whats the difference between red and blue 253

[HK1837]

With regards to the title which red 253 are we talking about? There are a few:

HT-HJ, LH-early LX high compression.
HT-HJ, LH-early LX low compression.
HX-HZ, late LX and VB high compression.
HX-HZ, late LX and VB low compression.

And there are minor variations amongst some of these as well with regards to auto vs manual (carbies on some, dizzys, no EGR valve on manuals etc). Aircon versions got different fans and in some cases different rocker covers and air cleaners. Rocker covers varied along the way too, and there were different oil filler locations and heater hose attachment points The last few years of red 253 construction also got a different block. The inlet manifold design also changed during the later style red 4.2. The water pump design changed late in red V8 as well, and the last design red 4.2L block and water pump carried on into the blue 4.2.

The main differences between the last of the red 4.2L and a blue 4.2L are:

Lower compression.
Heads have different exhaust crossover ports.
4BBL manifold and 4BBL Quadrajet.
HEI distributor.

However a HX style 4.2 is probably almost as different to an earlier 253 than a blue 4.2 is to a red 4.2.