Valve Adjustment on 1985 - L98
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Valve Adjustment on 1985 - L98
Because my #4 exhaust valve rocker arm nut back off almost completely, and caused the problem that's been diagnosed in a different thread, I am now replacing all the nuts and readjusting all the valves.
In my Chilton's Corvette manual, it says to appropriately, index the crankshaft to the compression & exhaust strokes for adjustment of various valves.
Also, it mentions that when the pushrods won't spin anymore by hand, then tighten the rocker arm nut, "One complete turn further", to finish the adjustment.
When I tighten the nuts down this far, it appears that the valves are being pushed down some in the process.
Is this normal or am I just imagining things?
When I took the valve covers off, it appeared that there was a little slack in most all of the rocker arms and pushrod. Not much but some.
Can you please advise??
thanks,
85_TPI
In my Chilton's Corvette manual, it says to appropriately, index the crankshaft to the compression & exhaust strokes for adjustment of various valves.
Also, it mentions that when the pushrods won't spin anymore by hand, then tighten the rocker arm nut, "One complete turn further", to finish the adjustment.
When I tighten the nuts down this far, it appears that the valves are being pushed down some in the process.
Is this normal or am I just imagining things?
When I took the valve covers off, it appeared that there was a little slack in most all of the rocker arms and pushrod. Not much but some.
Can you please advise??
thanks,
85_TPI
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
The point of tightening the nuts until the push rods don't turn, is to get the valve train to zero lash. Then you righten the nut the additional amount is to establish the preload you want in the lifter. The common way to find zero lash is by spinning the push rod until the drag in the spinning INCREASES, but not to the point where it is impossible. With many lifters, it is possible to spin the push rod, beyond zero lash. If you keep tightening until the push rod won't turn, you have gone past zero lash and ALL of the preload. Then, what you THINK is the preload, is actually opening the valve. It won't run like this, because the valves will never close. It may be difficult in some cases to determine zero lash, this way. What I find a little more fool proof, but also more awkward is to jiggle the push rod up and down as you tighten the adjusting nuts. When they don't go up and down any more, the lash is gone.
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Melting Slicks
Joined: Mar 2002
Posts: 3,252
Likes: 6
From: The Colony Tx
I have done this many times on sb's and found that you never tighten more than 1/4 turn after the push rod stops turning. After you start the engine, back the nut off (one at a time) till you hear a tap. Then slowly tighten the nut till the tap stops. At this point tighten 1/4 turn and you should be good to go. Only problem with this is the oil that spurts out of the push rod. If you have, or can find, an old set of valve covers you can cut the center out and do the adjustment with them on. Or you can buy a set of clips made to adjust valves.
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Originally Posted by 85_TPI
Because my #4 exhaust valve rocker arm nut back off almost completely, and caused the problem that's been diagnosed in a different thread, I am now replacing all the nuts and readjusting all the valves.
In my Chilton's Corvette manual, it says to appropriately, index the crankshaft to the compression & exhaust strokes for adjustment of various valves.
Also, it mentions that when the pushrods won't spin anymore by hand, then tighten the rocker arm nut, "One complete turn further", to finish the adjustment.
When I tighten the nuts down this far, it appears that the valves are being pushed down some in the process.
Is this normal or am I just imagining things?
When I took the valve covers off, it appeared that there was a little slack in most all of the rocker arms and pushrod. Not much but some.
Can you please advise??
thanks,
85_TPI
In my Chilton's Corvette manual, it says to appropriately, index the crankshaft to the compression & exhaust strokes for adjustment of various valves.
Also, it mentions that when the pushrods won't spin anymore by hand, then tighten the rocker arm nut, "One complete turn further", to finish the adjustment.
When I tighten the nuts down this far, it appears that the valves are being pushed down some in the process.
Is this normal or am I just imagining things?
When I took the valve covers off, it appeared that there was a little slack in most all of the rocker arms and pushrod. Not much but some.
Can you please advise??
thanks,
85_TPI
The indexing the crank part is where your problem begins.
In order to correctly find zero lash, the lifter has to be on the base circle (heel) of the camshaft lobe, not on it's ramps. Since you can't actualluy SEE the lifter (in most cases the intake manifold is installed blocking the view) what you need to do is watch the rocker arm movement. From that movement you can tell when the lifter is on the base circle.
Most guys get tripped up using the service manual procedure which only makes finding zero lash at lot more difficult because of the confusing instructions the manual gives.
I really don't know why some manuals still list that paticular procedure, unless it is designed specifically for those who are VERY familiar with the workings of the engine.
By far, the most fool-proof way is to either use the procedure recommended by the big cam companies, like CompCams and Crane (which is the EO/IC method), or use the "adjust while idling" procedure that some favor.
The lifter MUST be on the base circle of the camshaft lobe or any adjustment will not be correct. The Tech Tips section on this Forum has at least two procedures that, if followed, will direct you on how to do it correctly.
Hope this help.
Jake
