How much should I cut these heads?

[40cpe]

8CM heads on 3 5/16 X 4 block. I know they aren't the best choice, but they are what I have. There is no interference when placed on the bare block. With a new gasket torqued to 20 # to measure the clearance I have the tightest measurements of .068" on the edges of the piston and .100" at the dome. The Best fiber gasket now measures about .058" so I think I have about .005"-.008" to compress it. Should I cut enough to get about .040" clearance around the edges or cut more to tighten up the dome and then cut for clearance around the edges? Thanks for any advice you have.

[41LjH]

It really depends on the cam you are using. If you cut too much you will have valve interference. Does 3 5/16x4 mean you have a 4" crank? 8CM stock heads have lower compression with a 3 3/4" crank because they were designed for a 4" stroke.
Hope this helps.

[Bassman/NZ]

Shoot for 50thou over the centre of the dome and reshape the edges of the chamber so you get 50thou all over the piston. You will want that clearance over the valves too, but if your cam is stock you'll have no problem. I had exactly the same specs with mine, took 50thou off the heads, reshaped the chambers at the edges. Goes like a cut cat.

[40cpe]

Thanks for the replys. It is a L100 cam w/~.360" net lift and I have .065" above the deck. I checked the valve clearance, too, and the least clearance I have on them is .100" so i think I'm OK on the valves.

Bassman, What heads do you have?

[Ol' Ron]

Bassman/NZ

has the answer, I cut .040: off the EAB heads and clean up the edges with a Dremmil.
However I do have to clearance for the lift of an L-100 cam
ll

[40cpe]

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What kind of abrasive do you use in the Dremel?

[39topless]

Originally Posted by 40cpe

What kind of abrasive do you use in the Dremel?

Trump?

[40cpe]

Originally Posted by 39topless

Trump?

I couldn't control anything that aggressive

[Ol' Ron]

Now that the political jokes are over. I use a stone about 1/2x1/2". But I don;t think it maatters. I also use aluminum balls to check clearance. I also try for .040/.045"

[Bored&Stroked]

Another option is to have the domes recut with a cutter that matches the piston crown profile . . . only problem is that few folks have them. I believe that H&H has one, but not 100% sure.

I created a 3D CAD model in SolidWorks with the correct dome shape - and a variable value to set the specific depth to match my requirements (per job). We then generated a toolpath for a 1/4" ball, then recut the chambers to match the pistons. My pistons were custom from Ross - have a little .020 pop-up above the deck.

The heads I used were new Navarro (from Barney) 59ABs - and their dome shapes didn't match the consistent radius of the Ross pistons . . . probably were modeled after some of the original Ford cast pistons.

I see no reason you can't do the work by hand, just take your time, measure a lot and do the dome work before you mill the heads . . . as you'll undoubtedly have a few 'pecker tracks' on the sealing surfaces from when the cutter got away from yah! LOL

You might consider cutting an old piston in half - to use as a sort of 'dome gauge' to check your work along the way.

[Mike51Merc]

Here's a question for the experts: On OHV closed chamber heads, you typically want a 0.030"-0.040" quench, why does a flathead call for 0.050"?

And just as a sidenote to 40cpe, most head gaskets are sold with a compressed height spec so you don't have to guesstimate it.

[Charlie ny]

40,
Just my opinion.......I use a a pretty heavy duty air grinder and carbide 'burrs'
with 1/4" dia shanks to re contour chambers and clear valve pockets. I begin with
foil balls on top of the pistons and hang the head on 4 studs with a used Best gasket
between the block and head. With the head tightened down I crank the motor over
to determine the amount to mill off. After milling I reinstall the head after an application of a fairly dark grease on the valve heads and piston domes. With the head
hanging on the 4 studs and gravity holding it down I once again crank the motor over
and VOILA the grease leaves witness marks where clearance is needed.
This process usually takes 7 or 8 hours per head but I then know the chambers
are bang on over the piston crowns. When all is done there is pretty close to zero
clearance so with the compressed head gasket in place I'm comfortable that I've
achieved the .040 clearance required.
Charlie ny

[Ol' Ron]

I think the .050" clearance is my fault, I don't think there's a factory spec.. Many years ago when we were running these's lead pistons made by Jahn's we had rod strech that caused a bit of problems. We never measured piston clearance before and this was something new to us (Back in the 50's). When I wrote my book I spec. .100 clearance. On a trip to Bville with JWL back in '02, he questioned that. I also was aware that the SBC's ran .035 (Yes I build a few chebbys). I figgered that .050: would be a safe distance for the adverage builder and easy to get, Plus not all chambers are the same. I try for 40/45 Thou. Any way I think this shortens the flame front, creates the turbulents required to make a better running engine.

[Bored&Stroked]

Originally Posted by Mike51Merc

Here's a question for the experts: On OHV closed chamber heads, you typically want a 0.030"-0.040" quench, why does a flathead call for 0.050"?

And just as a sidenote to 40cpe, most head gaskets are sold with a compressed height spec so you don't have to guesstimate it.

I usually shoot for .040 on my flatheads. Yes, there will be some rod stretch when running skinny rods (like the 21A rods I have in my latest build), but not .040 worth!

Normal head gaskets are about .050 compressed - though you can buy all sorts of thicknesses from Cometic if you want to run (and pay for) MLS gaskets. I just put a set of these on a full-race 42 Merc motor . . . just to try them out. So far, so good. I think they'd be ideal on a blown engine.