A build group is the largest assembly of engine parts that can be conveniently removed or installed as a unit.
The JT8 engine was designed before the build group concept was fully developed. As such this does not apply to well to the JT8D compared to the newer engines. Because the JT8 has a full length fan duct it is not practical for the engine to be fully modular.

Major Engine Build Groups.

• Front accessory drive group
• Fan inlet group
• Front compressor group
• Compressor intermediate group
• Rear compressor group
• Diffuser group
• Diffuser outer fan duct group
• Combustion and No5 bearing section (group)
• Turbine nozzle group
• Rear compressor drive turbine group
• Front compressor drive turbine group
• Exhaust case group
• Accessory gearbox group
• Fan discharge section (group)

Front Accessory Drive Group
The front accessory drive group contains gears which are turned by the front compressor drive shaft, These gears turn the N1 tachometer and the scavenge oil pump for the No1 bearing, cast magnesium housing includes an oil nozzle

Fan Inlet Group
This directs the air to the fan blades at the correct angle with a minimum pressure loss, supports the No1 bearing and front accessory drive. The fan inlet case has two bosses for anti icing air. They are at the 2:00 and 10:00 positions. There are 19 equally spaced inlet guide vane which are hollow, anti-iced and support the No1 bearing. Eighteen of the vanes have two internal ribs run the length of the vane. The remaining vane is the master vane, which is at 6:00. (or six o clock vane), it has five internal tubes. The N1 Tacho tube, Breather tube, scavenge tube, pressure tube and the Pt2 tube. There is a Pt2 probe at the 5:00 position on the inlet case, this senses total temperature at the inlet (goes to PRBC). There is another Pt2 tube which goes to the nose bullet and EPR system associated. A Tt2 sensor at the 7:00 position senses total temperature at the inlet (for FCU).

Front Compressor Group
This group increases the pressure of the primary airflow. The fan stages accelerate the secondary air. The case is designed for fan blade containment, there are two cases Fan Front Case and Fan Rear Case. There are six stages in the front compressor, two stage fan (1 and 2) and a four stage low pressure compressor (stages 3,4,5,6).

Blades in This Group
First Stage Fan, 30 blades in the -7, 27 blades in the -9, -11, -15, -17 models, dovetail slots and retaining plate.
Second Stage Fan, 42 blades -7, 40 blades in the -9, -11, -15, -17 models, pin joint attachment with rivets
Third Stage LPC, 64 blades, dovetail slots and retaining plate.
Fourth Stage LPC, 62 blades, dovetail slots and tablocks.
Fifth Stage LPC, 64 blades, dovetail slots and tablocks.
Sixth Stage LPC, 62 blades, dovetail slots and tablocks.

There are five stator assemblies for compressor stage 1 thru 5 and are of a continuous ring construction.

Note that the fan exit case is an assembly part and not part of this group. It has a two piece split case.

Compressor Intermediate Group
This support the No2 and No3 bearings, accessory gearbox and attachment points for the engine front mount brackets. This group includes sixth stage stator, vanes in the secondary gaspath, some farings in the secondary gaspath for sixth and eighth stage bleed air.
Other ducts and cases in this section are, fan discharge front compressor inner duct, fan discharge front compressor outer duct, compressor intermediate case (includes tower shaft), Fan discharge rear compressor inner duct, fan discharge rear compressor outer duct.

Note that the fan discharge rear compressor inner and outer ducts have bosses for 6th and 8th stage bleed air, 6th stage for the Ps3 sense signal to the PRBC and aircraft use, 8th for engine anti icing and other aircraft use.
One or three antisurge bleed valves are attached to the exterior of the fan discharge rear compressor inner duct. (see the section on the antisurge bleed system valve numbers vary depending on model).

The compressor intermediate group has borescope ports that give a view of the 6th and 7th stage area.

Rear Compressor Group
Increases the pressure of the primary air, the rear compressor receives pressurised primary air from the low pressure compressor before passing it to the diffuser.
Seven stages of high pressure compressor blades (stages 7 thru 13). Six stages of high pressure compressor vanes (stages 7 thru 12)

Blades in This Group
Seventh Stage, 60 blades, pin joint attachment with rivets
Eighth Stage, 58 blades, dovetail slots and tablocks
Ninth Stage, 60 blades, dovetail slots and tablocks
Tenth Stage, 64 blades, dovetail slots and tablocks
Eleventh Stage, 70 blades, dovetail slots and tablocks
Twelfth Stage, 80 blades, dovetail slots and tablocks
Thirteenth Stage, 74 blades, dovetail slots and tablocks

There are six stator assemblies stages 7 thru 12 of a continuous ring construction.

Diffuser Group
Receives high pressure primary air from the rear compressor, reduces air velocity suitable for combustion. Supports the No4 bearing, holds nine fuel nozzles and mountings for nine combustion chambers. The diffuser case has nine hollow struts with holes in there sides. They are between the fuel nozzle supports and slightly forward of them. Thirteenth stage bleed air enters these struts through there side holes and flows outwards to the 360 degree manifold on the exterior of the diffuser case.
The following tubes pass through these struts, Two oil system breather tubes, one pressure oil tube, one scavenge oil tube, one seal air pressure tube (eighth stage, labyrinth seal only), Three seal air bleed tubes (if No4 bearing seal is labyrinth, -11 thru -17) Otherwise Two seal air bleed tubes (-1 thru -9).
Note the diffuser group also includes the 13th stage stator assembly and two fan duct fairings for the fuel inlet tubes.
Bleed Air -