Iron BBC w/ aluminum head growth

[Ron Gusack]

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]

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]

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]

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]

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]

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.

[Ed Wright]

Checking springs are easier to push the valve down against the piston, with the dial indicator on the retainer, It, of course, has more with with over 1000 lbs open spring pressure, but more of a pain to check that way. As long as I know how much clearance I can get away with checking springs, what does it matter how many thousandths more it has hot, with the race springs on? It's just a point of reference. I use .075" exhaust clearance.

To find what to set the valves to cold, I made a run, soon as I got back, I pull the covers and set them to the hot spec. Exhaust first, since exhausts lash will change more as the valves cool. Intake valve temp, thus length, won't change as much. Next morning I check to see what cold lash is. Cold, the first valve you set is the same temp as the last.
JMHO

[Ron Gusack]

Thanks Ed. My problem with light springs is that my variance between the light and real springs is too wide so I couldn't just add.0X. I think I narrowed my problem down to my lash adjustment with the light springs and that was probably because my checking springs were too light. My spring pressures are about 240 on the seat and I use an old Manley tool that was supposed to be used to check spring pressure with the heads on the engine. I'm sure you've seen them or most likely have one. I like not having to take the springs off and put them back on. And my numbers are accurate.

I find the cold/hot lash the same way as you. My question was weather or not the P to V clearance grow like the lash does. Maybe no one really knows. I wonder if the quench grows or does the piston grow too?

[Ed Wright]

As long as we have a cold reference that we know is as close as we can run them warm, what does it really matter how many thousandths it gains warm? I know that my intakes touch warm with racing springs, when they check .033" cold with checking springs, don't touch at .036" cold with checking springs, right now they have .040" cold with checking springs, what does it really matter what it has warm with racing springs?

And yes, I do have one of those old Moroso tools for checking springs on the engine with a flex beam torque wrench. You can use a 1/2" break over to check like that. Slips off my Jessel rockers. I check everything before I set up & install my springs. Last thing I do before I bolt them on to stay.