Isky 6005 valve Spring Rate is 275 lb/inch. Isky recommends these springs for all of their Y-block cams. Ted has said that he's not had a problem using these springs with any of the Isky cams. 

I intend to use PAC-1286 beehive valve springs in my G heads/w stock-size valves. The PAC-1286 spring rate is 283 lb/inch. The recommended installed height is 1.75. My net valve lift will be 0.485. The open pressure/w recommended installed height will be 262. I am considering shimming the springs to increase open pressure at .485 to 277. This will increase the seat pressure 15 pounds to 140. Considering the Isky springs have a seat pressure of 135 pounds at the recommended installed height, am I on the right track by shimming the PAC springs for the increased open pressure? In order to prevent premature wear of cam lobes and lifters, Ted recommended keeping open pressure below 300 lbs.

Keep in mind that the stock size Mummert stainless valves have undercut stems and are lighter than stock OEM valves. The PAC beehive springs and retainers are lighter than stock components as well. 

What is your actual seat pressure with the PAC spring at 1.750"?  Is that your actual/physical installed height?  Anything close to 130# seat pressure is sufficient for most valves that fit the Y.  Instead of increasing the seat pressure, you may need another/different spring for your heads depending on the actual installed spring height.  The actual spring pressure at lift may not be what you see on paper.  You will have to do an actual measurement and check the fit.   Joe-JDC

JDC

I double checked that spring specification, and it has 125# seat pressure at 1.750", and is listed for up to .600" camshaft lift with over the nose at 295# at .600".  If your installed height is corrected to 1.750", it should work just fine as is.   I would not suggest more than 130# seat pressure for the street.  Joe-JDC

JDC

Thanks, Joe.

I'm actually more concerned about valve float. According to Ted, it isn't the seat pressure that can do damage, but open pressure. On the other hand, I don't want to increase either, if it isn't necessary. At .485 net lift, coil bind isn't a concern. 

By the way, I think it was John Mummert who said that the guides will have to be cut for anything over .450 lift. How much? I've read 1/8 (.125). Comp Cams Viton .530 seals will be used. 

If you cut the guides down for .530" viton PC seals, then cut the guide boss down so that the beehive will fit over the boss, or down flat and use a spring locator/seat cup.  You will need to establish how you intend to locate the beehive spring to keep it stable on the seat.  I had to cut the spring seat down to allow a .060" spring seat cup locator for my beehive springs.  You must take that into consideration with the installed height of 1.750".   Manley has a cutter for the iron heads and .530" guide to cut the boss for PC seal conversion.   Most machine shops should be able to do that for you if you don't have the ability yourself.  It should be done in a mill to keep the seat perfectly 90* from the valve guide/valve stem.  Most iron heads valve guide boss can be cut down ~.030" below the factory seat without a problem.  I cut my G heads down .060" to use a seat cup and installed height of 1.800" for .600" valve lift.  I had to notch the cylinder bores for the valve radial clearance at that lift.  Joe-JDC   

JDC

Joe, originally I wanted to use PAC-1283 springs; they would have required to guide bosses to be cut down. However, the PAC-1286 springs have a larger inside diameter at the bottom and will fit over the guide bosses without cutting them. Both 1283 and 1286 are the same size on top and use the same retainer. I would have preferred using the 1283 springs, but I'm having a difficult enough time getting a shop to do the heads; there's also the added cost of having the bosses cut; not every shop has the correct size cutter to do it. The 1283 springs have the same recommended installed height as the 1286 springs (1.75), but the 1283 spring rate is 363 lb./inch vs 283 for the 1286 springs. The inside diameter of the PAC-1286 springs is 1.000. If I recall correctly, the bosses measure 0.970. Correct me if I'm wrong, but the 1286 ID (bottom) is the same as the stock Y-block springs. 

Manley valve guide seat cutter for 11/32" valve stems and .530" PC seals part number is Manley 41711.  Check the Manley catalog and you will find all those cutters for different valve stem sizes and seats.  That is the cutter I use for my iron heads.  Joe-JDC

JDC