valve clearance

[my367s]

We started to put my 390 together and things were going good until we installed a head and found we did not have enough valve to piston clearance.I have a comp cam which came with new lifters and valve springs as a matched set and the specs on the cam said we are supposed to have 100-120 thousands clearance,but we only have 13 thousands.We are using the stock pushrods and rockerarms which are supposed to work.Does anyone have any ideas or has anyone had this problem before.Thanks Jerry

[willowbilly3]

Did you check it with a head gasket? 13 thou is almost touching. Have your heads been milled? Block decked? What is the installed height on the pistons?
I looked through a factory service manual and couldn't find that spec but I am thinking around .025.
The only way you can run all those parts together is to have valve reliefs flycut into the pistons.
Also you might call Comp Cams tech line and get their take on what kind of clearance you should have with the cam.
Also look how far the lifter center is depressed when the valve is closed. Ideally you would want something like .010 preload but some builders like them about centered in the bore. If they are depressed too far this might be an indication that some prior head or deck milling had been done.

[68F250]

Or maybe your cam timing is off, did you check it with a degree wheel?

[my367s]

We solved my valve clearance problem,the previous owner had machined the towers that support the rockerarms down by about 50 thousands.Why he did that we are not sure but I found a set at the wreckers and we tried those and we had the proper clearances.But now I have another problem,in that my rebuilt heads need new seats because the valves are too far up into the head and thus with the cam I have its compressing my lifters too much.Hope that makes sense,anyhow by the time I pay for seats,installation,porting its almost worth buying a set of aftermarket heads.Has anyone used edelbrock heads on their 390?

[DuckRyder]

Probably the easiest way to deal with it is to install and adjustable valve train. You will end up with 300.00 or so in a stock style one (pushrods and rocker arms.

I have also been told that people have shimmed them using valve spring shims for (gasp) an SBC. You have to pay special attention to being sure there is sufficient seal on the two oiling towers to still allow oil to reach the shafts.

You did not mention the cams specifications but if it is very large or the spring pressures are very high, you need to look at upgrading the rocker assembly.