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Old 11-13-2012, 17:08   #1 (permalink)
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Valve Lash

When using a Hydraulic Flat tappet Cam with Adjustable Roller Rockers mounted on pedestals, what valve lash is to be used?

Long story short: This is an engine I used for a Drag car and am trying to detuned it for the street by using a different cam set up rather than the solid lifters type previously used.
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Old 11-13-2012, 18:01   #2 (permalink)
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Re: Valve Lash

Tried looking for an answer, got some definitive numbers for a racing engine, but a lot of vagueness for street. I'd venture to say 16 thousandth is a reasonable number from everything I've read. If anybody's got a better number, please speak out.
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Old 11-14-2012, 14:41   #3 (permalink)
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Re: Valve Lash

how " much " of a drag engine do you have ? If it is pretty beefy you will have to do a
lot more than changing the cam and lifters to get it streetable.
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Old 11-14-2012, 16:03   #4 (permalink)
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Re: Valve Lash

The only items 'leftover' are 0.030" overbore and 12:1 pistions.

Do you think that would make a big difference?
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Old 11-14-2012, 18:35   #5 (permalink)
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Re: Valve Lash

The .030" shouldn't be an issue, but I'd thinkn the 12:1 pistons might make it a little rough on the low speeds, not unbearable, but rough.
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Old 12-03-2012, 02:23   #6 (permalink)
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Re: Valve Lash

You don't run any "lash", you actually have lifter "preload", which means the pushrod is pushed into the Hydraulic lifter a certain amount. Usually around the 40 thou preload mark, but can be down to near zero preload with certain setups.

And be carefull trying to run too much compression on a small Hydraulic cam. People talk of what compression pistons they have, but what actual static compression do you have when its all worked out? Piston volume, head chamber volume, head gasket thickness, piston to deck height etc.
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Old 12-03-2012, 11:22   #7 (permalink)
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Re: Valve Lash

As xd said there is no valve lash on hydraulic setup.

There is a factory setting but this is with the lifter completely bled down or collapsed using a tool on the rocker arm to slowly bleed the lifter down, and that is 3/8-inch. This is hardly ever done and may not be so relevant with parts so different to factory but it could give some sort of starting point to measure to so perhaps helpful to you.
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Old 01-16-2013, 17:51   #8 (permalink)
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Re: Valve Lash

If you measured your pushrods with a solid hydraulic lifter (as you should - take the internals out etc) then there'll be no lash - you measured with zero pump so you should install with zero pump up.
How did you measure your pushrods? You know they will change length with the different cam and lifter setup.
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Old 04-05-2015, 00:14   #9 (permalink)
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Re: Valve Lash

This thread is old but the information is somewhat flawed as well. Maybe this can help.

I drag raced AMC cars but we worked on lots of GM and Ford at our family garage and lots of hot rods.

Normal stock type engine hydraulic running position is anywhere close to the MIDDLE of the available travel the lifter has built into THAT particular lifter brand used. The number can vary slightly in the brands there. When running in the middle there is a lot of room for normal valvetrain wear and also not holding the valve open. If you take the lifter internals out and then use just the top seat for pushrod you are running then with lifter essentially completely collapsed and right on holding the valve open and also allowing max opportunity for pumping up to occur since you still have all lifter internal travel left and not used up. Asking for trouble there.

Take a lifter and take it apart and use solvent to clean it to then be able to freely compress the pushrod seat in the top and then figure out a way to measure that total travel, then whack the number in half to be in the middle. Then take all lifters going into the motor and use a C-clamp and something in the pushrod seat and squeeze all the lifters down dry of most of the oil so they are bottomed out. That gets you to the bottom number and then you add the half number in feeler gauge to then set the adjustable rocker down on the collapsed lifter with the feeler gauge like with solid lifters. Now though you are setting the clearance to put the lifter in the middle of travel. When the engine starts up and the lifters fill, they will then be set about in the middle and good to go. Or, if motor already together do one lifter again to measure total travel, then set all lifters by backing off until they click then adjusting back down just enough to barely stop the click, you are at top end of travel at that point. Find the thread pitch of the rocker studs and convert one full rotation into a depth measurement and then turn all rocker nuts down enough to get one half your distance there.

Example? GM stud using 3/8"-24 thread, one full thread then uses .042", a standard lifter with .060" total internal travel has a half number of .030" to be in the middle. If lifter set at the very top of travel then using the .042" as a full turn means you would turn the rocker nut another 3/4 turn down to be about exactly in the middle or around .030" down. Pretty much what we used on most smallblock GM s we worked on.

Best to run stockers in the middle but they will run wherever, say 1/3-2/3 or 2/3-1/3, they can run anywhere in the middle as long as away from the ends of travel, or where the problems begin.

Anti-pump up lifters? We never used them, no need. When a valvetrain separates at valve float, the prevailing oil pressure then expands the lifter at that time to then try to take up as much space as it can in the float event. Then valve settling down and all the space closing back up lets the lifter then hold the valve open to run like crap. If you run the lifter close to the top like we did you get virtually no pump up at all, the lifter cannot as it is already at its' extreme extended limit. We ran regular dead stock Mellings grocery getter lifters with the lifter set as close to 1/4 turn or less as possible down from the lifter rattling and using good valve springs had no trouble hitting 7500+ rpm with absolutely zero lifter pump up issues ever. Often like .550 lift hydro cams too.

It's not 'valve lash' technically on hydraulic cams, rather called a collapsed lifter clearance, it guarantees you are in the middle of travel for successful long term use. The early Pinto/MII 2.3s were very bad about people not doing that measurement, then valvejob done and valve raised higher in the head then held the valve open to run bad at rebuild time and lots of cams and followers (rockers to you V-8 guys) torn up by doing that. I remember back in the day no machine shops wanted to touch them as they had not a clue of why cams kept tearing up over and over. Silly, there was no reason for all that damage at all other than the mechs did not know what they were doing.

By the way, using 12/1 pistons on an otherwise stock type motor is asking for severe detonation, the pistons are generally used with long duration cams and why the compression has to go up, simply to replace what gets lost with longer timing numbers. With short cams high compression can very easily damage an engine and problem even worse with todays' crap octane fuel.
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