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Technical Data		 Technical Data
for the MG YB Series only

We often get asked questions on technical matters relating to basic specification data on the MG Y Type, so it seems sensible to make available a printed sheet with all the usual data on it.

Source: Maintenance Manual and Instruction Book for the MG One and a Quarter Litre (Series 'Y') - available as a reprint in full from the MG Octagon Car Club - see Links page.  While every effort has been made to carefully copy and reproduce the data shown accurately, your attention is drawn to our DISCLAIMER.

NB This data is correct for the MG YB only - not the MG YA or YT!  Download this sheet as a pdf file here.

MG YB only

ENGINE:
Type:      XPAG/SC2. Four cylinders, water cooled, with overhead valves, push-rod operated, with detachable head.
Bore 66.5 mm (2.62 in.).
Stroke 90 mm (3.54 in.).
Capacity 1250 c.c. – (76.28 cu. in.).
Bore/stroke ratio 1.353 to 1.
Compression ratio 7.2/7.4.
Depth of head 45.5 c.c.
Cylinder head gasket 0.045 in (1.14mm.) thick copper and asbestos.
RAC Rating 10.97 hp.

  

CONNECTING ROD:
Type Steel.
Length 178 mm with loose steel shell white-metal bearings.
Bore at big-end 48.658 mm – 48.671 mm.
Bore at small end 18 mm.    +.03 mm.

                –.01 mm.
Width at big-end 27.865 mm – 27.890 mm.
Width at small end 19 mm.
Gudgeon pin Type: Clamp. Dia.: 18 mm. – .015mm.  Length 58.5mm.

                                                 – .010mm.

  

CRANKSHAFT:

Diameter

Width

Radius
Main journal (front) 52 mm     – .035 mm.
                – .015 mm.		 38 mm. 2.5 mm.±.15 mm.
Intermediate journal 52 mm     – .035 mm.
                – .015 mm.		 38 mm     – .02 mm.

                – .01 mm.

 2.5 mm.±.15 mm.
Main journal (rear) 52 mm     – .035 mm.
                – .015 mm.		 40 mm. 2.5 mm.±.15 mm.
Minimum regrind 50.50 mm.    
Crankpin journal 45 mm     – .035 mm.
                – .015 mm.		 28 mm     – .015 mm.

                – .010 mm.

 2.5 mm.±.15 mm.
Minimum regrind 43.75 mm.    

  

MAIN BEARINGS:

 

 

 
Bore size of crankcase for main bearings

 

56.34 mm. ± .02mm.
 

Diameter

Width

 
Main bearing (front)

52.020 mm.
52.005 mm.

35.12 mm.
34.88 mm.

 
Main bearing (inter.)

52.020 mm.
52.005 mm.

37.955 mm.
37.925 mm.

Radius 2.75 mm.
U/cut .25 mm.
Main bearing (rear)

52.020 mm.
52.005 mm.

35.12 mm.
34.88 mm.

 
Main bearing clearance on crankshaft diameter

.020 mm
.055 mm.
Main bearing (inter.) end-float on crankshaft

.035 mm.
.095 mm.

  

BIG-END BEARING:

Diameter

Width

Thickness
 

45 mm.

22.46 mm.
22.21 mm.

1.825 mm.
1.831 mm.
Big-end clearance on crankshaft diameter

 

.011 mm.
.056 mm.

  

CRANKSHAFT REGRIND

 

Bearing and crankshaft dia. undersize

Reference

Crankshaft main journal std. and regrind sizes

Crankpin journal std. and regrind sizes

Main bearings std. and regrind sizes

Standard

Standard

52 mm.

45 mm.

52.020 mm.
52.005 mm.

.5 mm.
(.020 in.)

R.2

51.5 mm.

44.5 mm.

51.520 mm.
51.505 mm.

1.00 mm.
(.040 in.)

R.4

51.00 mm.

44.00 mm.

51.020 mm.
51.005 mm.

 

CAMSHAFT DRIVING CHAIN:

Renold chain number 114036 (duplex)
                                 Type: Roller 3/8 in. pitch. 60                                  pitches endless.
CAMSHAFT BEARINGS:  

Front Bearing: White metal.  Outside diameter 43.61 mm. Press fit in cylinder block. Inside diameter to be reamed in position in cylinder block to 41 mm.+.05 mm/+.08 mm.
Centre and rear bearings: Zinc alloy Outside diameter 43.5 mm.+.00 mm.
                                         – .02 mm.

