On a mild hydraulic cam, what is the optimum preload? How much is too much?

Assuming you have adjustable rocker arms or adjustable pushrods.
Tighten until there is zero slack in the rocker to pushrod and then turn another 1/4 to half turn.
IF you are using the stock rockers and stock pushrods, then you are stuck with what you have unless you can get rocker shaft shims. This would move the rockers away from the valve tip and pushrod giving you less preload. OR order custom length push rods to give you the preload you want.
I assume you know about leaving the lifters sitting in oil over night to get all the air out of the lifter plungers. IF not, you will never get the prelaod right.

More than 1/2 turn is too much imo. 1/4-1/3 turn has always worked best for me.

Recomended is .020"-.040" preload. Fine thread 3/8-24 threads per inch. 1÷24=.0417" So 1/2-1 turn gets you there.
Doug

I set them that way on the engine stand and again with the motor warmed up or hot after breaking in the cam

Does anyone know who's lifters and what part number he is using? It could possibly make a difference.

.020 to .040 is what I was taught, Should be fine for stock replacement but it probably depends on the exact parts used.

What is the OP working on, and what is the parts/machine work combination?

Yep .020 to .040 is good. If you are going to race it then I like to set mine closer to almost no preload but they can be a little noisier. I had my old 383 at about .010 because I raced it some and it was less chance of lifter pump up if over reved. Years ago my buddy used to race me in my 273 Dart and he was not good with his 4-speed Chevelle and he missed gears a lot. One day when we raced my little 273 was right with his 350 Chevelle and right after I yanked third and jumped a 1/2 a car on him with my Hurst Super Shifter he droped way back and disappeared. I went back to his house and about a 1/2 hr later he showed up. He missed third and over reved his eng and his lifters pumped up and held the valves open and shut the eng down. He said he could not get it started for almost the 1/2 hr as the lifters had to bleed back down below 0 preload to get comp back. Ron

Sorry guys, should have been more specific. My good machine shop built the short block and the heads, so I know they were correct. The problem was the shop that installed the heads and installed the motor was terrible. It is a 70 383 with stock non adjustable rockers and very mild cam. I took on the job and found the carb was very rich, the the total advance was 80 degrees and the button was missing in the rear end and the brake drums were rubbing on the backing plates. Therefor, the test drive was limited but the motor ran terrible with very little power. The car is going back together now so when I pulled the valley pan to fix the leaks, I noticed the lifters had way too much preload. .110 to be exact. I think it was holding the valves open slightly above 2500 rpm. I use adjustable rockers on all my builds and I do the 1/4 to 1/2 turn and never actually measure preload. She does not want to spring for the adjustable rockers, so I’ll have to shim the stands or get new pushrods. I called Hughes Engines and they said they liked .080 to .090 preload. That seams high as well. My bud said .060 is fine for a street engine. This motor has less than 100 miles, if that, on this rebuild.

I should add that the lifters are not performance lifters. They have the small wire retainer for the plunger and I have seen them break if not enough preload is used.

there is a school of thought that haveing the cup a few tho up from the BOTTOM is the way to go. I do not think your preload is an issue. On the snap rings there is a snap ring HO-68 from www.huyette.com (iirc) that is cheap/works well & it was suggested to install em with the squared edge on the bottom which I thought was backward but the guy (on here but I forget who) sounded sharp. I previewed this & huyette ain't right but I have the receipt if needed. MORE EDIT it is www.huyett.com HO-68 housing ring internal 11/16 CS PH

Ok then. This raises an interesting question. What will happen at .110 preload? Will it slightly hold the valve open at high rpm or seat the valve anyway. The only motor I built that had trouble was a 318 in a truck. It ran terrible at idle. I installed adjustable rockers with 1/2 turn preload and it ran fine.

stock preload with a stock valvetrain is .110"+. Chrysler tappets have .200"+ plunger travel. I like the plunger somewhere between 25%-50% of the travel distance. I use .060"-.090" preload with an adjustable valve train. never had an issue. some performance tappets have short plunger travel; maybe .100" or less. best thing to do is measure the travel and then decide on a preload distance.

And any influence by the amount of oil pressure? As in might to much pressure makes hydraulic lifters act like solids?

2 schools of thought. #1;Very little preload so if it loses valve train control it cant pump up much. #2; With the lifter almost bottomed, valve train load can't collapse the lifter much losing lift/duration.
Doug

I see this preload topic brought up and battered to death with opinions and such.

Question for those that may have checked.
How much plunger travel exists on a production Chrysler lifter?
How much plunger travel exists on a late model aftermarket lifter?

Reason I ask.

The service manual calls out .060” - .210” dry lash.
That equates to .040” - .140” plunger preload.

If you have production level plunger travel - your engine should not run poorly, idle poorly, perform poorly with this preload.

I ran production juice lifters in Stock Eliminator in the 70’s with a GK cam, production pushrods and rockers and never floated a valve, broke parts, etc. I kept my preload at .060”. No issues for 5 years using the same parts without replacement. Of course I only spun it 6K back then but it never failed me.

Take a 1/8 of a turn out of all of them and see
if it acts better.. if not I would look at the
cam timing.. if its retarted it will be a mutt..
and dont take the word of your shop.. we are
all human and make mistakes

“Too much” preload is when the plunger is down far enough to plug off the feed hole in the side of the lifter.
Other than that........ any preload that doesn’t result in that happening should work fine in a daily driver type application.

In the OP’s situation, if he’s not comfortable with .110 preload, a set of .050 shorter pushrods would certainly put him well within the preferred range.

In a similar scenario as Rons Chevelle story....... I was out wailing on a car with a tired old 361 that did the exact same thing.
I wound it out hard in 1st...... it sputtered a bit, then just shut off.
I had no clue what it was.
I was under the hood looking at wiring and such. When I cranked it over it was like the plugs were out of it.

Then, after a period of time, I tried it again.......just because.........and I could tell some compression was back, at least on a couple cylinders........ and it started.
Running really rough at first, then started smoothing out, and then it was fine.

Any thoughts are appreciated on this subject. I’m trying to learn. I never had the car running long enough to get to the lack of power problems. I was afraid the rear end would loose an axle! I’ve always used adjustable rockers on all builds save my last 440 and it was all original untouched oem parts. So basically, any amount of preload short of a mechanical lockup if the lifter should never hold open a valve, correct? It would just loose lots of lift and duration.

Fastmark - how did you determine how much preload you have ? What method did you use ?