I am going to upgrade my cam and the two cams I am picking from are the Comp Cams Magnum 280H or the 286H. The 280 calls for valve springs 911-16 and the 286 calls for valve springs 924-16. The 924's are flagged saying that the heads require machine work. What is the machine work and could I get by without the work?

A little history:
The block is a true 1969 396/375hp block with 11:1 compression. The heads are NOT rectangle ports but are #3917215 oval close port, 101 cc chamber heads. I want a bigger cam and think the one in it now is a stock hydraulic (it is not the 375 hp Solid). I bought the car and have just discovered that the engine parts are not matching. When I removed the valve covers to get the head numbers I noticed that the internals are very very clean and looks like it was rebuilt not to long ago. The lifters and pushrods look new but I still want a bigger cam! I am also replacing the performer intake with a Performer RPM intake and Holley 770 street avenger. I also bought a compression gauge to test the compression. If I get over 190 lbs, will this verify that the compression ratio is at least 11:1? I do not really want to remove the heads or pistions. Like I said, the internals look really new and there is no kunk anywhere. What cam (these and others) would help things and also give a rough idle? I am also putting 3.73 gears into the 68 chevelle too!

Thanks!

Joe,
If you are going to use the 924's, the heads will have to be machined. The valve guides will have to be cut to accept a small positive seal that will fit inside of the inner spring of the 924. If you are going to use the bigger cam, my advice would be to take the heads to your machine shop BEFORE you buy the springs and make sure they will work on your head castings. The heads do vary some from GM and sometimes you have to use a different spring than the one the cam manufacturer lists.
Even if you use the small cam, the 911's are not always a "drop in" replacement.
There is absolutely no way of telling without removing the old springs and measuring.

Hope this helps,