Inside diameter 23 mm.        +.015 mm.
                                           – .005 mm.
Bore of cylinder block for bearings 43.5 mm. +.02 mm.
               – .01 mm.

  

CAMSHAFT:  

End-float: .005 in. – .013 in. (.125 mm. – .325 mm.).
Diameter of shaft for bearing: Front: 41 mm.        + .01 mm.
                              – .02 mm.

Centre and rear: 23 mm.       +.05 mm.
                                           – .08 mm.
Cam lift 5.334 mm. = 8 mm. at valve.
Cam Timing 5 – 45 – 45 – 5

  

VALVES AND GUIDES  
Valve port throat diameter Inlet: 30 mm.
Exhaust: 26 mm.
Valve head diameter Inlet: 33 mm.
Exhaust: 31 mm.
Valve stem diameter 8 mm.±.01 mm.
Valve seat angle 30˚.
Valve guides Bore: 8.06/8.08 mm.
Outside diameter: 14 mm.    +.02 mm.
                                           +.04 mm.
Valve guide length Inlet: 59 mm.
Exhaust: 54 mm.

 

Valve spring pressure

Inner

Outer

Shut

 31 lb. (14.1 kg) 62 lb (28.1 kg)

Open

 43 lb (19.5 kg) 80 lb (37.2 kg)
Valve lift 8.0 mm.  

 

Valve timing

 

 
Exhaust opens 45˚ B.B.D.C.  
Exhaust shuts 5˚ A.T.D.C.  
Inlet opens 5˚ B.T.D.C.  
Inlet shuts 45˚ A.B.D.C.  
Valve clearance (early models) .019 in. (.48 mm.). (engine hot)
Valve clearance (commencing Engine No. XPAG/SC2/18097) .012 in. (.30 mm.). (engine hot)

  

LUBRICATION:

 
Oil pump speed Half engine speed, gear type.
Oil pressure 40 lb./sq. in. to 45 lb./sq. in.
(2.8 kg./cm.2 to 3.2 kg./cm.2).
Oil pump by-pass 60 lb./sq. in. (4.2 kg./cm.2).
Oil sump capacity 9 Pints (5.1 litres).
Oil sump capacity (commencing Engine No. XPAG/SC2/17383) 10.5 Pints (6.0 litres).
Oil filter Full-flow type.

  

IGNITION:

 
Ignition timing T.D.C.
Order of firing 1, 3, 4, 2.
Sparking plugs Champion L.10.S. 14 mm. (Champion
NAB on later models)
Sparking plug gap .020 in. to .022 in. (.51 mm. to .56 mm.).
Contact Breaker Gap .014 - .016 in. (.36 mm. - .40 mm.)

 

PISTONS:    
Type: Aluminium alloy, tin coated, controlled expansion.  
  Width Diameter
Compression ring groove .0886 in. (2.250 mm.)
.0087 in. (2.253 mm.)		 2.381 in. (60.477 mm.).
2.385 in. (60.579 mm.).
Oil control ring groove .1576 in. (4.003 mm.)
.1577 in. (4.006 mm.)		 2.389 in. (60.680 mm.).
2.393 in. (60.782 mm.).
Side clearance of rings in grooves .001 in. (.025 mm.).
.002 in. (.050 mm.).		  
  Width Radial thickness
2 off. Compression rings .0885 in. (2.248 mm) .109 in. (2.768 mm.).
.101 in (2.565 mm.).
1 off Slotted oil control ring (8 slot) .1575 in. ( 4.0 mm.) .105 in. (2.667 mm.).
.097 in. (2.464 mm.).
Piston ring gap .006 in. to .010 in.
(.15 mm. to .25 mm.).		  
Gudgeon pin bore 18 mm.  – .0001 in. (.0025 mm.).
                – .0003 in. (.0076 mm.).

  

PISTON AND CYLINDER BORE SIZES (XPAG Engines)

REPLACEMENT CYLINDER BLOCKS (Normal Standard and Oversize Bores)

Cylinder bore oversize in thousandths of an inch

Cylinder bore size in mm.

Cylinder bore size in inches

Piston size reference

Standard

66.50

2.6181

A…OK

19.7

67.00

2.6378

C…OK

39.4

67.5

2.6575

E…OK

 

Note: After oversize “E” the cylinder bores are relined to standard.

