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Jaguar XK engine
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===Cylinder head=== The cylinder heads were made of [[Hiduminium|RR50]] aluminium alloy due to that material's high rate of heat conduction and light weight, the latter estimated by William Heynes to give a weight saving of some {{convert|70|lb|abbr=on}} compared with a similar head made of cast iron. Special attention was paid to the gas flow, with [[Weslake|Harry Weslake]] and Heynes designing a curved inlet port to impart swirl to the air-fuel mixture for improved combustion. The same basic cylinder head layout was preserved throughout the production life of the engine but with many detail changes. Valve and port sizes and angles, camshaft lift, compression ratio and carburetion were frequently amended from model to model, depending upon whether power or torque was being emphasised.<ref>Thorley, Nigel. ''Jaguar - All the Cars'', Haynes, {{ISBN|978-1-84425-693-8}}</ref> Very early XK engines fitted to the XK 120 sports car and Mark VII saloon lacked fixing studs at the front part of the cam covers, where they covered the timing chains, which leaked oil as a result. From 1951 onwards, all XK engines had studs around the full perimeter of the cam covers. The earliest cylinder heads are known as "Standard" or "A type" heads and are identifiable by their differing valve angles of 30 degrees inlet and 45 degrees exhaust, as well as by their unpainted aluminium finish.<ref>Porter, Philip and Isaac, Rowan. ''Original Jaguar XK, The Restorer's Guide'', Motorbooks International, {{ISBN|0-7603-1737-2}}</ref> Around 1954, the "B type" head appeared, with equal valve angles of 35 degrees on inlet and exhaust. The "B type" heads are painted light "duck egg" blue (early cars) to light green (later cars) in the 2.4 and 3.4-litre models and metallic dark blue in 3.8-litre models. A very few XK 120s and XK 140s were supplied to customers with the "C type" cylinder head, which was painted red and carried a plaque on each cam cover stating "Jaguar Type C". A supposedly more efficient "straight port" cylinder head, again with equal 35 degree angles on the inlet and exhaust was introduced later in the 1950s and this was painted pumpkin orange on the XK-150S. The cylinder heads in the Series 1 E-type and Mark X were painted gold whether the engine was a 3.8-litre or 4.2-litre. Around the time of the later series 1 E-Types and 420G in 1968, about the same time as the arrival of the ribbed cam covers, the practice of painting the cylinder heads ceased. Many sources still describe the subsequent unpainted aluminium finish as "silver". Jaguar's colour-coding of its cylinder heads of the late 1950s and early 1960s can be determined from Heiner Stertkamp's{{cn|date=October 2019}} list below. The only real anomaly is that the earliest 3.8-litre E-Types had an orange painted head (because they had 3 carburettors) but later changed to gold (because they had straight-port heads). Gold paint signified a straight port triple-carburettor setup from then until the demise of the Mark X. * Silver/bare aluminium => 3.4 A-type head (Standard XK120, XK140 and MkVII with two H6 carburettors) * Red => 3.4-litre with C-type head and 2 carburettors (XK 120 C and XK 140SE/M only) * Light duck egg blue and later light green => 3.4-litre with B-type head and 2 carburettors (may appear as light blue, light green or light blue-green) * Dark blue metallic => 3.8-litre with B-type head and 2 carburettors * Orange => 3.4 and 3.8-litre with straight-port head and 3 carburettors <!-- "AND earliest 3.8-litre with straight-port head and 3 carburettors" The prior passage (set off by quotemarks) was moved here as redundant. What is the difference between that left as is and that moved here with respect to 3.8-litre straight port heads with 3 carburettors? --> * Gold => later 3.8-litre and all 4.2-litre (until 420G and Series 2 E-Type) with straight-port head and 3 carburettors Cars with the straight-port head and only two carburettors did not have painted heads. See the complete table of XK cylinder head types as they relate to Jaguar and Daimler models at the end of this article.
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