Since the first Gipsy Major engine was flown in 1931, the engine has been subjected to a lot of development resulting in several marks with upgrading of performance. As can be expected, Gipsies in current use cover the whole range of modification standards and reducing an old engine to spares may not produce some parts to the latest standard. Details as to the compatibility of modification standards of the various components are not shown in the engine Maintenance Manuals but they may be found in old Modification Sheets, Technical News Sheets, etc.
The different marks of Gipsy Major can be divided into three groups; using the post war designations these are:
1. Gipsy Major 1
GM 1. The basic Gipsy Major engine. Fitted with aluminium bronze cylinder heads and only suitable for use with unleaded fuels. Variants of this engine were produced specifically for the Tiger Moth and Magister aircraft and these were designated Gipsy Major 1 Trainer and Gipsy Major 1 Magister. Rating 122hp at 2100rpm.
GM 1F. As the GM 1 but fitted with aluminium alloy cylinder heads for use with leaded fuels. Developed for Tiger Moth glider tugs operating in post-war France.
GM 1 Auster III. Similar to the 1F, has screened ignition and installational differences for the Auster III.
GM HC. Similar to the 1F but has different pistons, piston pins and connecting rods to give a higher compression ratio of 6:1. Rated at 130hp at 2100rpm.
GM 1C. As the GM HC but built to a later modification standard.
GM 1D. Similar to the 1C but fitted with fuel pumps, screened ignition, induction system priming and double scavenge pumps. Rated 142hp at 2400rpm.
GM 1G. Similar to the 1C but incorporates the accessory drives and timing gear cover from the GM 10 Mk 1-3. Rated at 130hp at 2100rpm.
GM 1Z Gipsy Major 1 engines modified to accept the splined crankshaft as fitted to the Gipsy Major 10 Mk 2 are identified as Gipsy Major 1Z. These engines are identical in all other respects to the original standard.
2. Gipsy Major 10 Mk 1
GM 10 Mk 1-1. Developed from the GM 1 and is similar to the 1D. Rated at 142hp at 2400rpm. Civil version of the Military Mk 7.
GM 10 Mk 1-3. Similar to the 1-1 but has a redesigned timing gear cover with accessory drives.
GM 10 Mk 1-4, Mk 1-7. Both engines are similar to the 1-3 but modified for particular aircraft installations.
GM10 Mk 1-1A, Mk 1-3A, Mk 1-4A, Mk 1-7A. Identical to 1-1 etc, but modified to accept the splined crankshaft from the GM 10 Mk 2 with Hoyt white metal main bearings.
3. Gipsy Major 10 Mk 2
GM 10 Mk 2. The 10 Mk 2 is a development of the 10 Mk 1 and is fitted with a strengthened crankshaft with a splined propeller shaft. The main bearings are thin wall Vanderval type in place of the Hoyt white metal bearings in earlier Gipsy Majors. Other differences include redesigned tachometer drive, exhaust valves and induction manifold. Rated at 145hp at 2550rpm.
GM 10 Mk 2-1. Similar to the 10-2 but modified to make it interchangeable with the 10 Mk 1-3 in the Auster J5/1.
GM 10 Mk 2-2. Introduced by modification G 2901 for the Auster 6A and A61.
GM Mk 8. Military version of the 10 Mk 2, except that it is fitted with Hoyt white metal main bearings.
Cylinder heads can be divided into three groups and it is important that they are not mixed but fitted in sets:
Aluminium bronze. Aluminium bronze cylinder heads are for use on Gipsy Major 1 engines only. They are not compatible with leaded fuels, although they may be modified by fitting steel valve seats. Modified engines run successfully with leaded fuel (100LL).
Aluminium alloy. Aluminium alloy cylinder heads may be fitted to any mark of Gipsy Major engine and they are compatible with leaded 100LL fuel. These heads are normally fitted with 12mm spark plug inserts but may be modified for use with 14mm plugs by Mod G1989. The aluminium alloy heads used on GM 10 Mk 2 engines have larger valve guides to accept sodium cooled exhaust valves.
Aluminium alloy - strengthened design. The latest cylinder heads introduced by modification G2197, are manufactured from 'Y' alloy and may be fitted to any mark of Gipsy Major. These heads must be fitted in sets and not be mixed with those of an earlier standard. They have been redesigned to incorporate webs to strengthen the valve rocker platform. A new rocker bracket is attached to the head by three studs and one bolt and the valve rocker cover attaches directly to the rocker bracket which dispenses with the stirrup bracket of the earlier cylinder heads.
The sodium cooled exhaust valve is standard for all marks. A 14mm spark plug adaptor is fitted as standard. If these cylinder heads are intended to be used on engines with unscreened ignition, screened end fittings are still required.
A version of this strengthened cylinder head with 12mm spark plug adaptors was introduced by modification G2282 but are in short supply.
