Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification : MGB & GT Forum : The MG Experience

Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #1
marklindsay MARK THOMPSON ERARING, NSW OZ, Australia Sign in to contact	Topic Creator (OP) Mar 31, 2015 05:10 PM Joined 12 years ago 65 Posts

Can someone please outline what this sentence means
"These heads do require exhaust valve cut-outs in the block ..."
Has anyone some detailed info and or visuals
Thanks

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #2
warekl Keith Ware White Bear Lake Mn, USA Sign in to contact 1972 MG Midget 1973 MG MGB GT 1978 MG Midget MkIV 1979 MG MGB	Mar 31, 2015 05:14 PM Top Contributor Joined 14 years ago 3,724 Posts

The design of the combustion chamber does not allow the valve to come in contact with the surface of the block.

In the smaller valve/combustion chamber heads, that could occur and the blocks have a small crescent shaped cutout by each cylinder to prevent contact.

USMC 1967 - Always ... Semper Fi
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73 GT in Black Tulip.. Engine by Hap Waldrop, OD Transmission by Dick Moritz Distributor is "Schlemmerized"
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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #3
dickmoritz Dick Moritz Philly 'burbs, PA, USA Sign in to contact	Mar 31, 2015 05:15 PM Top Contributor Joined 14 years ago 26,273 Posts

Mark,

On MGB engines the exhaust valve overhangs the edge of the cylinder. When they went to the big head and newer, they made the combustion chamber smaller, which moved both the intake and exhaust valves closer to the block, presenting possible interference between the exhaust valve and the edge of the cylinder. So on these engines, all 18V engines, the factory machined eyebrow-shaped reliefs in the block for added clearance for the exhaust valve.

You can find many images if you simply Google "MGB eyebrow valve reliefs." However if your block is an 18V block, it almost certainly already has the valve reliefs machined in. If not, it's not a big deal to machine them into the block, but you must be careful not to go too deep into the cylinder because it's important that the top piston ring not contact the relief area.

Dick

Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #4
Rod H. Amity, OR, USA Sign in to contact 1964 MG MGB 1968 MG MGB GT	Mar 31, 2015 05:20 PM Top Contributor Joined 20 years ago 32,210 Posts

Well, I was all ready to post a link to a thread where Hap explains the situation nicely, but Dick just explained it. (-:

Here's the link anyway if you want to take a look.

http://www.mgexp.com/phorum/read.php?1,2269082,2269533#msg-2269533

Friends talking around a fire is the history of mankind.

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #5
riley1489 Bruce H Great White North, QC, Canada Sign in to contact 1953 Jaguar XK120 1959 Riley 1.5 "King George" 1973 MG MGB	Mar 31, 2015 05:44 PM Top Contributor Joined 12 years ago 26,302 Posts

Mark,

Voila,

B

Life's most persistent and urgent question is, "What are you doing for others?"

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #6
ohlord Rob C A tiny Island off the coast of Washington State, N.W., USA Sign in to contact 1957 Land Rover Series I "EYEYIYI" 1957 Land Rover Series I "OVRLND" 1971 MG MGB 1971 MG MGB "Bedouin 2" & more	Mar 31, 2015 05:58 PM Top Contributor Joined 12 years ago 28,878 Posts

Hand done
http://www.mg-cars.org.uk/peterburgess/exhaustcutouts.html

needed when using later small 39cc chamber 18V head on earlier non 18V engine block

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #7
Perdido Rut Rutledge Tuscaloosa, AL, USA Sign in to contact 1960 Austin-Healey Bugeye Sprite "Scarlet" 1962 Triumph TR4 1966 Triumph TR4A 1966 Triumph TR4A & more	Mar 31, 2015 09:45 PM Top Contributor Joined 16 years ago 5,263 Posts

Or, if you bore .060 over there's no need to cut the reliefs!
Rut

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #8
Checkmate Jim R None, Left Coast, USA Sign in to contact 1954 MG TF 1960 MG MGA 1600 1967 MG Midget Conversion	Mar 16, 2016 05:48 PM Joined 10 years ago 922 Posts

In reply to # 2953568 by Perdido Or, if you bore .060 over there's no need to cut the reliefs!
Rut

Is that enough? increasing the radius of the bore .030" is only going to move the edge .762 mm farther out. A photo on Peter Burgess' website shows the relief being more like 2 mm past the original cylinder perimeter.

http://www.mg-cars.org.uk/peterburgess/exhaustcutouts.html

I'm trying to determine if I need to make the cutouts on an 18GB block with a 12H 2923 big-valve head.

