General information

The Petter M is a two-stroke spark-ignition Oil-engine designed, after starting on petrol, to run on what we would now call Paraffin and for which 28 second heating oil is a reasonable cheap substitute. The engines are almost as happy on Petrol. For  most pleasing (and cheaper)  performance, and an absence of the dreaded "Petterspots ™", engines are best, and most economically, run on oil at between one-third and half power. A Petter M off load is an unhappy thing that runs unevenly, oils the spark plug, dilutes the crankcase oil with fuel, dribbles from every pore, and  coughs out irremovable spots of oil. If off-load running is unavoidable I respectfully suggest that draining the exhaust pot before starting and refraining from over-oiling will enable you to live, more or less, at peace with your neighbours on the rally field (and your spouse!).

Production began at Petters Yeovil factory in 1914 (possibly 1913) but switched to Loughborough during 1939. From that time the "Yeovil" cast into the diaphragm cover was roughly ground out. A specific "Loughborough" cover does not seem to have been made (not least because the longer word would not have fitted). 

Serial numbers are usually stamped on the flywheel rim, flywheel keys, sometimes on the crankshaft end, and on the serial plate which is screwed or later riveted above the carburettor. Prior to 1920 or 1921 it was usual to stamp the last two or three digits of the serial on the flywheel/keys and each serial number usually has 1 or 2 stamped with it to indicate on which side it should be fitted. The serial plate (sometimes cast and sometimes pressed) always carries the full serial number From 1920 a casting date can appear cast into the reed valve housing aperture in the crankcase. Generally an engine was built about six months after that date but much longer intervals are known. Casting dates which postdate the selling date are usually indicative of a rebuild.    Around 20,000 were made between 1913 or 1914 and 1939 or 1940. The serial number ranges used were 40100-49999 and 70000-79999. I am reliably informed that the first 101 engines of any run were regarded within Petters as 'prototypes' and it is therefore likely that the serial range actually began at 40,000.

    VERY approximate production figures are: pre Jellymould-1000, Jellymould-4000, Victory-4000, 1922 Model-1000, 1923 Model 7000, New Model-3000.

Rebuilds were performed by local agents and by Petters themselves. Petters supplied 1/16" (approx 0.060") oversize pistons and usually stamp the top face of the cylinder with the oversize and rebuild date. Other oversizes appear to have been made as one-offs. Engines were often updated at the same time and thus acquired later features. Returned engines were often rebuilt and resold. If an  engine has screw in aluminium covers for the ring-oiler wells then it may have been rebuilt. However this feature also seems to have been introduced for main production for a short period in 1930-1 prior to the introduction of the 1931 Calibrater model. If a liner has been fitted a rebuild was probably not done by Petters or an approved agent since an oversize piston would have been a cheaper option.

Colours The norm appears to have been close to Mid Brunswick Green (BS381C-shade 226). However many were supplied to OEM (Original Equipment Manufacturers) and to Military and Government organisations who painted them a wide range of colours. Grey and  Red are known but it would be unwise to be dogmatic about any colour. Modern restorers sometimes use Land-Rover Bronze Green which certainly look very well.

Horsepower Generally 5hp. The 1930s option of 6hp seems to have been available from about 1923 but was rarely selected. A Town-gas version is also shown as 5hp in a 1922 (Australian) catalogue but at unknown rpm. The gas model does not appear in the 1927 or 1931 catalogues.

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