[Will]

Let's say you want to put some big valve 74cc heads on a 350 'cause that's what you have laying around.

We know the block needs chamfers.

We know that at some point, the intake, exhaust, or both valves will contact the block and that point will be determined by which heads are used and the size and position of the chamfers.

How far away from that point of contact should you stay to be "safe" while the engine is in operation?

For example, let's say I measure and find that the intake valve touches the block at .500" lift. What lift can I safely run in that scenario? Any reason the exhaust valve clearance would be different?

[AG]

The usual rule of thumb is a min of 0.060" clearance for the intake valve and 0.090" for the exhaust valve.

[Will]

Originally Posted by AG

The usual rule of thumb is a min of 0.060" clearance for the intake valve and 0.090" for the exhaust valve.

That's quite a bit. So in my hypothetical, you'd stick to .440" on the intake and only .410" on the exhaust.

just curious where this rule of thumb comes from/what the rational for it is. That's more than I would've expected.

[PontiacJim1959]

Originally Posted by Will

That's quite a bit. So in my hypothetical, you'd stick to .440" on the intake and only .410" on the exhaust.

just curious where this rule of thumb comes from/what the rational for it is. That's more than I would've expected.

I think it will depend on your cylinder bore chamfers, how deep they go.

Here is a picture of a 350CI with the chamfers, typically the 1969-70, but this can vary over the years.

The 1969 350 HO Firebird that Royal Pontiac set up used a solid roller cam having a lift of .512" at the valve, with the intake clearance of .016" and exhaust .018".

So without the chamfers, I suspect the lift would be less as pointed out.

[tom s]

OK,I just ran into this yesterday on my 301-383 build.4.040 bore.2.19 and 1.73 valves.Both I&E hit the block.Short block is together,coming apart for clearance,also need to get a set of 1.50 rockers instead of my 1.65s.I thought I was OK,NOT!Be sure!Tom

[Will]

Ok folks, again - this isn't a question about how much lift you can run, but how much clearance you need from the point where the valve touches the block.

Every time I ask this question I get people telling me about how much lift they ran with this or that combo. I appreciate that people are trying to be helpful, but that's not answering the question. I will measure to find out where contact takes place on my particular combo. What I need to know is how far away from that point I need to stay.

So tom s - how much clearance do you want? Is the rule of thumb AG posted sensible?

I'm only questioning this because my instincts would tell me .030" - .060" max would be plenty, but that's just a semi-educated guess and if you really need .060" - .090" I want to confirm that.

Thanks!

[Formulas]

Valve clearance with other moving parts ( pistons ) is different than stationary parts, If a valve floats it has essentially changed timing in relation to a moving piston and can be struck

If you open a valve .020 from the deck and the valve floats is it going to come in contact with the deck? As in open further?

I have #48s on my .030 350 put them on along time ago but going from memory the intake will not have issues with deck or bore within reasonable lifts

The exhaust valve will interfere with the deck in scallop area and I have under .030 lift clearance

Things may wear in time but will be gradual IE valve guides so if clearance is lost later the parts will probably clearance as required

Mock it all up evaluate and go from there

.

[Will]

That's an interesting question about valve float. Does the valve go open a little further? I'd almost think it would. A spring that can't keep the lifter on the cam lobe might allow the cam to "toss" the lifter off the lobe slightly. The whole "object in motion" physics thing. Hmmm....

[dmac]

Just curious, but where is the piston when max lift is reached with the came timing advanced or retarded? Would that few degrees add the max cam lift, or could you advance the cam more to allow more lift? I know some of the people are much more able to picture this mentally than I can.

[steve25]

Here's two pictures, one is of a 2.11" Intake valve in a -4 6X head @ .500" lift., the other picture is of that same 2.11" valve @.500" in a 1968 high compression head.

It's clear to see which head needs more VTPC then the other I think you would agree!

It also shows how much more shrouded the intake valve is in the deeper chambered low comp heads, which is also something that needs to be addressed above a certain stock flow rate, but that's another story!

If your fighting for every scrap of compression then like I have done you can run as little as .055" VTPC, but you had better be darn sure you have 100% good valve springs and a rev limiter, but even besides that if the rear OD of the Intake valve notch is of the normal type then it itself will shroud the high lift flow out of the Intake valve at high cycling rates.

Also here's a picture of the type of Bore notch that's needed in a stock Bore 350 to work very well with just a 2.05" valve, no less a 2.11" valve.

My flow test show that if on your 350 with a a high comp head that your Cam will lift over .450" lift then you are better off starting with a small valve head (1.96") and then going up to a 2.05" valve , not a 2.11" valve.

[steve25]

P.S, Exh valve to cylinder wall clearance is not a issue even with the usage of a 1.77" valve.

[tom s]

It is if it hits the block!Tom

[tom s]

My builder wants 50 thou.We had no issue with the pistons at all,it the contact with the block.Im running the SPII cam.Tom

[Formulas]

I currently have Lunati 30706, 270/220dur .465 lift - 280/230dur .489 lift camshaft and 1.52 rockers
.030 over 1970 350 #48 heads

[steve25]

Tom, my reply was to what the OP of this post might be questioning in regards to iron heads with a 1.77" valve , because with the valve spacing of the iron heads it's not a issue.

[Stan Weiss]

There are 2 different thing look one can look at. How much clearance do you need

1) so that parts clear each other

2) to help flow.

Stan

[tom s]

I understand Steve,just saying my iron heads with a 1.73 valve hits the block on a 4.040 bore but my intake is 2.19.When added up it looks like they would fit?Tom

[Will]

Originally Posted by tom s

I understand Steve,just saying my iron heads with a 1.73 valve hits the block on a 4.040 bore but my intake is 2.19.When added up it looks like they would fit?Tom

Well, they would if they were perfectly centered in the bore, had less than .080" spacing between them (probably do?) and they didn't open at an angle (opened straight down into the bore).

Thanks Tom!

[Will]

Originally Posted by Stan Weiss

There are 2 different thing look one can look at. How much clearance do you need

1) so that parts clear each other

2) to help flow.

Stan

Good point Stan. I suppose it's possible that opening the valve right up to where it's about to touch the cylinder wall could cause more shrouding at that lift and reduce flow than if you used less lift to keep it further away.

Interesting. Would like to see some flow tests of that.

[tom s]

I made the decision we will cut the valves down just enough to clear.