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5-main 1800 block and head assembly differences over the production run...

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David A. Avatar
David A. David Auerbach
Canaan, NH, USA   USA
1979 MG MGB
Are there differences in the various years of the 5-main 1800 block and head assembly? I am re-building a ‘79 engine with mild improvements and notice in the catalogs that parts are listed for various years, but don’t seem to be different or they can be used interchangeably. For example, I have a 25D distributor currently in the car and the engine (18V883AEL31032) seems to run just fine. In addition, it was recommended that I use a head with 43 cc chambers (12H2389 casting) without the smog drilling to replace the cracked smog head I currently have – heads seem to be interchangeable as well.

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ghnl Avatar
ghnl Silver Member Eric Russell
Mebane, NC, USA   USA
1961 MG MGA "Calvin"
Any B-series cylinder head will fit any B-series block. The differences are in valve sizes and combustion chamber volume.

Although written for the MGA, much of the info in the MGA Guru's Tech pages also applies to the MGB. See: http://www.mgaguru.com/mgtech/engine/ch100.htm



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

bills Avatar
bills Bill Spohn
W. Vancouver, , BC, Canada   CAN
In reply to # 3680479 by ghnl Any B-series cylinder head will fit any B-series block. The differences are in valve sizes and combustion chamber volume.

Watch it though - you have to use a rocker set that has a pedestal that matches the location of the head oiling hole or you ruin the valve train. Can't think of any other issues except that you also have to match pistons and combustion chamber volumes as both varied over the years. Failure to do that can result in lower or higher than anticipated compression ratios.



Bill Spohn www.rhodo.citymax.com/carstuff.html
Current: 1958 MGA Twincam (race car (170 bhp)),1962 MGA Deluxe Coupe (98 bhp)
1957 Jamaican MGA (200 bhp)1965 1971 Jensen Interceptor (350 bhp)
2009 Pontiac Solstice GXP Coupe (350 bhp)
2007 BMW Z4M coupe (340 bhp)
Recent: 1969 MGC roadster (175 bhp),Jensen CV8 (375 bhp),
1969 Lamborghini Islero S (350 bhp), 1988 Fiero GT turbo (300 bhp)
North Vancouver BC

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riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
The smaller capacity combustion chamber is actually shallower. This brings the exhaust valve rather close to the cylinder bore and a cutout was introduced into the 18V engine blocks to aid gas flow, or un shroud the valve. (below)
One year cylinder head casting #12H2923 sported a larger diameter intake valve.

B



Check your ego Amigo!


Attachments:
BH_valve reliefs.JPG    42.7 KB
BH_valve reliefs.JPG

Fairfield, CA, USA   USA
there are also internal differences, rods, and such. But if not digging into the insides probably not an issue.



1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

boggsy64 Avatar
boggsy64 Platinum Member Ken Boggs
Brooksville, KY, USA   USA
1977 MG MGB "Dad's Hand Me Down"
Bruce, Thanks for the pics. I have those cutouts on my block and was not sure but thought it was a valve thing.... I am not sure but guess I have the "smaller capacity combustion chambers" as well, do you have a head casting number for that? I have gone up in compression with my pistons from original as well.

fast-MG.com Avatar
fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
David(OP), a few changes come to mind. 18G_ blocks have studded main caps and "camel hump" caps which are more fracture prone than the 18V blocks which have stronger caps and use good quality bolts. Either cap design is good for road use, racing engines require(IMO) mods to use camel hump caps.

18G_ cranks are forged with roundish counter weights, most 18V cranks are forged with flat sided CWs but a few late 18Vs have cast cranks with bell shaped CWs and only one(vs two) FW dowel. IMO, these are the best cranks for road motors due to CW-ing.

Pre18V heads had 43cc chambers(deeper) than later 18V heads(37cc). The 12H2923 head came with 1.625" intake valves vs 1.67" for most of the others. The CAM 1106 head has offset oiling to the #4 rocker stand and these do not interchange with the inline rocker stand with out modification. All heads can be easily fitted with the larger intakes although any with shallower chambers need to be checked for E-valve fouling the block and the block notched if necessary. Starting with casting 12H2389, all heads are smog design with A.I.R. ports.

18V oil pans have a rear corner skived away for access to the torque converter bolts on the rare AMT equipped cars. These pans have less oil capacity.

There is a rubber plugged hole in the back plate on these for that same bolt access. 5-main engines used with 4-synchro trannys have a larger FW and revised back plate for the relocated starter.

Late 18V engines have a larger crank pulley and a shorter water pump with a larger shallower pump pulley to maintain pump RPM.

18V engines have single row timing chains with advanced cam timing for better low end torque to go along with the single carb/catalyst emission control system and lower C/R.

18V engines have 8:1 C/R vs 8.8:1 for the 18G_. This was done via piston dish volume.

Some 5-mains have bushed rods with floating pins while many are pressed in place with no bushings. Pistons to match. Early engines have angle cut rods vs straight cut. The angle cut use 3/8" cap screws vs 11/32" press in bolts with 12 point cap nuts.

Early engine pistons can be found with 5 rings, later have 4 rings.

smoking smiley


Member Services:
Dave Headley, dba FAB-TEK offers full service race car parts and preperation for MGB & MGA race cars, SCCA and Vintage. Dave is a mechanical engineer and has raced MGBs since 1963.
riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
In reply to # 3681084 by boggsy64 the "smaller capacity combustion chambers" , do you have a head casting number for that?

at risk of high jacking the OP thread query;

The cylinder head casting identifiers that require the valve reliefs, (shallower combustion chamber) in the block are 12H2923, and CAM1106.

The 12H2923 cylinder head uses an inline oil feed rear rocker pedestal, (below) whilst the CAM1106 requires an offset oil feed rear rocker pedestal. (below)

I repeat only the 12H2923 casting is fitted with te larger diameter intake valve.

B



Check your ego Amigo!

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BH_oilfeed2.JPG    44.3 KB
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