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #9
dipstick Kenny Snyder (RIP) La Center, WA, USA Sign in to contact 1941 Ford N-Series 1958 MG MGA 1500 Coupe "Rosie" 1970 MG MGB GT "Pat's GT" 1971 MG MGB "Gifted To Me" & more	Mar 16, 2016 06:15 PM Top Contributor Joined 15 years ago 7,564 Posts

"I'm trying to determine if I need to make the cutouts on an 18GB block with a 12H 2923 big-valve head."

- Yes. The 12H2923 head has a shallow (depth) combustion chamber and the exhaust valve will contact the block, exhaust valve reliefs are required.

"Or, if you bore .060 over there's no need to cut the reliefs!"

- Not necessarily so. I just finished a .060" over block that had just enough core shift in to cause two of the exhaust valves to contact the block.

Something to consider when cutting exhaust valve reliefs into the block:
Does the front-to-rear centerline of the exhaust valves run directly on top of the front-to-rear centerline of the cylinder bores? cool smiley

Be safe out there.
Kenny

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #10
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA Sign in to contact	Mar 16, 2016 06:19 PM Top Contributor Joined 15 years ago 9,605 Posts

No, the valve centers are offset toward the pushrod side of the head, just a little. smoking smiley

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #11
jwatsonjr James Watson ODENVILLE, AL, USA Sign in to contact 1967 Sunbeam Alpine 1980 MG MGB	Mar 16, 2016 09:49 PM Joined 9 years ago 50 Posts

does anyone have a visual reference to the 2923 head and it's clearances?

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #12
Checkmate Jim R None, Left Coast, USA Sign in to contact 1954 MG TF 1960 MG MGA 1600 1967 MG Midget Conversion	Mar 16, 2016 10:55 PM Joined 10 years ago 922 Posts

In reply to # 3218665 by jwatsonjr does anyone have a visual reference to the 2923 head and it's clearances?

I have the Flowspeed link to some general information on all the different heads, but no dimensions.

http://www.flowspeed.com/cylinder-ident.htm

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #13
Perdido Rut Rutledge Tuscaloosa, AL, USA Sign in to contact 1960 Austin-Healey Bugeye Sprite "Scarlet" 1962 Triumph TR4 1966 Triumph TR4A 1966 Triumph TR4A & more	Mar 16, 2016 10:58 PM Top Contributor Joined 16 years ago 5,263 Posts

I got my info from Hap when I bought my rebuild kit and .060 over worked fine for me.
Rut

Edit: Mine was the stock pre 18V engine that came in the car originally. As others have said you really want to measure your engine to make sure there is no interference.
Rut

1960 Bugeye,1275, 5 speed
1970 MGB, Pale Primrose
1967 Triumph TR4a
1966 Triumph TR4a
When I went to school, they asked me what I wanted to be when I grew up. I wrote down 'happy'. They told me I didn't understand the assignment, and I told them they didn't understand life. John Lennon

Edited 1 time(s). Last edit at 2016-03-17 07:23 AM by Perdido.

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #14
Checkmate Jim R None, Left Coast, USA Sign in to contact 1954 MG TF 1960 MG MGA 1600 1967 MG Midget Conversion	Mar 16, 2016 11:21 PM Joined 10 years ago 922 Posts

That's good to know, because I'd rather not do it if I don't have to.

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Re: Big valve head 12H-2923 . Exhaust valve cutout in block . Clarification #15
ohlord Rob C A tiny Island off the coast of Washington State, N.W., USA Sign in to contact 1957 Land Rover Series I "EYEYIYI" 1957 Land Rover Series I "OVRLND" 1971 MG MGB 1971 MG MGB "Bedouin 2" & more	Mar 17, 2016 12:57 AM Top Contributor Joined 12 years ago 28,878 Posts

Non 18v safe needs eyebrows
Peter Burgess website has measurements and cutting procedure

In reply to # 3218700 by Checkmate That's good to know, because I'd rather not do it if I don't have to.

LNDRVR4X4.COM
Home of Project "INCARN8'

1957 Series 1 Land Rover electric VEHICLE CONVERSION

FIXITUPCHAP.COM
FIXITUPCHAP INCORPORATED

RD3 Radar/ Electronic Warfare Technician
VIETNAM 1969-1972

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