What will happen is pushrod preload is too high?

Myasylum

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I am running HD stock rockers with RPM heads.

The preload I am getting is .072. I have Lunati cam and lifters. Lunati states should be .020 to .060.

can I run them at .072? Can I use .030 shims?

I've already spent a lot of $ on this car.

Options?

Thanks!
 

Mr. Cranky

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Only if you don't want your valves to close all the way....imo, you taking a big chance on that
 

Myasylum

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Only if you don't want your valves to close all the way....imo, you taking a big chance on that
You mean running them with the .072 preload or using .030 shims?

Thanks!
 

RemCharger

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Can we assume you're checking with stock pushrods ?
Or what length was specified? How did you measure
 

Myasylum

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Stock length yes, but aftermarket chromoly. Which is what? 8.57 on a 400?
I checked at lowest point on the cam at zero preload and made a mark, then torqued down 25ft pounds and make a mark. Then I measured the distance between... which appeared to be about .072, which I admit wasn't as easy as it seems.
I was tempted just to run it, but I figured I'd get opinions here first.

Head was milled .005 and with the cam I figured it threw the geometry off from stock.
 

Wietse

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The best is to get a length checker pushrod and redo the measurement on several places to get an accurate reading.
Then order the correct length rods if required to be safe.
If the manufacturer recommends .020 - .060" better stick with it, i think with rpm (alu) heads you might want to stay on the .050" - .060" range.
 

Curiousyellow71

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I suspect .020 to .060 preload from Lunati is a number pulled out of a stock rebuild book.
.072 will work fine. In fact hughes recommends .080-.095.
Hydraulic lifters will compensate as long as they aren't bottomed out when the engine warms up.
I pulled this from a hughes tech sheet...(they obviously aren't to worried...)

Screenshot_20220809-045945_Chrome.jpg
 
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Myasylum

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That's what I was afraid of was them bottoming out. I noticed when the cam is on the top of the lobe there is no give at all, so that made me nervous. Maybe it's supposed to be that way? There is no oil in the lifters yet either though since the top end is still fresh.
 

Curiousyellow71

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Your over thinking it. The preload is for when the valve is shut and the cam is on the smallest part of base circle. With no oil in the lifter what is described is doing exactly what you should see. Once the engine is running with oil, the lifter will take up the .072 clearance you measured.
 

gtxrt

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most lifter travel is 120-150 thousands, ask lunati or measure your total travel. Hughes one time saying 20-30 thousand preload now there saying 80-95 thousands. your's are perfect and should run quiet.
 

493 Mike

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The less preload you use, the less likely that you will have lifter pump up at valve float conditions, which could cause valves to crash into piston tops. It's a safety measure for engines subjected to high rpm operation.
Mike
 

gtxrt

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if you want to go past 5700 rpms use the pro magnum lifters like comp sells or better yet put a solid lifter cam in. your going to start losing HP at higher rpms with leak down or pump up with hydraulic lifters. there had to be a reason why Hughes changed his tune about preload.
 

Mr. Cranky

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I've always run my hydraulic cams at the minimum numbers. That way there's less of a chance of pump up when run hard. You usually notice it when the engine comes back down into the lower rpm range after a hard run. Generally it'll idle really rough when it happens....and I've always adjusted the rockers after the lifters are full of oil. And I have run over 6k with hydraulics....6200 was my max rpm and know others that have gone more than that.
 

pnora

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I would get a push rod length checker or make one out of an old push rod and get a true accurate measurement.
 

padam

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On my Lunati cam, the base circles are different, since the intake and exhaust lifts are different. It confused the **** out of me for awhile.
Using non adjustable rockers I had to kind of split differnce on pre load.
 

Myasylum

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On my Lunati cam, the base circles are different, since the intake and exhaust lifts are different. It confused the **** out of me for awhile.
Using non adjustable rockers I had to kind of split differnce on pre load.

Yea, I only quickly did one exhaust valve and the preload seemed less than the intake, but like I say I did it quickly so I didn't put much stock in it... But since you mentioned it?!

I'm running them!
 

RemCharger

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If this build is what i am remembering...old engine with new parts bolted on.
Running it high rpms time after time would likely not go well. I like the preload on the minimal, I don't really care for what hughes is recommending.
I have to agree. Might even speculate they recommend that to cover all forms of amateur engine building techniques.
 

451Mopar

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Be careful using shims on aluminum heads, you don't want to split the rocker stands.
Most likely you will be OK because the lifters usually have 0.090+" travel.
I use checking pushrods and custom length pushrods that give 0.040" preload.
 
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