From: WallysFlattys (Original Message)
Sent: 3/10/2007 9:14 AM
I'm building a "stock looking" 265. I have 0.060" over pistons. I am using stock heads. I measured the radius of the piston dome: 5 inches. I measured the radius of the relief in the heads: 7 inches. I plan to have a clearance of 0.100". Therefore to me the radius of the relief in the head should be a 5.1" radius. That way I would have a uniform clearance between the piston and head. It will also improve my compression.
What do you think guys?
From: Russ/40 Sent: 3/10/2007 11:17 AM
Your attempt to reduce the radius of the head will be counter-productive to your goal of increasing the compression, unless you shave your heads to compensate. Your squish area of .100 is rather large the conventional thinking is it should be .050 or less. You did not indicate what cam your using so there likely will be some valve pocket work. Is that why you are shooting for the .100, to assure valve clearance? In the discussions I've read through the years, I don't recall any emphasis on uniform piston to head clearance. Go down to my "Two Questions" post and read JWL's comments.
From: WallysFlattys Sent: 3/10/2007 8:03
Russ, I am going to use a '53 Mercury cam. But I will deal with the valve to head clearance at the same time I mill the heads and machine the radius for the piston clearance.
My thought was in order to get a uniform piston to head clearance that the radius of the head relief and the piston dome need to be almost the same. The stock 7 inch radius relief in the heads with 0.050 clearance in the center of the piston would give me almost twice that clearance at the other edge of the piston.
So here is my plan, radius the relief in the heads at the same 5 inch radius as the piston dome and shoot for about 0.060" clearance between piston and head (gasket plus 0.010).
From: OriginalJWL Sent: 3/10/2007 9:04
The stock Ford heads are designed with a 6 inch radius. I have never seen a piston which was cut to a 5 inch radius. But, anything is possible. The most important aspect of the piston to head clearance is the outer edge opposite the valves. This is the area where residuals and detonation become a problem. There are at least 2 methods for accomplishing the end: Make the head radius smaller than the piston radius, or, angle mill the heads. The later is my usual choice, but, be aware the stud holes must be re-reamed to be perpendicular to the new gasket face and the spot faced areas, for the washers, will need to be retouched, if you angle mill.
From: Russ/40 Sent: 3/10/2007 9:16 PM
Curious Wally, whose pistons will you be using? Also, how have you been determining/measuring a radius? Got a formula you can share?
From: Speedbumpauto1 Sent: 3/11/2007 12:44 PM
For my own knowledge, how would you re-radius either the chamber or the piston? What type of machine or process? I can't seem to get my mind around how this would be done in an accurate, reliable way.
JWL, I like the angle mill idea a lot. do you do it to most of your rebuilds? I think you were the one who finally woke me up to the difference in quinch areas between the OHV and flathead. Now that piston clearance thing makes perfect sense. I've been 0 decking OHV blocks for a while now to achieve the same purpose.
From: Elmo Rodge Sent: 3/11/2007 2:46 PM
The chamber is done with a "doming tool" on a mill. It is a cutting tool cut to the exact radius. For the pistons I'd just do it on a lathe. There is a trick I know how to do but couldn't begin to explain it. Wayno
From: Russ/40 Sent: 3/11/2007 3:43 PM
Wally, when I clayed my heads recently, I did not see any difference piston to head clearance across the entire piston. I used Egge 4 ring pistons.
From: WallysFlattys Sent: 3/11/2007 7:52 PM
Russ, my pistons are 4 ring 0.060 over out of the Speedway catalog, I assume their Egge. I have been scribing various radii on 0.030 plastic sheet then cut them out with a scissors. I have been varying the radii every 0.250 and you can tell when 5.25 doesn't fit and 5.00 fits. Just the reverse in the heads, 6.75 was real close but 7.00 was dead on.
JWL, are you saying I want less clearance around the outer edge? Right now with a five inch radius on the piston and a seven inch radius in the head, I figure I will have about twice the clearance on the outer edge as I do in the top of the dome.
Keep in mind I am using stock cast iron heads here. As far as how to cut the new radius, I will leave that up to a buddy of mine that can do wonders on his CNC mill.
From: OriginalJWL Sent: 3/11/2007 9:00 PM
Speedbump, as already pointed out by Wayne, one way to do the head dome is with a block style cutter ground to the form of the radius you want and chucked in the spindle of the vertical mill. Another method is helical, circular interpolation on the CNC mill with a small stepover. Pistons can be done manually or in a CNC lathe and even on the CNC mill, but I have not done that. I can say the block cutter, on cast iron heads, is a real bear. Aluminum is much easier to deal with. I'm trying to attach a pic of a cast iron "rub-off".
Yes, Wally, I am telling you the MOST important area is the outside edge.
Having more clearance there is undesirable. Shoot for about .040 in that area.
From: OriginalJWL Sent: 3/12/2007 12:51
Oh, forgot to respond to the head question: I think that is a 59A head I had made to fit my 265 engine. Then later, out of necessity, I had to go up to 276 and the pistons had a different crown, and made contact at the outside where I had it very tight for the 265.
The head holding fixture, for the mill, has adjustable supports under and into the sparkplug counterbores. After the supports are adjusted against the head, the head is clamped to the center posts of the adjustable supports. What you are viewing is the bolt and copper washer that is aiding in the clamping of the head into the fixture. These prevent the head from deflecting when heavy tool pressure is applied. Such as is the case when cutting the domes.
