Background

by Chris Draper 2001-11-15

Basic Technical data taken from an article originally appearing in French, based on information supplied by Salmson.

The S4 series of engines originally appeared in 1928, replacing the earlier "Petit" designs. The first type the S4 used a separate alloy crankcase and bolted on cast iron head and block. It was made in three basic versions. The bore size being S4 65mm, S4C 69mm, S4D 72mm and the final S4 61 with 75mm bore. There were also minor differences with the crankcases depending on starters, dynastarts and distributor drives. From the S4C onwards the internal dimensions of the crankshaft and other components were unchanged. British Salmson produced the S4C and S4D versions. The S4D is using a modified crankcase to suit the unique British gearbox.

There was a further S4E engine of around 2.3 litre that used a one piece block and crankcase. This was produced as a cast iron engine, pre-war and as an alloy one post war. It was never manufactured by British Salmson.

Technical data

Valve timing

Inlet opens 0.2mm before the piston reaches top dead centre (TDC) or 10mm before TDC on a flywheel of 280mm diameter.
Inlet closes 17mm after bottom dead centre (BDC) or 128mm on the flywheel.
Exhaust closes 0.5mm after TDC or 20mm on the flywheel.
Exhaust opens 14.4mm or 20mm on the flywheel.

Note: the opening of the inlet and closing of the exhaust are the critical measurements.

Valve clearances

Inlet 0.1mm
Exhaust 0.1mm

Firing order

1, 3, 4, 2.
Maximum advance 1mm before TCD on the piston.

Cylinder block

Bore size is always nominal +0. Wear allowances are +0.02mm for ovality and 0.02mm for bore taper.
Pistons
75mm bore
Weight 300 +/-3 grams.

Clearances 0.02mm at the top of the piston, 0.03 at the bottom.

Connecting Rods

Distance between centres 210mm
Bore of small end 22mm
Bore of big end 50mm
Weight 680 +/-4 grams

Crankshaft S4C and later engines

Three main bearings length 50mm diameter 50mm. Side clearance 0.10mm. Radial 0.05mm.
Big ends length 30mm, diameter 50mm, Side clearance 0.035mm.

Oil pressure relief spring

Free length 11.1mm, exterior diameter 9.6mm, wire diameter 0.8mm, number of coils 4, fitted length 7mm when it should exert a pressure of 0.875 kg.

Cylinder head

Depth of head 113.05mm.

Valve seat angle 90° for inlet and exhaust.

Valve guides exterior diameter 12mm, interior bore 8mm. Length 37.5mm. Clearance of valve stem in guide 0.02mm to 0.03mm. NOTE: this applies to the later cast iron valve guides. British Salmson engines originally used bronze guides and a larger clearance will be required dependent on the exact specification of material chosen.

Valves overall length 72.17mm head diameter 37.6mm stem diameter 8mm, width of valve seat 2mm.

Valve springs
External spring: Free length 40mm, number of coils 7, exterior diameter 26.2mm, wire diameter 2.5mm. length under load of 4kgs 25mm.
Internal spring: Free length 40mm, number of coils 8, exterior diameter 18.8mm, wire diameter 1.8mm length under load of 4kgs 25mm.

Camshafts three bearings, diameter of shaft 20mm, lateral clearance in bearings 0.04mm, end float taken on rear most bearing 0.10mm.

Typical Concerns experienced with these engines

Lack of oil pressure. Oil leaks from the rear of the crankshaft. Cylinder head gasket failure. Oil pump drive gear failures. Cracking of the cylinder heads and blocks in various places. A number of articles related to these matters have appeared in BSOC Newsletters over the years. Please use the Newsletter texts by Car Part pages of this site to read more.