Astron heads explained.
Posted: Sun Jul 10, 2011 10:47 am
Astron I (1.85, 2.0 and 2.6)
GE - GK Sigma 1979-85.
The 2.6lt has a small combustion chamber but ran dished pistons to lower the compression; it uses 35mm diameter exhaust valves and 43mm diameter inlet valves. These were all designed for leaded fuel.
Astron II
The Astron II had a few changes, notably hydraulic tappets, replacing the manual tappets in the previous Astron I.
In December 1985 the Astron I 2.0-litre was dropped, as MMAL did not believe it warranted the conversion to unleaded fuel introduced in January 1986.
M1 - M5 - GN Sigma 1985-87 & TN – TP Magna 1985-91.
Larger hemi style head all much the same 40mm diameter ports using a 38mm diameter exhaust valve and a 46mm diameter inlet valve.
Pajero and Sigma GN.
Ports are much the same as early Magna as above.
M6 - M7 - TR Magna 1991-94.
Uses the same large open hemispherical chamber as above but bigger ports at 42mm diameter with a better flow to the inlet port. They use the same 38mm diameter exhaust and 46mm diameter inlet valves.
This head is the best flowing in stock form.
M8 - M9 – TS Magna 1994-96.
This is the last of the range the open chamber heads; they were dropped for a kidney shaped chamber of smaller size matched to a kidney shaped dish in the piston. They use the same big ports as the M6 and M7 and same nice inside radius (flow) to the inlet port, the same sized valve as all the other Astron II heads. Flow is reduced slightly only because of some masking of the port due to the combustion chamber.
M28 is a new casting on the old Astron II rwd design
Note; the Magna head has a large water gallery at both front of the head and toward the rear whilst the rwd head has the same front gallery but has a small rear gallery.
The 2.4lt 4G64 in TE-TF Magna's is not compatible.
When fitting a Pajero or Triton series II head you will need to use an electric fuel pump as the mechanical pumps are not compatable.
GE - GK Sigma 1979-85.
The 2.6lt has a small combustion chamber but ran dished pistons to lower the compression; it uses 35mm diameter exhaust valves and 43mm diameter inlet valves. These were all designed for leaded fuel.
Astron II
The Astron II had a few changes, notably hydraulic tappets, replacing the manual tappets in the previous Astron I.
In December 1985 the Astron I 2.0-litre was dropped, as MMAL did not believe it warranted the conversion to unleaded fuel introduced in January 1986.
M1 - M5 - GN Sigma 1985-87 & TN – TP Magna 1985-91.
Larger hemi style head all much the same 40mm diameter ports using a 38mm diameter exhaust valve and a 46mm diameter inlet valve.
Pajero and Sigma GN.
Ports are much the same as early Magna as above.
M6 - M7 - TR Magna 1991-94.
Uses the same large open hemispherical chamber as above but bigger ports at 42mm diameter with a better flow to the inlet port. They use the same 38mm diameter exhaust and 46mm diameter inlet valves.
This head is the best flowing in stock form.
M8 - M9 – TS Magna 1994-96.
This is the last of the range the open chamber heads; they were dropped for a kidney shaped chamber of smaller size matched to a kidney shaped dish in the piston. They use the same big ports as the M6 and M7 and same nice inside radius (flow) to the inlet port, the same sized valve as all the other Astron II heads. Flow is reduced slightly only because of some masking of the port due to the combustion chamber.
M28 is a new casting on the old Astron II rwd design
Note; the Magna head has a large water gallery at both front of the head and toward the rear whilst the rwd head has the same front gallery but has a small rear gallery.
The 2.4lt 4G64 in TE-TF Magna's is not compatible.
When fitting a Pajero or Triton series II head you will need to use an electric fuel pump as the mechanical pumps are not compatable.