From: WallysFlattys Sent: 3/12/2007 4:52
Thanks for all the advice JWL. I had planned to take the gasket thickness into consideration. I'm still in the planning stages for the valve clearance work, if needed.
I'll keep you posted on my progress.
Thanks again, Wally
From: Blown49 Sent: 3/14/2007 8:37 AM
I drew this up in a cad program this morning.
Based on a 7" head radius, a 5" piston head radius and a 3-3/16" bore.....for 0.060 piston dome clearance in the center you'd end up with 0.137" on the outside of the pistons.
I feel you're right if you want 0.050" gasket thickness and 0.060" dome clearance the radius of the dome cutter should be 5.010".
From: OriginalJWL Sent: 3/14/2007 6:13
I constructed the head fixture. I do not know of any such items available from suppliers.
I grind the cutters that I use.
I'll have to dig out a set of Lincoln heads and see if I can understand the area you mentioned as being unequal? The first thing I do is insert a dowel pin in several bolt/stud holes and check for perpendicularity to the gasket face. Most are nearly perfect. If ok, and this is just to be a "cleanup" job I put them accross the Platten resurfacer. If I am machining for performance I indicate the head to get it positioned where I want it to tighten up the clearances in the right places.
Chamber volume equalization is an over emphasized performance tool. The fact is the work done to equalize volume can reduce the efficiency of the chamber while making it the same size as others. There are so many things which create cycle to cycle differences in each cylinders performance that equalizing volume, to some finite element, should be placed low on the list important factors.
From: Jim Marlett Sent: 3/17/2007 9:49 AM
JWL, I couldn't agree with you more about chamber equalization, although I am probably one of the least qualified to actually comment on it since I haven't actually put a flathead together since the '60s. (What a lazy bum I am!) I have never understood why, if you had seven "perfect" small chambers and one "imperfect" slightly larger chamber, you would make them all match the "imperfect" chamber. I can't imagine that all the cylinders breathe the same on a flathead. There has to be a difference between left and right sides and, at least on the exhaust side, the center and outer cylinders. I suspect perfect intake manifold distribution is pretty hard as well. I believe the perfectly balanced flathead would have slightly different combustion chambers on each cylinder, but who knows what the differences should be?
Of course, I could be wrong.
And, see, I do still occasionally drop in, even though this photography thing is eating me alive
From: WallysFlattys Sent: 3/30/2007 4:55
Just an update, I found the formula for the volume of a section of the a sphere, in other words the piston dome. Turns out the volume of the dome on the 0.060 over piston with a 5" dome radius is 16cc.
As a sidenote, measured the Felpro big bore copper gasket. The area is right at 16 square inches for each cylinder. If you figure the thickness after installation is 0.050 that amounts to 13cc added to the head volume.
I'm getting closer to making the final compression ratio calculations.
From: OlRon1 Sent: 3/30/2007 7:04 PM
Thanks for going over all this stuff again. I did some of this math when I wrote my book, the reason was the fact that many compression ratio charts do not represent the actual comprssion you have in your engine. You have to Measure it. I've never heard of a piston with a 5" dome, but I've never measured many, actually very few. Learn sompin every day. Keep up the good work.
From: redned10 Sent: 3/30/2007 7:46 PM
Hi Wally -
"I found the formula for the volume of a section of the a sphere,
in other words the piston dome."
So what is the formula Wally? Just like to note it down for future reference.
From: WallysFlattys Sent: 3/30/2007 9:42 PM
Here is what I did, first calculate the height of the dome or section of a sphere: h
h = the square root of (R squared - D/2 squared)
where R = the radius of the piston dome
and D = the diameter of the piston
Once you have h, the volume of the section, V, is:
V = 3.1416/3 times (h squared) times (3R - h)
If you are working in inches your answer will be cubic inches, for CCs just mulitply by 16.387
For the volume of the gasket I just traced it on squared paper and counted all the full squares and estimated the total of the pieces. Close enough for my crude measurements.
From: FlatV8 Sent: 3/31/2007 6:49 PM
I get a different value for the radius of the piston dome. Closer to 6-1/2 inches than 5 inches. Several years ago I found on the web somewhere the dimensions of a standard cast piston for a 239 flathead. That chart and diagram noted 0.210 inches of projection from the edge of the piston to the crown. Recollection was it was the Clevite web site, but it could of been Egge or even someone else. Can't seem to find the same page to provide documentation or verification. Using that dimension as a basis I performed the attached calculation of what the radius would be for the dome on a 0.060 over 3-3/16 piston. Now does anyone know of someone who will recut the dome of some milled EAB heads so they provide the necessary 0.040 to 0.050 inch clearance for a 0.210 piston dome projection? Or is this the time to learn how to be artsy-craftsy with a hand grinder?
From: OriginalJWL Sent: 4/1/2007 8:29
Just a couple of things to be aware of: First, the dome on many of the pistons I have seen is not a true radius. Measuring the height at center and calculating would be a very rough estimate. Even pistons with a "FORD" stamp can vary at the dome. Also, many, replacement, oversized, pistons are "decompressed" with extra material removed at the perimeter of the dome so only a portion is a true radius. And finally don't forget the actual deck height when trying to decide the CR.