So you've decided to build an Jaguar XK120 replica and you need a Donor car.  Easy!..., There are lots of them about, all with XJ6 Badges on the back of them and at reasonable prices. Rarely is anything that simple, as I found out.  The true Jaguar XJ6 evolved from 1968 (Series1), through 1973 (Series 2) to 1979 (Series 3) until 1986. All powered by the 6cyl XK engine (developed from the original engine that powered the XK120) There was also an XJ12 variant powered by a 5.3L V12 engine and  this is definitely not suitable and easily dismissed.  To make matters worse there are also Daimler Sovereign versions of the XJ6 and XJ12. Broadly speaking  these were up-market versions with extra "fluffy dice" and mechanically identical to the XJ6 & XJ12. After 1986 is where the confusion starts since the cars continued with XJ6 badges but were really XJ40's powered by a lighter (Aluminium) AJ6 engine. At a glance XJ40's (1986-94) and X300 (1995-2003) look like XJ6's but underneath they are extensively re-designed.   And what of the Jaguar XJS (1975 - 1995). Again you can eliminate anything with a V12 engine. But some XJS's were fitted with a 3.6 L, AJ6 power unit and could be a possibility. Confused? You should be? So let's see if we can throw some light on the subject.

The XJ6 provides most of the parts needed to build a Classic 120. The most useful donor is the XJ6 Series 3. Why not series 1 or 2? They could be used, but mostly on the grounds of build quality and age should be examined very carefully.  Prior to 1982 the build quality of XJ6's was variable to say the least. Post 1982 there was a definite improvement. "But I don't care about build quality - It's a Donor". Well, you would at the point where you have soaring build costs because of the amount of replacement or refurbished parts you are using.  Basically you are looking for a car in sound mechanical condition The body work is immaterial, it is just a big tin box containing most of the parts you need.

90% of all XJ6's have automatic transmission but it is possible to find some manual gearbox examples and the engine can be converted to manual if required. Power plants were 2.8L, 3.4L and 4.2L. They will all run on unleaded fuel without modification.  The 4.2L unit is probably preferable since it is fuel injected and thus slightly more economical. Traditionalists could opt for  2.8 or 3.4 L with carburettors.  Under current SVA regulations regarding emissions, carburettors would be allowed, since they would meet the emission standard for pre 1994 vehicles of 2.5% CO . Fuel injection, however , would be more environmentally friendly.

The Jaguar XJS was based on the XJ6 using the V12 engine but in 1983 - 1991 a 3.6 L version was offered using the AJ6 engine with a choice of  automatic  transmission or 5 speed manual box.  The AJ6 version can be used as a donor since it uses the XJ6 suspension, with the exception of steering rack, upper steering column. switch gear, brake pedal and instruments, which must be alternatively sourced.  The advantage of using the AJ6 engine is it is lighter and more powerful (225 bhp) than the XK unit (205 bhp).

Some  XJS's were designated XJR with a 4.0L supercharged power unit. In general it is felt that the 4.0L unit is not as long lived as the 3.6L unit and will probably expire before the 3.6L unit. Some XJ40's were fitted with the 3.6L AJ6 engine as well, mated to a 4 speed automatic gearbox,  Only the engine, gearbox and prop shaft are useable. If you really are interested in going down the AJ6 route then sourcing the engine alone or using the XJS as the donor vehicle is the only viable solution.

So after all that, find yourself an XJ6 Series 3 that is in good mechanical condition.  Do all those checks you would normally do when buying a 2nd hand car.  Brakes, Tyres, Lights, Steering, Suspension, Oil leaks, Engine Smoke, Electrics, Test Drive it,  The XK engine is pretty much thought of as indestructible, so high mileage is not necessarily a problem. However some engines of the mid 80's show signs of burning oil (blue engine exhaust smoke at high revs `and on overrun) above 60,000 miles. This is a reflection of the build quality of the time.

You have to make your own judgement if the smoke is excessive. A good one will do well in excess of 100.000 miles   If it has a FSH so much the better. A short MOT  would be a bonus, so that you can drive it for a while, as an extended test, and find out it's true condition.  If you are not sure, take someone experienced with you. Haggle over the price you are going to throw lot's of this car away