300 SiX

1980 - 86 FORD TRUCKS
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These are the specification sheets from the light truck facts book which Ford gave the dealerships.

​This is the one from the 1980 book:
The one below is from the 1981 book:
These are the 1982 specifications:
These are the 1983 specifications:
These are the 1984 specifications:
These are the 1985 specifications:
The ones below are the 1986 specifications:
  • 1981 FSM Section
  • 1985 FSM Section
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This section is from the 1981 Factory Shop Manual.
This section is from Ford's 1985 factory shop manual.
  • Carb Spacer
  • Compression Thoughts
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Ford used at least two types of spacers for the carburetors - with and without EGR.  The ones with EGR had a hole that allowed exhaust gas to enter the intake, and those w/o EGR had a tab that covered that hole.  Some people are looking for the non-EGR spacer, and there are two known sources, as shown below.  But if you find another please let me know so I can document it.
Here you can see the tab covers the exhaust gas hole, a spacer w/o the hose barb'd vacuum fitting, and one with the vacuum fitting.
In considering the varying engine output ratings (see the Specifications tab) over the years and the vehicle weight ratings it seems like there are several things in play, including the carburetor, manifolds, cam, and compression ratio.

Toward the latter I've done some sleuthing: The connecting rods didn't change from 1980 through at least 1989, so the CR changes are elsewhere.

According to the 1994 version of the Master Parts Catalog there were four heads used, as shown below:
  1. One for all 1980 through 1984 trucks
  2. Another for light-duty 1985 & 86 trucks with Thermactor ports
  3. A third for heavy-duty 1985 & 86 ports w/o Thermactor ports
  4. One for all 1987 - 89 trucks​​
WE SHOULD PROBABLY DISREGARD EVERYTHING IN RED, BELOW.  BUT I'M PRESERVING IT FOR THE MOMENT.....
So, what about pistons.  Here's what I can determine from the catalog, along with the compression ratios shown in the yearly dealer fact books, as shown on the Specifications tab.

Given the heads used and the changing pistons it is hard to tell exactly what caused the compression ratio changes, but I'll hazard these guesses:
  1. Since the same head was used on the trucks from 1980 through 1984 (from 1978 actually) we can see that the new piston used on the LD trucks in 1983 dropped the compression by 0.5:1
  2. However, I'm confused with the compression ratio of the HD trucks as the same pistons, rods, and heads were used from 1980 through 7/23/84.  And yet the 1983 and '84 compression ratios for the HD trucks supposedly increased by 0.4:1?
  3. A new LD head was introduced in 1985, but I don't see any pistons that were used then, so there's something missing in the catalog.  However, the head/piston combo supposedly raised the compression ration 0.1:1
  4. There was also a new HD head, w/o T/E ports, introduced in '85.  And this time there were pistons listed for the HD engines.  But the combination of the new head and pistons reduced the compression ration by another 0.5:1
Today (12/13/18) I climbed up to the microfiche tower and reviewed the 1990 version of the MPC on fiche.  And, there were some significant differences in both the piston and head areas.

On the piston info, I've added the info to the spreadsheet, and I've shown the changes in red.  Note that there's another piston (E4-L) that doesn't even show in the '94 MPC.  And, there was a lot of clarification regarding the other pistons.

And, the info on the heads was different as well.  I added it to the spreadsheet, as shown, but haven't gotten my head around it yet.