Question about spring installed heights
 

I've just fitted a Comp XE274 hyd flat tappet cam in my 350 Chevy and have a used set of Holley SysteMAX heads to install. I want to change the Holley springs for the proper Comp 981-16 springs/retainers/locks, but I'm not sure how the installed height will work-out. Can anyone tell me if I can just 'plug n play', or if I'll likely have to get the spring seats shimmed/machined?

First thing to do is pull out one of the old springs and retainers.

Then put the new retainer and lock on the valve (with out the spring). And measure from the bottom of the retainer to the spring cup with the valve closed. If the installed height is too much then add some shims, if not enough then it will need some machine work to lower the spring cups.

Then measure how far the retainer will go down before hitting the oil seal. There should at least 0.060" clearance between the seal and the retainer at max lift.

Then measure the spring pressure at the installed height and at max lift. Also make sure it will not stack at the max valve lift. Give at least 0.080" clearance between spring stack and max valve lift.

Yea, I'm familiar with the process (but thanks for the info). What I'm asking is how likely it is that I'll need to shim or machine. Thing is, I'm in England and even the spring shims are hard to come by without ordering them from over there and waiting 2-3 weeks for them to arrive. How likely is it that I can just install the Comp springs/locks/retainers and be right at the recommended installed height right off the bat? Tempted just to stick my old (but refaced, pressure-tested and hot-tanked) O.E. heads back on.

only takes a few minutes to check. But I would think the springs would have a very good chance to have the right install height.

I bet the stock holley springs may even be good enough. Check with holley's website and see what their stock spring pressure is and how much lift they can handle. Going a little light on the spring is better than going too stiff (especially for the break in)

I think the holley heads would make much more power than your old GM heads.

Yea, the Holley springs are a little TOO good enough, lol. They're good for .600" lift and are duals. They're so stiff, I can't even get them off with my (hand tool) spring compresser. Wanna swap them out for the Comp springs... don't wanna tempt fate with the break-in.

Will check piston-to-valve clearance using the Holley springs and, if that checks-out, I'll order a set of Comp springs/locks/retainers and keep my fingers crossed that the installed height works out OK right off the bat.

Thanks.

http://www.holley.com/data/Catalogs/...er%20Heads.pdf

According to that they have a 1.78 installed height and a seat pressure of 132 pounds. The 981 Comp springs have a 105 pound seat pressure at an installed height of 1.7, so no, it probably wont be plug and play. The only way to find out what's needed is to mock up the comp retainer and measure that height. You can add shims, and they also make keepers that move the installed height up or down .050 inches. For break-in I'd just take the inner springs out, you can also call Holley, they might be able to give you the spring rates of both the outer and inner spring, it's possible that the rate of just the outer might be close to the specs on the Comp springs.