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Milling a -206 BBC head?

General engine tech -- Drag Racing to Circle Track

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pdq67
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Milling a -206 BBC head?

Post by pdq67 » Tue Jun 30, 2020 12:41 pm

Can an OLD, -206, BBC head be milled down around 90 cc's or so OK? (97 or so cc chambers here)...

I think it is a way early 396 true closed chamber head with 2.07"/1.72" valves.

I want to use them on a stock, mid 70's, 454 PU engine to raise it's really low CR up so that I can install a decent cam in it is all.

pdq67

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Tue Jun 30, 2020 2:07 pm

When you do, use a factory 454 LS6 (454hp) steel shim head gasket .022" thick.. new old stock Ask around..

I think Detroit gasket had a similar thingy ..

You must check valve to piston clearance if shaving down a bbc head for cr.

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Tue Jun 30, 2020 3:33 pm

Unless you own the #206-215 head already its probabily cheaper to replace the pistons with dome piston for the open chamber 454 heads.

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Re: Milling a -206 BBC head?

Post by falcongeorge » Tue Jun 30, 2020 3:37 pm

Just a guess, I would say yes. I used these heads and laid back the chamber around a 2.19/1.88 combo to a quasi-open chamber configuration and milled the heck out of them, wound up with a sort of heart shaped 103cc chamber, used them with a massaged L72 style dome and .032 piston/head, and wound up at 12.7/1 with a very nice shaped chamber good burn characteristics, colour right to the very back of the quench, and made quite good power. IMO this is superior to an open chamber head with a big ugly-ass dome. If I could remove that much material from the chamber and end up at 103, you shouldn't have much trouble getting a stock chamber to 90.

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Tue Jun 30, 2020 4:25 pm

When you only want (pumpgas friendly) 10:1 cr on a 454 with the typical 118cc oval port open chamber head the required piston dome is not big ugly ass. It does not impead combustion.
Its not that big a dome.

The big domes needed for 12:1++ cr is a different story.
don't equate.

The dome will be 26cc+/- depending on overbore, chamber volume Pt deck clearance and gasket thickness.

If you a bit under 10:1 cr with the 118cc oval head use the 18cc dome 454 piston (9.5:1 +/-)

Any all domed pistons must be clearance checked on the gm oval port open chamber heads. 049/781.

If you shave the hell out of a bbc head u will likely see valve to piston clearance issues with any cam that has generous valve overlap/duration
The factory bbc pistons and the FM/TRW bbc pistons are natorious for lack of vtp clearance with racy cams
Gets worse as you shave the heads ..
Last edited by F-BIRD'88 on Tue Jun 30, 2020 4:47 pm, edited 1 time in total.

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Re: Milling a -206 BBC head?

Post by pdq67 » Tue Jun 30, 2020 4:41 pm

I have a good pair of old -781 large oval heads but really want to up the sorry CR that old mid '70's 454 PU engines have is all W/O buying new pistons.

And my 496 has W/P's big valve C/I Merlin large ovals on it.

I am thinking like mill the old -206's down to around 90 cc's would do this along with .020" thick steel shim head gaskets.

And imho, the only reason Chevy dropped the true closed chamber BBC heads was because they are smog-wise, dirty!!

Closed chambered heads are actually more eff. that the open chamber heads at combustion, but dirty, smog-wise.

pdq67

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Tue Jun 30, 2020 4:55 pm

On a 427 or 454 bbc the open chamber oval heads are better even on a "tork motor" . The combustion is the same uses the same typical 38 deg timing at WOT as the closed chamber heads do... Been there with both on 454's.
By the time you buy heads and rebuild them and pay for all that shaving too the piston swap will be cheaper.. If the cylinders are good just hone them.
The old 396 206-215 heads lack hardened exhaust seats...

The KB257 26cc dome piston is $380 US a set.
do the math

Start by CC ing the chamber on your 781 heads.

