.Huh?! I just got my heads from the machine shop last night and they told me the highest lift cam I could use would be a .440. I thought the stock heads would take .480? Should I have the seats ground down? I've got the comp 08-302-8 that has .480 lift but now I'm afraid to put it in.....

 

BTW, I plan on using a set of LT-4 springs too.

 

The machine shop I used said that the heads would only clear .460 lift, so it probably justs depends on how much clearance they are calculating.

Though normally if they have to cut the seat for a bigger spring the valve guide will get cut at the same time.

It should not cost very much at all to have the guides cut down.

nick

 

Cut the guides, should be cheap. - CP(competition products) sells the GM HP Z28 valve spring kit(easily safe to .510) for $29.95. The kit has the springs, retainers, and locks. - just my $.02.

later, justin...

 

Thats cool, but how does just a spring with the stock guide height increase the lift?Are the springs taller to increase the distance between the retainer and the top of the guide? Regardless of the spring's tension, this is what ultimately detirmines the max lift.

 

The spings tension and installed height determine max lift. Go to the comp cams catalog and look at their spring chart. You can have 2 springs with the same diameter and height, but have different pressures because of the spring rates.

 

So how exactly do I detirmine how much lift the heads can handle? Are the aftermarket springs taller than a stock spring? I plan on using a set of LT4 springs with a comp 210/220 .480"/.480" cam.

 

You sill need to cut the guides down a little. I said that and the springs, because you don't want to have any valve float. If you push the vavle open further, and the spring isn't stong enough to close it any quicker, you will have valve float, which is death to cylinder heads. Comp and Crane make a tool that you can put in you drill to cut the guides down. The Comp part # is 4726. Thier vavle seat tool is 4718. The Crane part # 99402-1, but I think you also need 99027-1 to do all the work(not sure about the crane tools) -

later, justin...

 

Thank you all for all of your input. I think I'm gonna take them to the machine shop to have the guides cut down quite a bit. Thanx. :hail:

 

I used the same comp cam in my old 350 and only changed the springs with my stock heads.I had no problems with the retainers hitting the guides.I guess I was just lucky

 

I just went through this recently on a set of L-98 castings. There was contact between the retainer and the valve guide seal at .475". You should NEVER run valve lift right up against that spec unless you want the occasional valvetrain breakage or enjoy replacing vavle guide seals every few months. I wouldn't have run anything over .455" lift on these heads without machining the guides down. Every head is a little different, though, so a blanket statement can't really be issued. Gotta measure and see for yourself.

The springs would have taken more lift- they were not the reason for the "top stop."