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Old 01-14-2011, 02:39 PM
aricanderson aricanderson is offline
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need help checking/adjusting valve lash

hi everyone, i got a 78 302 engine in a 86 f250. Supposedly the engine was rebuilt although i would like to check the valve lash. It runs although i'm missing something somewhere. After reading i understand that i need to start at tdc and check/adjust valve lash then turn crankshaft 1/4 turn and go in order of firing. now my question is when i check the valve lash with a feeler gauge it should be around 2.4mm. When i take the feeler gauge should it be on the bottom of the rocker arm and on the top of the valve itself? Or should i check from the bottom of rocker arm to the top of the valve spring? because right now i cant even get .7mm of a feeler gauge in from the top of the valve to the bottom of the rocker arm. after adjusting should any of the pushrod turn or should they be tight?
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Old 01-14-2011, 03:15 PM
xlt4wd90 xlt4wd90 is offline
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The lash should be the space between the tip of the rocker and the tip of the valve stem. Where did you get that 2.4 mm specification, and what engine was it for?

Typically for valves with hydraulic lifters, there should be 0 lash, minus 3/4 turns for an adjustable nut. So a 2.4 mm is way too big for any engine I've ever heard of.

I don't remember if your 78 has an adjustable nut on the stud, or if it's a positive-stop nut, or a bolt-down pedestal. The latter 2 types can not be easily adjusted.

To make an adjustment, you turn the engine and watch the rocker motion to let it get to the point where it closes the valve (all the way up). At any engine position where one valve is like that, there should be at least 3 other valves like that. So you do not need to turn the engine to bring each cylinder to TDC, though that is more systematic.

For the adjustable nuts, you tighten it down until there is no gap between the tips of the rocker and the valve stem. You feel this by trying to move the rocker with your fingers; tighten it down until you hear no clicking between the parts as you try to move the it. Then you turn the nut another 1/2-3/4 turns down.
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Old 01-14-2011, 04:28 PM
aricanderson aricanderson is offline
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Click the image to open in full size.Click the image to open in full size.

This is what my rocker arms look like. Thats not my engine although its similar. Its just one nut on the top. I dont have a bleed down wrench.


This is what i was looking at from autozone.com's repair guide

GASOLINE V8 - ALTERNATE PROCEDURE

Follow Step 1 of the preferred procedure given above, but instead of collapsing the lifter as in Step 2, loosen the rocker retaining nut until there is endplay present in the pushrod; then tighten the nut to remove all pushrod-to-rocker arm clearance. When the pushrod-to-rocker arm clearance has been eliminated, tighten the stud nut an additional 3 / 4 turn to place the lifter plunger in the desired operating range.
Repeat this procedure for all of the cylinders, using the firing order sequence as a guide. It takes 1 / 4 turn of the crankshaft to bring the next piston in the firing order sequence up to TDC at the end of its compression stroke. Collapsed Tappet Gap Clearance:




Allowable: 0.071-0.193 inch (1.8-4.9mm)

Desired: 0.096-0.165 inch (2.4-4.2mm)


Should i even worry about the clearance, or are they talking about a clearance somewhere else?

What i'm concerned with is that the rocker arms seem loose to me, although i don't want to tighten them too much. I can move the rocker arm side to side a little. I just don't think im getting the power that i should. and i'm getting some weird exhaust noise or valve chatter, im not sure which one though.

Should i just loosen all rocker arm and then go to zero lash and tighten 3/4 a turn? How do i know what type rocker arms i have? I know they are not rollers.
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Old 01-14-2011, 11:27 PM
xlt4wd90 xlt4wd90 is offline
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Now I understand the instructions; they want you to measure the clearance when you collapse the lifter. You could do that, or you can do what you said; loosen up all the nuts and set to zero clearance, plus 3/4 turn. I should have said "rock" the rocker when I said "move" the rocker to check clearance. There will be some side-to-side looseness due to the clearances in the valve alignment devices, whether it's a set of rails on the rocker arm, a slot in the head, or guide plates. The bolt-down pedestals work with sled-type rockers, so there should be no side-to-side motion.

If you have a bolt-down pedestal rocker, it's obvious. If you have a nut on a stud, you can take off the nut and see if the stud body goes straight down its entire length or has a shoulder. The shoulder is the positive-stop, and that version cannot be adjusted. The other type is adjustable.

If there is too much rocker to stem tip clearance, the tip of the arm will tap against the tip of the valve, which you will hear with the engine running. If you do not turn the nut down enough, it can loosen up with the engine running, make really loud tapping noises, and possibly come off the stud and cause all sorts of damage.

If you tighten it too much, it will not allow the valve to seat properly, which will cause the valve and seat to burn, the engine will back fire and run poorly.
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Old 01-14-2011, 11:27 PM
 
 
 
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