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Valve Lash Adjustment on BB Aluminum Heads on Iron Block

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Old 09-28-2005, 10:01 PM   #1
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Default Valve Lash Adjustment on BB Aluminum Heads on Iron Block

I installed Chevy L89 aluminum heads on my 435Hp BB. with solid lifter cam.

When I had the iron heads on I used to set the valves cold and added .002" to the hot lash setting which is .021, so I'd lash them at .023 cold.

I am reading articles now that suggest when you have aluminum heads on an iron block that you must subtract lash from the hot lash setting when setting the cold lash.

From the Crane Cams website for setting cold lash:

Iron Block Iron Heads Add .002"
Iron Block Aluminum Heads Subtract .006"
Aluminum Block Aluminum Heads Subtract .012"

Does this make sense to any of you out there? If my hot lash is .021 then this info above suggests that I set them cold at .015. This concerns me.

If anything should I just set them cold and deduct .002" or do them the same way I have with the iron heads, add .002???

What is common practice concerning this?
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Old 09-29-2005, 12:01 AM   #2
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Set them to cam specs, warm it up and do it all over again if you want to use the cam as suggested. Otherwise, the tighter they are set. it will improve top end trap speed. Looser then suggested builds bottom end off the line power. After a few runs you can adjust the cam as is needed.
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Old 11-05-2005, 07:38 AM   #3
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I have an aluminum head sb and routinely subtract .004/.006 from my cam specs when adjusting, I never set them hot and have never had a problem, been doing it this way for 10 years. I follow the Crane method of setting the valves also, it takes more time but by pulling all the plugs it isn`t too bad.
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Old 11-05-2005, 01:01 PM   #4
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Set them to the hot spec. Come back in a few hours or tomorrow and measure the clearance, cold. That is where you want to set them cold, from now on.

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Old 11-06-2005, 01:00 AM   #5
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I set my valves cold too. With my aluminum heads I also subtracted .006". I could never adjust them fast enough to feel confident that the last valves adjusted had the same clearance as the first. Perhaps I am also a woose and just don't like getting my fingers burned.

I don't think there is much risk being a few thousandths one way or the other as long all they are all close to the same. The only danger I can think of for lashing too tight is that with iron heads, the valves are known to eventually sink into the heads from wear because the seats are much softer - possibly making them not close all the way when the clearance is gone. Not so with your hardened seats used in the aluminum Chevy heads.

Too tight may make your idle slightly lumpier, but as has been said earlier, your top end power will be better. I just don't like loose mechanical parts. Generally, the greater the slop, the greater the strain or abuse to parts.

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Old 11-13-2005, 10:31 PM   #6
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al (soft) heads grow taller at operating temp, so u sub 005-006 to cold lash cause the head gets higher and the pushrod is steel (coef of exp is much less) so the pushrod grows less = looser.
vary hot lash less max 006 or more max 004 to tune powerband,this has been sop for many years.
do not go tighter than 006 from mfr rec or cam base circle tolerance variation may lift valve slightly off seat at wrong times in otto cylce,causing loss of heat transfer from valve (valve warpage and cracking) or hot gas erosion of valve and seat.
do not go loose more than 004 from rec or u risk loss of acceleration ramp designed into cam lobe and excess wear will result.
u must adjust solid lifters regularly. daily driver once a month, more often if you play a lot. racers every race day at least once, maybe every run if u r really buzzing it.
if u have EFI, lifter noise may agitate knock sensor/retark spark-alum rocker arms may deaden noise enuf to avoid falsing.
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Old 11-20-2005, 04:26 PM   #7
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I'm adjusting valves on my BB & SB this weekend- both iron blocks w/ aluminum heads. Setting the BB hot yesterday at .024 I find that they're at .022 cold today. The SB was at .016 hot yesterday and is at .014 cold today. Both only varied .002 so it's probably more accurate to check your motor this way to make sure you've got them where you want them.
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