Valve Lash Setting Question with Aluminum Heads
10-16-2006, 05:27 AM
#1
Melting Slicks
Thread Starter
Valve Lash Setting Question with Aluminum Heads
I have a after market solid lifter cam on my 67 L89 435HP C2. Chevy rectanglur port Aluminum heads on cast iron block.
Usually when you set the valve lash cold with Iron heads and iron block you add 2 thousanths (.002) to the recommended lash, (When hot)
The lash on my cam is recommended at .021 hot so if these were iron heads you would set them at .023 cold.
For aluminum heads on a cast iron block, Cranes website recommends DECREASING the cold valve lash setting by .006 when setting the cold lash (since aluminum heads "grow" when hot, addling valve lash). So this suggests that I set them cold at .015? When the engine gets hot the lash will increase to .021???
http://www.cranecams.com/?show=faq&id=4
If I set them at .015 cold they will be very tight until the engine warms up and I do not want to cause any problems with hard idling and valve clearance, etc... Will aluminum heads really grow .006 taller when hot??
What do you recommend? Thank You Forum Members!
Usually when you set the valve lash cold with Iron heads and iron block you add 2 thousanths (.002) to the recommended lash, (When hot)
The lash on my cam is recommended at .021 hot so if these were iron heads you would set them at .023 cold.
For aluminum heads on a cast iron block, Cranes website recommends DECREASING the cold valve lash setting by .006 when setting the cold lash (since aluminum heads "grow" when hot, addling valve lash). So this suggests that I set them cold at .015? When the engine gets hot the lash will increase to .021???
http://www.cranecams.com/?show=faq&id=4
If I set them at .015 cold they will be very tight until the engine warms up and I do not want to cause any problems with hard idling and valve clearance, etc... Will aluminum heads really grow .006 taller when hot??
What do you recommend? Thank You Forum Members!
Last edited by babbah; 10-17-2006 at 12:26 AM.
10-16-2006, 08:15 AM
#2
Burning Brakes
I've never heard of iron heads contracting when hot....I suspect maybe they just don't expand as much as the rest of the valve train, like the push rods and valve stems....works out the same I suppose.
And yes AL heads do grow and the expand more than the typical steel valve train parts so you do often set them up tight when cold. I follow the comp cams recommendation but off the top of my head I don't know it, sounds about like the one you listed. I have never had an issue with interference or poor cold idle due to the valves being a wee bit tight, however I recommend a through warm up before getting on the motor. AL heads do effectively run cooler, though they heat more evenly and to me seem to heat quicker, but you really need for everything to reach it's normal temp before flogging it.
One thing I do is set things cold, thoughly warm and then rev the motor a bit shut her down and then quickly pull the covers and double check...doing this I usually get just about the right hot numbers every time (sometimes just a bit tight which I think is because the heads are cooling rapidly) unless I blew it on the initial set up. I am sure one of the engine gurus here will be able to chime in with more exact recommendations.
Darren
And yes AL heads do grow and the expand more than the typical steel valve train parts so you do often set them up tight when cold. I follow the comp cams recommendation but off the top of my head I don't know it, sounds about like the one you listed. I have never had an issue with interference or poor cold idle due to the valves being a wee bit tight, however I recommend a through warm up before getting on the motor. AL heads do effectively run cooler, though they heat more evenly and to me seem to heat quicker, but you really need for everything to reach it's normal temp before flogging it.
One thing I do is set things cold, thoughly warm and then rev the motor a bit shut her down and then quickly pull the covers and double check...doing this I usually get just about the right hot numbers every time (sometimes just a bit tight which I think is because the heads are cooling rapidly) unless I blew it on the initial set up. I am sure one of the engine gurus here will be able to chime in with more exact recommendations.
Darren
Last edited by macdarren; 10-16-2006 at 08:20 AM.
10-16-2006, 12:56 PM
#3
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Because aluminum heads expand more that steel or cast iron the clearance will open up when hot. It then becomes a matter of how much.
I think .006" is too much for aluminum heads - maybe that's what to use for aluminum heads AND block. My Cosworth Vega's clearance opens up .002", but block expansion has no effect - just head and cam carrier expansion because it is OHC. The Chevrolet Power Manual recommends cold lash .010" tighter than hot lash on ALL ALUMINUM big blocks
Suggest you set the clearance .002" less than recommended with iron heads, then install the covers, and thoroughly warm up the engine. Then quickly remove one cover and measure a few to see how much they open up from the cold setting, and that will be the number you subtract from the cold setting for all cast iron construction. If it's .002", you're done.
I've never measured a significant hot - cold difference on a cast iron pushrod engine. The steel valve stem and pushrod expand about as much as the iron head an block. .
The only adjustment I make to mfgs. clearance specs is due to rocker ratio geometry, which is not constant. It has to be measured throughout the lift cycle. Then you factor the clearance by the actual lash point ratio divided by the advertised ratio.
Duke
I think .006" is too much for aluminum heads - maybe that's what to use for aluminum heads AND block. My Cosworth Vega's clearance opens up .002", but block expansion has no effect - just head and cam carrier expansion because it is OHC. The Chevrolet Power Manual recommends cold lash .010" tighter than hot lash on ALL ALUMINUM big blocks
Suggest you set the clearance .002" less than recommended with iron heads, then install the covers, and thoroughly warm up the engine. Then quickly remove one cover and measure a few to see how much they open up from the cold setting, and that will be the number you subtract from the cold setting for all cast iron construction. If it's .002", you're done.
I've never measured a significant hot - cold difference on a cast iron pushrod engine. The steel valve stem and pushrod expand about as much as the iron head an block. .
The only adjustment I make to mfgs. clearance specs is due to rocker ratio geometry, which is not constant. It has to be measured throughout the lift cycle. Then you factor the clearance by the actual lash point ratio divided by the advertised ratio.
Duke
Last edited by SWCDuke; 10-16-2006 at 01:14 PM.
10-16-2006, 06:50 PM
#5
Race Director
Good discussion. I've wondered about different theorys for lashing my BB aluminum head valves. Now I wish I could just remember how I did it the last time.
Dan
Dan
10-16-2006, 11:27 PM
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I set mine .006 tighter cold for street use. I've double checked them after complete warmup and they end up dead on. By complete warmup, I mean about an hour of riding around on the street or highway...not just until the temp gauge reads something.
I set them slightly looser than that (cold) when going to the drags because it never actually gets that heated up. I can fire it up cold...let it warm up to about 130* and then make a full throttle, gear banging run. When I get back to the pits (driven-not towed) I find they loosen up about .003. Of course it gets cool downs inbetween runs, so that setting works well.
But normal cruising street driving where it will get thoroughly warmed up gets .006 tighter when cold.
JIM
I set them slightly looser than that (cold) when going to the drags because it never actually gets that heated up. I can fire it up cold...let it warm up to about 130* and then make a full throttle, gear banging run. When I get back to the pits (driven-not towed) I find they loosen up about .003. Of course it gets cool downs inbetween runs, so that setting works well.
But normal cruising street driving where it will get thoroughly warmed up gets .006 tighter when cold.
JIM
