Valve Stem Heigths Are Different - L92
Valve Stem Heigths Are Different - L92 Ok, so I'm setting up the valvetrain with Yella Terra rockers on a pair of L92 heads.
I check the intake............zero lash.
I check the exhaust.........and .020 to go before zero lash?
I am using 2 adjustable pushrods.
Surely I must have made a mistake and accidentally set them to 2 different lengths.
I pull the rockers and pushrods. Re-check the pushrod lengths. It is the same.
I swap the pushrods, intake for exhaust, re-install the rockers and check zero lash again.
Again I am at zero lash on the intake and have .020 to go before zero lash on the exhaust.
Maybe the Yella Terra pedestals are 2 slightly different lengths?
I remove them, measure them. Nope, they are the same.
Maybe the spacers I have under the pedestals are 2 different lengths?
I measure them. They are within .001 of each other.
This leaves only one thing.
I get the straightedge out and sit it on top of the valve stems.
Some of the valve stems are touching the straightedge.
Primarily the exhaust valves, there is a gap between the straightedge and the tip of the valve, in some cases approaching .020.
For a set of pushrods of all the same length, the preload on some of the lifters will be .025 while on others it will be .055.
Doesn't seem right to me for a brand new pair of heads.
And why should anybody have to buy TWO sets of pushrods to try to keep the preload the same on all of the lifters?
BTW, the intake valves are the optional hollow stem valves.
It appears that maybe they are all a bit longer than the exhaust valves?
It appears that the intake valve stems are an honest .015" higher than the exhaust stems:
I check the intake............zero lash.
I check the exhaust.........and .020 to go before zero lash?
I am using 2 adjustable pushrods.
Surely I must have made a mistake and accidentally set them to 2 different lengths.
I pull the rockers and pushrods. Re-check the pushrod lengths. It is the same.
I swap the pushrods, intake for exhaust, re-install the rockers and check zero lash again.
Again I am at zero lash on the intake and have .020 to go before zero lash on the exhaust.
Maybe the Yella Terra pedestals are 2 slightly different lengths?
I remove them, measure them. Nope, they are the same.
Maybe the spacers I have under the pedestals are 2 different lengths?
I measure them. They are within .001 of each other.
This leaves only one thing.
I get the straightedge out and sit it on top of the valve stems.
Some of the valve stems are touching the straightedge.
Primarily the exhaust valves, there is a gap between the straightedge and the tip of the valve, in some cases approaching .020.
For a set of pushrods of all the same length, the preload on some of the lifters will be .025 while on others it will be .055.
Doesn't seem right to me for a brand new pair of heads.
And why should anybody have to buy TWO sets of pushrods to try to keep the preload the same on all of the lifters?
BTW, the intake valves are the optional hollow stem valves.
It appears that maybe they are all a bit longer than the exhaust valves?
It appears that the intake valve stems are an honest .015" higher than the exhaust stems:
I have the same heads ( Prc ls3 with the hollow intake valves) and they are exactly the same as yours . I ordered different length pushrods for my combo. I bet most people with these heads dont check it so it hasnt even been brought up before. Atleast not that I have seen .
Last edited by offaxis; Mar 7, 2010 at 09:35 PM.
I have seen a few guys mention this. Go with the Manton 11/32 pushrods and order two different lengths. Stronger pushrods, made to order and reasonably priced. Thunder Racing can get them for you or you can go straight to Manton.
I posted this question last year about mine and didn't get any answers. Somebody else finally noticed. I was going to have the tips machined to the same height as the exhaust but was afraid since they were holllow. Running the same length pushrods though. Was thinking of ordering anther set of pushrods to even everything out.
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TECH Regular
Joined: Dec 2004
Posts: 440
Likes: 0
From: Alpharetta, Ga
I remember when I was rebuilding my heads after a TC break on my LS3 and I noticed the exact same thing. I ended up using the same pushrods all around as well, but its nice to see other people finding the same discrepancy.
Your correct the hollow stems are a little bit longer on the intake side. Our VJ is setup for stock length valves, but the hollow stems are just a hair longer. We can help you with a different length pushrod if you need them, but the difference probably won't require a pushrod change.
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Jason
Co-Owner, Texas Speed & Performance, Ltd.
2005 Twin Turbo C6
404cid Stroker, 67mm Twins
994rwhp/902lb ft @ 22 psi (mustang dyno) www.Texas-Speed.com
Jason
Co-Owner, Texas Speed & Performance, Ltd.
2005 Twin Turbo C6
404cid Stroker, 67mm Twins
994rwhp/902lb ft @ 22 psi (mustang dyno) www.Texas-Speed.com
The varying tip heights have an insignificant impact on hyd. lifter preload. Remember the rocker arm ratio is 1.7:1. A tip height difference of .021" would only move the lifter plunger .003".
A 1:1 ratio would have a significant impact but that's not the case here.
You have nothing serious to worry about.
Good luck.
Richard
A 1:1 ratio would have a significant impact but that's not the case here.
You have nothing serious to worry about.
Good luck.
Richard
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
The varying tip heights have an insignificant impact on hyd. lifter preload. Remember the rocker arm ratio is 1.7:1. A tip height difference of .021" would only move the lifter plunger .003".
A 1:1 ratio would have a significant impact but that's not the case here.
You have nothing serious to worry about.
Good luck.
Richard
A 1:1 ratio would have a significant impact but that's not the case here.
You have nothing serious to worry about.
Good luck.
Richard
Jon
Jon,
Yes, you are correct. Evidently my brain got stuck in neutral before the coffee kicked in. Good catch.
None the less the tip height difference is not enough to force a different pushrod length here.
Richard
Yes, you are correct. Evidently my brain got stuck in neutral before the coffee kicked in. Good catch.
None the less the tip height difference is not enough to force a different pushrod length here.
Richard
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
It's nice to have exactly the same but many heads are like this when you check them. That's why you check them! Shouldn't be cause for any problems though like Richard said .012 preload different is not going to do much on a regular hydraulic deal.
Check each end cylinder and then each side both drivers and passengers side too and you might have another small difference but again it shouldn't be a problem. We have heads from CFE that are like that that cost 10K so just check but something that small should be fine as long as you are aware of it.
This is where people trying to setup preload at like .010 can have BIG issues though and trash whole engines as they may be in lash on some valves or one one side if they only check a few valves!
Check each end cylinder and then each side both drivers and passengers side too and you might have another small difference but again it shouldn't be a problem. We have heads from CFE that are like that that cost 10K so just check but something that small should be fine as long as you are aware of it.
This is where people trying to setup preload at like .010 can have BIG issues though and trash whole engines as they may be in lash on some valves or one one side if they only check a few valves!
