Iron BBC w/ aluminum head growth

Ron Gusack · 03-18-2016, 04:52 PM

I've read for many years that valve lash will open .004 to .006 with this combination. Will the piston to valve clearance grow too?? I'm at .044 with everything as it will be when running except the heat.

63corvette · 03-18-2016, 05:59 PM

In my experience it is more like .002 to .003 on the valves.
Not sure on the piston clearance. If it is a alum rod it will be different than a steel rod.
Just My 2 Cents
Rick Cates

Dragsinger · 03-20-2016, 04:30 AM

Ron, in my experience .044 is in the danger zone and will most likely bend valves.

I am still in recovery from valve to piston clearance interference

which caused a very expensive rebuild.

Ed Wright · 03-20-2016, 11:58 AM

With mynaluminum SBC (LT1) heads, intakes are right at .005", exh more like .003", as the exhaust valve runs much hotter than the intake, making up for some of the head growth. With greater mass of a BBC head, I would think the difference would be greater.

I have run my intake valve-to-piston clearance as close as .036", using "checking" springs. Nothing touched, but no carbon formed where the valve would have touched. I normally use .040", but you had better have everything right, and stay on top of your springs. I would not try that with stud mounted rockers.

Tom Goldman · 03-20-2016, 12:56 PM

My 468" Mule has 074 GM/AFR heads which grow more than modern high density castings.
I get by with .040" on the intake with no kissing , but I have titanium intake valves and very high spring pressure.
I've never chanced anything less than .050 on the exhaust , but they have never touched , and it runs at 8500 rpm.
Also have a belt not a chain which I think helps .

Ron Gusack · 03-20-2016, 03:09 PM

Thanks for the comments. I've never been closer than .052 on the intakes and about .075 on the exhaust. I've heard the rule of running the same intake clearance as quench is always safe. Knowing the intakes are chasing the piston is a safety factor too. I stopped using checking springs a long time ago because I couldn't get them to read anywhere near the actual spring. I think it takes much less time to check with the race spring and they're more accurate. My garage isn't heated and checking things cold always concerns me when numbers are tight.

03-18-2016, 04:52 PM	#1
Ron Gusack Member Join Date: Feb 2009 Posts: 244 Likes: 1 Liked 7 Times in 6 Posts	Iron BBC w/ aluminum head growth I've read for many years that valve lash will open .004 to .006 with this combination. Will the piston to valve clearance grow too?? I'm at .044 with everything as it will be when running except the heat.

03-18-2016, 05:59 PM	#2
63corvette Senior Member Join Date: Mar 2007 Location: Canyon, Tx Posts: 551 Likes: 127 Liked 353 Times in 102 Posts	Re: Iron BBC w/ aluminum head growth In my experience it is more like .002 to .003 on the valves. Not sure on the piston clearance. If it is a alum rod it will be different than a steel rod. Just My 2 Cents Rick Cates

03-20-2016, 04:30 AM	#3
Dragsinger VIP Member Join Date: Jul 2008 Location: Liberty City [East Texas] Posts: 1,760 Likes: 5 Liked 99 Times in 60 Posts	Re: Iron BBC w/ aluminum head growth Ron, in my experience .044 is in the danger zone and will most likely bend valves. I am still in recovery from valve to piston clearance interference which caused a very expensive rebuild. __________________ Larry Woodfin 471W

03-20-2016, 11:58 AM	#4
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 390 Times in 170 Posts	Re: Iron BBC w/ aluminum head growth With mynaluminum SBC (LT1) heads, intakes are right at .005", exh more like .003", as the exhaust valve runs much hotter than the intake, making up for some of the head growth. With greater mass of a BBC head, I would think the difference would be greater. I have run my intake valve-to-piston clearance as close as .036", using "checking" springs. Nothing touched, but no carbon formed where the valve would have touched. I normally use .040", but you had better have everything right, and stay on top of your springs. I would not try that with stud mounted rockers. __________________ Ed Wright 4156 SS/JA Last edited by Ed Wright; 03-20-2016 at 12:22 PM. Reason: Forgot something....

03-20-2016, 12:56 PM	#5
Tom Goldman VIP Member Join Date: Jan 2003 Location: Allentown,PA Posts: 2,476 Likes: 887 Liked 870 Times in 236 Posts	Re: Iron BBC w/ aluminum head growth My 468" Mule has 074 GM/AFR heads which grow more than modern high density castings. I get by with .040" on the intake with no kissing , but I have titanium intake valves and very high spring pressure. I've never chanced anything less than .050 on the exhaust , but they have never touched , and it runs at 8500 rpm. Also have a belt not a chain which I think helps . __________________ Tom Goldman 1500 SG , 1506 STK

03-20-2016, 03:09 PM	#6
Ron Gusack Member Join Date: Feb 2009 Posts: 244 Likes: 1 Liked 7 Times in 6 Posts	Re: Iron BBC w/ aluminum head growth Thanks for the comments. I've never been closer than .052 on the intakes and about .075 on the exhaust. I've heard the rule of running the same intake clearance as quench is always safe. Knowing the intakes are chasing the piston is a safety factor too. I stopped using checking springs a long time ago because I couldn't get them to read anywhere near the actual spring. I think it takes much less time to check with the race spring and they're more accurate. My garage isn't heated and checking things cold always concerns me when numbers are tight.