      Bore sizes will be found stamped on the top front left-hand side of the cylinder block.

STANDARD AND OVERSIZE RANGES

When fitting new pistons selective assemble is necessary, and to facilitate this the pistons are marked on their crowns with an indication of their bore size.  Note particularly that the pistons markings indicate the correct size cylinder bore for which they are suitable, the correct working clearance having been allowed in the grading operation.  The piston size should therefore correspond with the marking on top of the face of the cylinder block on the right-hand side, which indicates the actual size of each cylinder bore.

The bores and pistons are graded in four sizes: –

Bores of normal size ±.000 in. to +.00049 in.–marked “STD”

Bores of normal size ±.0005 in. to +.00099 in.–marked +.0005.

Bores of normal size ±.0010 in. to +.00149 in.–marked +.0010.

Bores of normal size ±.0015 in. to +.00199 in.–marked +.0015.

The piston clearance is .0021 in. minimum to .0029 in. maximum (.056 mm. to .073 mm), measured at the top of the skirt, immediately below the oil control ring, and across the thrust faces, i.e. at 90˚ to the gudgeon pin axis.  This is important as the piston skirt is tapered and oval, and the extreme edge of the skirt and clearance can only be measured in this one position.  There is .005 in. (.127 mm.) ovality at the extreme bottom edge of the skirt and the clearance measures at 90˚ to the gudgeon pin centre line at the bottom of the skirt should be .00125 in. (.0317 mm.) less than that at the top.

To facilitate correct measurement of the bores and pistons, the actual sizes of the various  gradings are given at the top of page v (sic see below).

The markings on the top face of the cylinder block will indicate these sizes clearly.

Oversize bores on reconditioned engines supplied under the M.G. reconditioned engine scheme are limited to two oversizes: –

+.020 in. graded in 4 sizes as the standard settings

+.040 in. graded in 4 sizes as the standard settings

The actual sizes of these pistons and bores are provided in the tables on the following page (sic see below).

Note : Badly worn cylinder bores which will not clean up at the maximum oversize bore must be relined to “standard” size and limits.

STANDARD PISTON SIZES

Production engines with bores .002 in. oversize or over are made into +.010 in. bores and graded in the same steps are the standard bore engines.

Piston size
(across thrust faces below oil rings)

Piston marking

 Suitable fore bore size

in.

mm.

in.

mm.

2.6156
2.6160

(66.436)
(66.446)

To suit “STD” bore

2.6181
2.6185

(66.500)
(66.510)

2.6161
2.6165

(66.449)
(66.459)

To suit +.0005 bore

2.6186
2.6190

(66.513)
(66.523)

2.6166
2.6170

(66.462)
(66.472)

To suit +0010 bore

2.6191
2.6195

(66.525)
(66.535)

2.6171
2.6175

(66.474)
(66.484)

To suit +0015 bore

2.6196
2.6200

(66.538)
(66.548)

  OVERSIZE PISTON SIZES (+.020 in. RANGE)

Piston size
(across thrust faces below oil rings)

Piston marking

Suitable fore bore size

In.

mm.

in.

mm.

2.6356
2.6360

(66.944)
(66.954)

To suit +.0200 bore

2.6381
2.6385

(67.008)
(67.018)

2.6361
2.6365

(66.957)
(66.967)

To suit +.0205 bore

2.6386
2.6390

(67.021)
(67.031)

2.6366
2.6370

(66.970)
(66.980)

To suit +0210 bore

2.6391
2.6395

(67.033)
(67.043)

2.6371
2.6375

(66.982)
(66.992)

To suit +0215 bore

2.6396
2.6400

(67.046)
(67.056)

  OVERSIZE PISTON SIZES (+.040 in. RANGE)

Piston size
(across thrust faces below oil rings)

Piston marking

Suitable fore bore size

In.

mm.

in.

mm.