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Tue Jun 30, 2020 5:11 pm

Look at ALL the modern cylinder heads LS motor V6 etc etc non use the bathtub style chambers of the old 60's 396's.
They are not more efficient than the open chamber bbc heads.

The modern L29 vortec BBC head is a "Open chamber head" too.

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Re: Milling a -206 BBC head?

Post by pdq67 » Tue Jun 30, 2020 9:59 pm

F-BIRD'88 wrote:
Tue Jun 30, 2020 5:11 pm
Look at ALL the modern cylinder heads LS motor V6 etc etc non use the bathtub style chambers of the old 60's 396's.
They are not more efficient than the open chamber bbc heads.

The modern L29 vortec BBC head is a "Open chamber head" too.
OK, so flatten the BBC open chamber heads combustion chambers down so that they are around 90 cc's or so!!

Then we wouldn't need to worry about using closed chamber heads, now would we.

And as a side note, I have a new pair still in their boxes EQ big valve cast-iron SBC heads that are 49/50 cc's so have looked into this a lot.

You know flat top piston usage...

pdq67

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Wed Jul 01, 2020 1:52 am

Be aware that that EQ head is a milled down 64cc head. ir will have the same issues VTP clearance, manifold fit, pushrod length, Dizzy fit.

U gonna finish that lil motor in this lifetime.

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Wed Jul 01, 2020 2:14 am

Those EQ heads were intended for 350 358 sbc oval classes that rule use of a flat top piston . to get 12:1++ cr

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Wed Jul 01, 2020 2:52 am

You are sitting on the major part of a beauti 10.:1 oval port 454 hi perf engine combo (usable shortblock 781 heads) all you need is the right piston to bring it together.
Don't goof this up. The open chamber heads (you already got there) are flat out better and the 10:1 cr 454 piston for this has a moderate dome that does not effect combustion time.

You could sell this to fund $$$ finishing your lil buzin sbc project.
for your 67 Camaro

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Re: Milling a -206 BBC head?

Post by F-BIRD'88 » Wed Jul 01, 2020 4:37 am

Yes you can mill down the crap out if a bbc cylinder head but is this really the best path? You still will not have effective quench action with the oem 454 dished piston so where are you at all said and done.?
A 10:1 cr 454 without active quench (oem dished piston) is a sloppy inefficient knock probe build.

Changing the pistons for the open chamber heads and 10:1 cr is a much better deal, all in

The smart money is on changing the pistons VS changing heads, especially considering you ALREADY got the good oval port heads there on hand.
It is a BETTER hi perf 454 combo in every way including ¢o$t

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Re: Milling a -206 BBC head?

Post by vortecpro » Wed Jul 01, 2020 6:39 am

I DO NOT like the 206 casting. A 702 casting is a MUCH better head.
Racing a NA NHRA stocker should be mandatory before any posting.

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Re: Milling a -206 BBC head?

Post by tenxal » Wed Jul 01, 2020 8:51 am

pdq67 wrote:
Tue Jun 30, 2020 12:41 pm
Can an OLD, -206, BBC head be milled down around 90 cc's or so OK? (97 or so cc chambers here). I think it is a way early 396 true closed chamber head with 2.07"/1.72" valves. I want to use them on a stock, mid 70's, 454 PU engine to raise it's really low CR up so that I can install a decent cam in it is all. pdq67
We used to do quite a few of those head swaps on 454's for a our tow rigs back in the '80's(454 Suburbans, dualies, etc). The small chamber heads, an RV-style cam like the Comp 262, a cast iron factory tall QJet intake ('66-'69) and a '69-later QJet carb really woke these up. Throttle response was better, torque and power were much improved and the fuel mileage would generally pick up by 2 mpg when towing the race car. We didn't use the steel head gaskets...just the Fel Pro blue Perma Torque ones. I'd not use the steel shim gaskets for a lot of reasons.

You'll have no issues with valve to piston with any cam that's appropriate for this application.

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