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Old 03-10-2012, 06:45 PM
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336781 Heads With Flat Top Pistons

Have A 454 With Flat Top Pistons ,SLR Cam. Lift Is 532-522 Duration Is 282-292,with Roller Rockers . Had Some Head Trouble, They Were 702's Now Have 336781 Open Chamber Heads . 206-172 Valves Will This Combination Work Just As Well ? Thanks....................gus

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Old 03-10-2012, 08:22 PM
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flat tops and 781s make a pretty low CR
i dont know what cc the old heads were
have the 781s and/or the block been cut/milled ?

but it will probably work ok with that cam



*I am no expert
there are a few guys on here that know more than me
maybe they will chime in
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Old 03-10-2012, 09:20 PM
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The 702's Are 98 Cc . The 781 Are Supposed To Be 113 Cc. The Block Has'nt Been Decked ,but The Heads I'm Not Sure About As They Are Still At The Head Shop &the Bore Is Still Stock. Thanks For The Comeback...........gus
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Old 03-11-2012, 09:11 AM
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Quote:
Originally Posted by GUS00
The 702's Are 98 Cc . The 781 Are Supposed To Be 113 Cc. The Block Has'nt Been Decked ,but The Heads I'm Not Sure About As They Are Still At The Head Shop &the Bore Is Still Stock. Thanks For The Comeback...........gus
the 781s are more like 118 - 120 cc
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Old 03-11-2012, 09:41 AM
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I dont know what the deck height or the volume of piston or head gasket thickness is gonna be but the flatops with those heads will make that big block sick (wont go!) compresiion,Ithink ,will be in the 8:1 range at best...
Try to get some closed chamber heads or change pistons.
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Old 10-19-2012, 09:06 PM
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How much can I mill the 781s down .They have 219 188 valves now .I want to try them on a 454 with a comp extreme marine cam - lift-544-547 duration 226-236 & flat top pistons. stock bore & deck. .......
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Old 10-20-2012, 03:02 PM
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It won;t matter. You won;t get the cr up high enough with a flat top piston and those heads.

Get a supercharger or different pistons with a 18-26cc dome or different heads with a small combustion chamber.

A modern gen VI 454 vortec BBC head will work well. Any of the good old 96 to 100cc closed chamber oval port heads work well with flats tops in a 454. like 063, 215 castings etc.

the 781's will work well with a piston swap.
heads will work well.
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Old 10-20-2012, 03:43 PM
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If you want to use the big valve good 781 ovales it will be well worth it to do it right and get the right dome piston
for this combo. The right pistons are not that much money.
A KB hyper or speed pro hyper or probe will get it done.

You want a 18 to 26cc dome depending on the desired cr, actual measured head volume and assembled piston to head clearance @ TDC. (deck height)
Piston dome to chamber wall clearance must be checked. minor chamber wall clearanceing grinding may be required.

This is a really good combo when done right.
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Old 07-02-2013, 01:11 PM
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This is an old thread I know ... but I have just researched this as well, and this is what I think I have found for my setup:

454 bored .030 = 4.280" bore x 4.00" stroke

Clevite Forged flat-tops, P/N 224-2519.030
Specs in the Clevite catalog say these have a -4.9cc dome volume and that the stock deck clearance is .025" (My block was decked what my machinist only described as "minimally" ... approx .005, so I'm using .020")
Comp.Distance is 1.640"

336781 Cyl heads.
ALMOST ALL lists that I have found say these are 113cc.

Felpro Permatorque head gaskets have a compressed thickness of .039"

So, using Summit's compression calculator with these numbers:
4.280 (Bore)
4.00 (Stroke)
113 (Cyl Head Volume)
4.9 (Dome Volume)
0.020 (Deck Clearance)
0.039 (Compressed Gasket Thickness)
8 (# of Cylinders)

... I get a CR of 8.15:1
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Old 07-02-2013, 02:11 PM
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I get 8.7 with 113cc head.
All the 781 I have done were 188cc to 122cc.
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Old 07-02-2013, 02:18 PM
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Quote:
Originally Posted by lmsport View Post
I get 8.7 with 113cc head.
All the 781 I have done were 188cc to 122cc.
188cc ???


Duke
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Old 07-02-2013, 02:34 PM
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Typo i"m sure. Most average about 120 cc. stock.. I don't know where they are getting the 113 figure.
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Old 07-02-2013, 03:01 PM
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Quote:
Originally Posted by BOBCRMAN@aol.com View Post
Typo i"m sure. Most average about 120 cc. stock.. I don't know where they are getting the 113 figure.
Well, these are a few of the places that I found:

Chevymania Casting and Suffix Tables

Chevrolet Big Block Cylinder Head Casting Numbers

Big Block Chevy Engine Parts casting numbers and Identification

Looking on Google, though, there are websites that list them as 118cc ...
Mortec web archive


Has anyone actually cc'd these heads for themselves?
They sure seem to be popular (as far as cast heads go ...)

Last edited by 66GMC; 07-02-2013 at 03:11 PM.
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Old 07-02-2013, 04:11 PM
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Yes other people have CC'ed these heads. Do what everyone else in the world does . Combine these
781 heads with a dome piston. Usually 24 to 26cc is just right.
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Old 07-02-2013, 06:40 PM
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Quote:
Originally Posted by 66GMC View Post
Well, these are a few of the places that I found:

Chevymania Casting and Suffix Tables

Chevrolet Big Block Cylinder Head Casting Numbers

Big Block Chevy Engine Parts casting numbers and Identification

Looking on Google, though, there are websites that list them as 118cc ...
Mortec web archive


Has anyone actually cc'd these heads for themselves?
They sure seem to be popular (as far as cast heads go ...)

I read all three of the above sources. LOTS of incorrect information on them..

Example. 049 heads are also listed as 113cc. They are in actuality 120-122..
Another example. Stated in article that tall deck 427 trucks had 7/16" rod bolts. Completely false!

Plus many more. In this case don't believe what is printed..

Yes, I have actually cc'd many BBC heads and they are usually larger than GM stated spec.. I usually start with a .005" cut before measuring the chambers. A sloppy repair valve job can add cc's of volume to the chamber. Sink that valve/seat and add combustion chamber volume..
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