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Old 08-20-2012, 07:14 AM
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cobalt327 cobalt327 is offline
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Quote:
Originally Posted by Silver Surfer View Post
I am mocking up the valve train to measure for correct pushrod length and I need some clarification from the pro's. I have read up a lot of valve train geometry lately and pretty much given up on the idea of setting up for midlift (smallest wipe area). Given my hardware I just do not think I can make it work, so I am just going for the center wipe.

First, here is my hardware:
  • SBC350, decked .020"
  • L31 Vortec heads, milled (.010" ?), OEM pressed in studs
  • Howard's Cams full roller rockers
  • Comp Camps XE274H (230/236 @.050", .490")
  • .026" Compressed thickness head gasket (although I am NOT using it in the mock up phase
Use the head gasket while doing the push rod measuring. The gasket won't be hurt; you don't need to torque the heads down to full specs, 40 ft/lbs is enough.

Are yiou using self aligning rockers?

Quote:
When I tighten the polylock it is very clear when it bottoms out/zero lash is achieved. Is it from this point that I should go another 1/2 to full turn?
Just so we're on the same page, "zero lash" is when the clearance between the cam lobe, lifter plunger, push rod, rocker arm, and valve tip is removed. The lifter plunger must not be compressed until it bottoms out- only remove the clearance w/o any compression of the plunger. The ONLY compression of the plunger allowed is the 1/2 turn you're adding after reaching the zero lash point. If the plunger is bottomed out THEN 1/2 turn more preload is added, the valve will now be held off the seat and the engine will not be able to make any compression, so will not start.

Quote:
Where does the plunger on a hydraulic lifter normally operate at in a running engine (as in what height)? Doesn't it normally stay at the very top of the lifter, and when it is bumped by the cam it goes down a few thousands of an inch?
The basic idea is for the lifter plunger to be compressed enough that the lifter will compensate for changes in the valve train due to small amounts of wear and to changes in clearances due to temperature. This gives a fairly wide range of adjustment. But it has been found that under most circumstances, using 1/4 to 1/2 turn of preload works well (unless the manufacturer's instructions say otherwise). You may have read that the plunger should be midway between zero lash and fully compressed, or in the older factory manuals as much as 1-1/2 to 2 turns may have been recommended. The problem w/using that much preload is the lifter can "pump up" under some circumstances, causing a loss of performance and possible damage.

Because motor oil isn't compressible, as soon as there's pressure applied to the lifter's pushrod seat, the lifter is basically solid. There is clearance that will allow the lifter to "bleed down" over time, but in the brief period of time that the lifter is cycling, the tiny amount of movement can be dismissed as far as setting the geometry goes.

Quote:
Maybe I should consider lash caps? Are those OK to run in a street engine?
Lash caps can be used, but they're usually not needed when using a street-type cam/valve train. You can simulate the use of lash caps by carefully positioning a feeler gauge between the rocker's roller and the valve tip to see what changes this makes.

Quote:
In this pic you see the where I am at with the 7.600" pushrod. The lightweight valve checking springs I got are pretty stiff. They are pushing the plunger on the hydraulic lifter down, so when I get to peak lift I use my hand to push the rocker down the rest of the way. I stop when the plunger reaches the top of the lifter. Is this OK?
I don't see how you're getting a good swipe mark using the method you've described. But in any event you need to use a solid lifter. You can use one like you are using and stack small washers into the center until it's a solid or you can use a solid lifter- as long as the seat (where the push rod sits) is the same height as the hydraulic lifters you're using.

Quote:
Right now I am getting a good centered roller wipe path with my adjustable pushrod set to 7.600". This "seems" short. When I tighten the polylock it is almost bottomed out on the stud shoulder. Is this normal/OK?
I would suggest you get or make a solid lifter, install the head gasket w/the bolts tightened to 40 ft/lb, and recheck the measurements to see if they vary from what you saw earlier.

I'd be curious to see what the results would be using a different rocker. If you have a different rocker arm you can use- even a stocker- give it a try to see what it gives you.
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