|The 6X head was mounted on the L-78 400 cubic inch displacement engines between 1977 through the final installment of 400 engines in 1979. However, it appeared to many that the Pontiac 350 engine shared the same head with the standard L-78. This was not the case, and an identity crisis ensued. People started to wonder what head they had on their engine, or what head was on that engine and so on. Some wondered if they had a big and small chambered head. There were plenty of discussions about the heads in Pontiac circles and what I have written below is gleaned from those discussions and my own study. [The above photo is driver's side 6x-4 head that sits atop my L-78 W72, 1979 T/A 6.6 engine.]
1. There are two and perhaps three versions of the 6X head.
2. The small chambered head version was designed to fit the bore of the Pontiac 350 engine and was used on the L-78, W-72 engines.
3. The large chambered head was designed to fit atop the standard L-78 Pontiac engine.
4. The larger chambered head is distinguishable from the smaller one on the head's exterior. (Read below)
5. The exact chamber size of any 6X head cannot be assumed and requires measurement to eliminate uncertainty. Too much inaccurate information on chamber size and time has made information less than exact.
6. All stock 6X heads have 2.11" intake valves and 1.66" exhaust valves.
7. All stock 6X heads can be reconditioned, milled and performance enhanced.
A 6X head has 6X cast into the two center exhaust ports. Locate the vertical boss as shown in the photo above. If the head has the number 4 punched into the boss it's the smaller chambered head. If there is an 8, it's the larger chambered head.