tighter for power gains .

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LOL
How many more hp do you get?
I used to build 5hp B&S engines for WKA stock, stock appearing and outlaw classes. I have degreed and plotted many stock and aftermarket cams and put engines on a dyno. If you can tell a power differece of a couple thousands lash on a fairly low compression 3600 RPM governed engine I'd like to see that on a dyno. The old flat head briggs with the "easy spin" bump on the cam it did make a difference. Opening up the lash to defeat the compression release bump would give more low end torque and minimum lash gave more lift and duration and higher hp. All this at the 4000 to 6500 RPM powerband measured on a dyno and the difference is still small.

I only know one case here where I slip the camshaft a tooth on an old 12.5 l-head where there a considerable notice performance that only shifted the power band of the engine to more torque at the working speed. I don't even remember if it was advanced or delayed by one tooth but I do remember doing it but heck that was 40 yrs ago,

Hammermechanicman said:

LOL
How many more hp do you get?
I used to build 5hp B&S engines for WKA stock, stock appearing and outlaw classes. I have degreed and plotted many stock and aftermarket cams and put engines on a dyno. If you can tell a power differece of a couple thousands lash on a fairly low compression 3600 RPM governed engine I'd like to see that on a dyno. The old flat head briggs with the "easy spin" bump on the cam it did make a difference. Opening up the lash to defeat the compression release bump would give more low end torque and minimum lash gave more lift and duration and higher hp. All this at the 4000 to 6500 RPM powerband measured on a dyno and the difference is still small.

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Where did you race? My dad and I built and raced WKA stock & modified using Briggs 5HP flathead engines. We raced in upstate SC and western NC. I re-set the lash on the modified engine after every race, and we raced weekly. I can tear down a horizontal shaft Briggs flathead in my sleep. I'm sure you can too.

My butt was too fat for me to race LOL. I just built engines for folks in southwest ohio. There was a local guy that quit building 5hp briggs to concentrate on 2 strokes as that was where the money was. I picked up a lot of his 5hp briggs business and i had access to his dyno and engine building books.
You must have been running one of those super fast ramp speed Dyno Cams that beats the hell out of the valves and tappets

Hammermechanicman said:

My butt was too fat for me to race LOL. I just built engines for folks in southwest ohio. There was a local guy that quit building 5hp briggs to concentrate on 2 strokes as that was where the money was. I picked up a lot of his 5hp briggs business and i had access to his dyno and engine building books.
You must have been running one of those super fast ramp speed Dyno Cams that beats the hell out of the valves and tappets

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I was a skinny teenager at the time. "super fast ramp speed Dyno Cams"... Nope. I think my "career" was before those. Late 70's. Most of the aftermarket engine parts were Horstman.