2.6556
2.6560

(67.453)
(67.463)

To suit +.0400 bore

2.6581
2.6585

(67.516)
(67.526)

2.6561
2.6565

(67.465)
(67.475)

To suit +.0405 bore

2.6586
2.6590

(67.529)
(67.539)

2.6566
2.6570

(67.478)
(67.488)

To suit +0410 bore

2.6591
2.6595

(67.541)
(67.551)

2.6571
2.6575

(67.490)
(67.500)

To suit +0415 bore

2.6596
2.6600

(67.554)
(67.564)

  

PROPELLER SHAFT:

 
Type Hardy Spicer needle bearing (balanced).
Length Face to face 45 5/16 in. (1.151 m.).
Tubular shaft 2½ in. diameter (6.35 cm.).
Joint size KR–1111.   GB 22/1118–GB 65.

  

CLUTCH:

 
Type Single dry plate–Borg & Beck
Diameter 8 in. (20.3 cm.).
Facings RYZ
Thrust springs 150 to 160 lb./in.
(26.8 to 28.6 kg./cm.).
Colour “Brown”
Clutch plate damper springs 6 off (Black/Green)

  

GEARBOX:      
Four speed, synchromesh second, third, and fourth gears  
Oil capacity  

1Ό pints (0.7 litre)

Gearbox ratios

 Overall ratios  
Top 1 to 1 5.125 to 1  
Third 1.385 to 1 7.098 to 1  
Second 2.07 to 1 10.608 to 1  
First 3.50 to 1 17.937 to 1  
Reverse 350 to 1 17.937 to 1  

  

INSTRUMENTS:  
Speedometer British Jaeger Model 58597.  Type SC.52
Reduction gear for speedometer 6/15 ratio

  

HYDRAULIC DAMPERS:  
Girling piston type front and rear  
Type:      Front
                Rear		 Luvax Girling PVA6X.
Luvax Girling PVA6.

  

COOLING SYSTEM:  
Cooling by radiator, pump and fan with thermostat control
Total capacity of system 13½ pints (7.7 litres).
Capacity of radiator 7½ pints (4.3 litres).
Circulation By pump fitted in front of cylinder block.
Drain taps 1. Bottom of radiator
1. Front end of cylinder block below exhaust manifold

  

FUEL SYSTEM:  

Fuel tank capacity (level checked by electric gauge on instrument panel)

  
8 gals. (36 litres).
Fuel delivery S.U. electric pump.  12-volt. Type “L”.
Carburetter S.U. 1Ό in. semi-downdraught.
Type H.2 Spec. 456. 
 
Carburretter needles		 Standard               Richer                    Weaker
   “F.I.”                      “D.K.”                    “E.F.”

  

STEERING:  

Rack and pinion

 2.625 turns from lock to lock.

Steering wheel

 Adjustable for length 3 in. (76.2 mm.).

  

FRONT AXLE:  

Camber of wheel

 Nil. ±1˚.

Castor angle

 1˚ ±½˚.

King-pin angle

 9˚- 10.½˚.

Toe-in

 Nil.

Knuckle angle

 10˚.

  

FRONT SUSPENSION:

Independent, with coil springing.

WIRE DIA.(GROUND) .538”
MEAN COIL DIA. 3.238”
RATE 435 LB/INCH
No OF EFFECTIVE COILS 7.34
FREE LENGTH 9.82’ ± 1/16”
TO CARRY 1023 LB. AT:- 7.47” ± 1/32”
TO CARRY 1390 LB AT:- 6.63” ± 1/32”
SOLID HEIGHT 5.025

 

REAR AXLE:  

Type

 Semi floating.

Crown wheel and pinion

 Hypoid.

Oil capacity

 2Ό pints (1.3 litre).

Axle tooth ratio

 5.125 to 1 (8/41).

Crown wheel and pinion backlash

 By spacers and special tools.

No OF LEAVES

7 x Ό" x ½"
WORKING LOAD

650 LBS
STATIC DEFLECTION

3"
MEAN RATE

216.6 LBS/INS
FREE CAMBER

4"
MAX. DEFLECTION

6Ό"

WHEELS:  

Type

 Dunlop No. CDM.317 Ventilated disc.

Wheel size

 4J X 15.

Tyre size

 5.50–15.

Tyre pressures

 Front: 22 lb. per sq. in. (1.5 kg./cm.2).
Rear: 24 lb. per sq. in. (1.68 kg./cm.2).

  

BRAKES:  

Type

 Lockheed hydraulic.  Hand brake cable to rear only.

Linings

 Ferodo M.R.19.

Brake-drums

 9 in. diameter +.005 in./ – .000 in.
(22.9cm. diameter +.127mm./ – .000mm.).

