MGB 18G/18GA Engine General Data Specs - Bore, Stroke, Crankshaft, Bearings
Reference · 1962-1974 BAll 6 data & spec plates
MGB 18G/18GA engine General Data spec sheet listing bore 3-16 in, stroke 3-5 in, 1798 c.c., 8.8:1 compression, torque 110 lb ft at 3,000 rpm, plus crankshaft, bearing and piston clearances.
Part of 1962-1974 MG B Data & Spec Plates
This is page 47 of the MGB workshop manual (marked "MGB. Issue 7. 80619", "General Data 1"), presenting the General Data specifications for the 18G/18GA engine. It lists core engine specifications: 4 cylinders, 3.16 in. (80.26 mm) bore, 3.5 in. (89 mm) stroke, 1798 c.c. capacity, firing order 1-3-4-2, pushrod overhead valve operation, compression ratios of 8.8:1 (H.C.) and 8.0:1 (L.C.), compression pressures, torque figures (110/105 lb. ft. at 3,000 rpm), 500 rpm idle speed, and oversize bore limits (.010 in. first.040 in. max). Detailed dimensional data follows for the crankshaft (main journal 2.1262-2.127 in., crankpin 1.8759-1.8764 in., end-float.004-.005 in.), the three thinwall steel-backed copper-lead main bearings with clearances and undersizes, connecting rods (angular-split big-end, 6.5 in. between centres), shell-type big-end bearings with lengths, clearances, undersizes and crankpin end-float, and aluminium solid-skirt pistons including cylinder clearances, ring count (3 compression, 1 oil control), ring groove widths, and gudgeon pin bore dimensions. A reader can use this page as the factory reference for machining tolerances, bearing selection, and engine rebuilding specifications for the three-main-bearing 18G/18GA MGB engine.
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Frequently asked questions
- What is the bore and stroke of the MGB 18G/18GA engine?
- Bore is 3.16 in. (80.26 mm) and stroke is 3.5 in. (89 mm), giving a capacity of 1798 c.c. (109.8 cu. in.).
- What is the compression ratio of the 18G/18GA engine?
- 8.8:1 for high compression (H.C.) engines and 8.0:1 for low compression (L.C.) engines.
- What is the firing order and idle speed?
- The firing order is 1, 3, 4, 2, and engine idle speed is approximately 500 r.p.m.
- What are the crankshaft journal sizes for the 18G/18GA?
- Main journal diameter is 2.1262 to 2.127 in. (54.01 to 54.02 mm) and crankpin journal diameter is 1.8759 to 1.8764 in. (47.648 to 47.661 mm). Crankshaft end-float is .004 to .005 in.
- How many main bearings does the 18G/18GA engine have and what undersizes are available?
- Three thinwall, steel-backed copper-lead main bearings, with undersizes of −.010, −.020, −.030 and −.040 in.
- How many piston rings does the 18G/18GA use?
- 4 rings per piston: 3 compression rings and 1 oil control ring.
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