I'm going to be running a cam with 0.460" lift. I have a set of stock spec 340 valve springs which physically fit the stock valve seats. Will I need to have the seats machined down to give enough valve spring travel, and if so, by how much?

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Matt Cramer
1966 Dodge Dart turbo / EFI project

You should be fine. Just check one or two after you assemble.

Lou

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Home of Slant6-powered fun machines since 1988

At .460 lift it is more important to check for the top of valve guide to spring retainer interference.
Be sure to allow additional room for the thickness of the oil seal (valve stem seal) you will be using.

I always trim the tops of all the valve guides on a SL6 head to ensure there is a clean, flat surface for the umbrella seal to touch... and to be sure there is enough guide to retainer clearance.
DD

Agreed. Valve seals are what will get destroyed if retainers too close to guide.

Lou

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Home of Slant6-powered fun machines since 1988

What is the height of lift over which this would become a"have to" do ?

to determine if there is clearance between the bottom of the retainer and the top of the valve stem seal...

1) install a stem seal that will be used on the final head assembly
2) install a valve into that valve guide, again a valve that will be used in the final assembly
3) pull the valve into the valve close position and install the retainer that will be used and keepers that will be used in the final assembly
4) use one hand to pull up on the retainer to fully seat - close the valve
5) use a caliper in the other hand to measure the distance between the bottom of the retainer and the top of the stem seal.

the distance measure must be greater than the cam lift at the valve or the bottom of the retainer will crush into the top of the stem seal when the valve is open.

if the clearance is close, < .030, I would check all the valves as there will be some difference in the seat heights, depending upon the compentance and care of the person and machinery doing the valve seat work.

Your initial question may also have to do with coil bind,
I always re check that when manually rotating the engine during final engine assembly, but you don't want to find the issue then.
1) use a spring height tool.
https://www.summitracing.com/parts/sum- ... 29e8f8c24c

3) Install a valve, retainer keepers, use the tool to measure the installed valve height.
4) put the valve spring in a sturdy bench vise and compress the valve to the installed height. ( be careful to keep the valve in the vise)
5) compress the valve further to the valve open distance ( open height minus the cam lift at the valve)
6) verify clearance between the coils is at least .060,,, depending upon how the coils compress some coils may touch, that is ok, but the clearances that are available should be .060
or more with in reason. You definitely don't want the coils to go solid, but having crazy excess coil clearance ( .200) is not good either.

And lastly, concerning machining spring seats,, I buy springs that fit within my installed height to cam lift to coil bind requirements and are close on open / closed spring pressures then machine or shim the spring seats to get to the target for valve open-closed spring pressures.

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Thanks for the responses, everyone! I wanted to take the head to a machine shop for a three angle valve job while the engine is apart, so I'll see if I can check clearance off the engine.

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Matt Cramer
1966 Dodge Dart turbo / EFI project

Measurement results - looks like the valves will open 0.400" or maybe 0.410" before the keeper hits the valve guide. So it looks like I'll need to have the top of the valve guide machined along with the valve job.

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Matt Cramer
1966 Dodge Dart turbo / EFI project

Wow that isn't much over stock lift. Looking like I'll have to do the same with "only" a .435-.440" lift cam.

Very surprised at that with stock 340 retainers. Yep, that would smash things up.

Lou

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Home of Slant6-powered fun machines since 1988

Just have the machine shop cut down the guides for positive seals and tell them you want it for .500 lift. It really only takes them minutes with their cutter. Then your good to go and more in the future if needed. You can always run the stock type seal also.

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