Length of lining

 8Ύ in. (22.22 cm.).

Width

 1½ in. (3.81 cm.).

Thickness

 3/16 in. (4.76 mm.).

Number per car

 8.

Rivets

 Hollow countersunk head (brass).

Number of rivets per lining

 12.

  

GENERAL DIMENSIONS:  

Wheelbase

 8 ft. 3 in. (2.515 m.).

Track:     Front
          Rear

 3 ft 11 3/8 in. (1.203 m.).
4 ft. 2 in. (1.270 m.).

Overall length

 13 ft. 8 in. (4.165 m.).

Overall width

 4 ft. 9 in. (1.45 m.).

Overall height

 4 ft. 9 in. (1.448 m.).

Ground clearance

 6 in. (15.2 cm.).

Turning circle

 33 ft. (10.21 m.). L/H and R/H lock.

 

WEIGHTS:  

Complete car

 20 cwt. 3 qr. (1054 kg.).

Front

 10 cwt. (506 kg.).

Rear

 10 cwt. 3 qtr (508 kg.).

Chassis

 10 cwt. 2 qr. 22 lb (543 kg.).
Bumpers fitted, less wings and lamps.

Power unit (with gearbox)

 3 cwt. 2qr. 5lb. (180 kg.).

 

PERFORMANCE:  

Brake horse-power

 46 b.h.p. at 4,800 r.p.m.

Maximum torque

 702 lb. in. (8.1 kg.). at 2,400 r.p.m.

Litres per ton-mile

 2480.  Top gear.

M.p.h per 1,000 r.p.m.

 14.5.  Top gear.

K.p.h. per 1,000 r.p.m.

 23.33.  Top gear.

Engine speed per 10 m.p.h.

 689 r.p.m.  Top gear.

*Petrol consumption

 36.5 m.p.g. at 30 m.p.h.
(13 km. per litre at 48 k.p.h.)
32.7 m.p.g. at 40 m.p.h.
(11.5 km. per litre at 64 k.p.h.)
29.0 m.p.g. at 50 m.p.h.
(10.3 km. per litre at 80 k.p.h.)

 

*ACCELERATION:  
0–30 m.p.h. (0–40 k.p.h.) in 7.5 seconds
0–50 m.p.h. (0–80 k.p.h.) in 18.8 seconds
0–60 m.p.h. (0–100 k.p.h.) in 29.5 seconds		  

Brake horse-power per litre capacity

 36.8.

Maximum speed

 69 m.p.h. (100 k.p.h.) average.

 

*BRAKING.  DRY CONCRETE:  
30 m.p.h. (48.2 k.p.h.) 31 feet (9.45 m.).
50 m.p.h. (80.4 k.p.h.) 92 feet (28.04 m.).
70 m.p.h. (112.6 k.p.h.) 227 feet (69.19 m.).		  
Hand brake braking.  Dry tarmac.  30 m.p.h. (48.2 k.p.h.) 45 feet (13.7 m.).

* Extracts from Motoring Press

 

ELECTRICAL:  

12-volt ignition and equipment; coil ignition controlled by automatic advance and retard mechanism, incorporated in the distributor.  Dynamo output controlled by automatic compensated voltage control unit.

Starter type

 Lucas Model M.35G1.

Starter switch

 Lucas Model ST.18.  Type L/15.

Distributor type

 Lucas Model DKYH.4A. (Part No. 40089).
              Later models are Part No. 40058F.
              Rotation – anti clock.

Dynamo

 Lucas Model CP.39.PV2.

Dynamo speed

 1.16 engine speed.

Coil type

 Lucas Model Q12–8. Type L/O.

Control unit

 Lucas Model RF.95/2.  Type L/2, 12 volt.

Battery

 Lucas 12-volt, 51-amp.  Type GTXW.9A.

Windscreen wiper

 Lucas Model CR.2.DA.36.

  

LAMPS:  

Headlamps

 Lucas Model MBD.140.

Sidelamps

 Lucas Model 1130.

Fog-lamp

 Lucas Model FT.57. Type L/1.

Stop and tail-lamp

 Lucas Model ST.50.

Reverse and tail-lamp

 Lucas Model RT.50

 

Reprints of the full Maintenance Manual And Instruction Book for the MG YA can be obtained from the MG Octagon Car Club. 
See the Links page for contact details.

 

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Technical Data