The cylinder head assemblies have been subject to over 60 modifications during the life of the engine. While the majority are of a minor nature, introducing changes in material and design to give performance improvements, some are operationally important and give improved reliability. The modifications and TNS of particular interest are:
Mod G1448. Introduction of increased diameter rocker bracket bolts to reduce the possibility of failure. The triangular locking plates introduced by Mod G1158 is superseded by three plain nuts and a one piece locking tab washer. Mod G1500 and 1536 must be embodied concurrently with this modification. The bolts are 9mm diameter, Part No. 34680.
Mod G1500. Introduction of tab washers in DTD 171B material superseding those in Mod 1448. In addition the enamel finish of the valve gear casing was found to shrink and allow the rocker bracket bolts to work loose. The finish under the rocker bracket is removed to prevent this.
Mod G1534. Introduction of rocker bracket bolts in improved material. Bolt Part No 35175 supersedes 34680, and Nut Part No 376 supersedes No 372.
Mod G1536. To ensure satisfactory seating of the rocker bracket bolt tab washers, the diameter of the spot facing on the bracket is increased.
Mod G2217. This modification was introduced to ensure that the correct valve seat pressure was maintained throughout the life of the engine. A range of different thickness valve spring bottom collars was introduced so that the spring tension can be altered to ensure that the load on the end of the valve required to unseat the valve is between 25.5 and 34lbf. This modification should prevent rough running caused by the exhaust valve being held off its seat during the induction stroke.
Mod G2329. This modification introduces torque loading figures for the cylinder holding down nuts and gives details of machining operations to provide clearance for the special spanners. The nuts should be evenly tightened down to a torque of 350 to 400Ibf in using the tools introduced by Mod G2353.
Mod G2353. Introduction of special tools to torque load the cylinder holding down nuts.
Mod G2362. Introduction of a redesigned valve collet to prevent heavy wear taking place between the valve stem and collet. The outside taper of the collet was altered and the grooves re-dimensioned. The new collets can be identified by pairing numbers etched on the top of each half collet. The numbers are prefixed with the letter 'L' and are numbered LA 1 to LA 999 and continuing LB 1 to LB 999 etc. The modification also gives wear limits for the valve stem grooves. Collet Part No 43425 supersedes 42170.
Mod G1099. Introduction of aluminium alloy cylinder heads for GM 1 to allow use of leaded fuels. The GM 1 then becomes the GM IF.
Mod G2197. Introduction of strengthened aluminium alloy cylinder heads and redesigned rocker brackets. These heads have a 14mm spark plug adaptor fitted.
Mod G2202. Introduction of a strengthened cylinder head to Mod G2197 standard but with 12mm spark plug adaptors.
TNS G No 2. This TNS gives details of the valve clearance setting with the various cylinder head and push rod standards. In all cases the exhaust valve clearances are 0.005in, and the inlet valve is 0.005in in aluminium alloy heads. In bronze heads the inlet valve clearance varies between 0.005 and 0.015in depending on push rod material and modification standard.
TNS G No 10. Introduction of an examination of rocker brackets on pre-mod G2197 cylinder heads. The brackets should be examined for cracks at the radius between the nut spot facing and the bracket stem.
TNS G No 29. Details of acceptable fin damage on a head. Damage up to 2sq in per cylinder head is acceptable and the damage may be blended out and cracks in fins stop drilled.
TNS G No 42. The correct assembly procedure for the cylinder head gasket is detailed. The latest type of gasket, Part No 37257, should be assembled with the joint in the copper towards the cylinder barrel. If the gasket is fitted with the joint towards the head, there is a tendency for it to open out when tightening down the head, leading to ridging of the copper and subsequent blowing of the gasket.
INLET AND EXHAUST VALVES
The various standards of inlet and exhaust valves can be divided into groups broadly relating to the three standards of cylinder head. Modifications are generally applicable to all groups and the main differences between the valves are a change in material, change in dimensions or, with exhaust valves, the sodium content. A brief review of the differences is as follows:
Stem diameter. The stem diameter of exhaust valves fitted in Mod G2197 and G2282 cylinder heads is a nominal 12.5mm. The stems of all other exhaust valves and of all inlet valves is a nominal 11mm diameter.
Stem length. Valves manufactured in material specification DTD 49B were introduced by Mod G1861 (exhaust) and G1862 (inlet). These valves have a lengthened stem with a hardened end and the valve thimble is deleted. Valves to this and subsequent standards can be readily identified by comparing the length of stem from the collet grooves to the end.
Sodium Content. Exhaust valves having sodium filled stems for cooling were introduced by Mod G 1861. The original content was reduced by Mod G2691 to allow the heat to dissipate more quickly and prevent excessive carbon build up around the valve stem.
Interchangeability. All valves are interchangeable within the three groups.