45" Head Identification

[BigLakeBob]

The heads pictured are the style to fit on '36-'50. 8 holes about .410 diameter.
'39 parts book says 6.8 low comp & 5.5 high comp, '36-'39.
'41 parts book says 6.8 for '29-'35, not these because they are not 7 holes, also says 5.5 for '36-'41 .
'46 parts book says 4.5 & 5.5.
Johnny Sells mentions confusion on part # differences in the early parts books.
Bruce Palmer mentions 6.8 & 5.5.
I know there is something to do with how many 'bars' the head has next to the #. Are the 6.0 heads I have 5.5? Any help appreciated.
Thanks. Bob

[Rubone]

Bob,
The numbers on them relate to volume, the bigger the number the lower the compression. The bars go along with that with markings from one bar to three bars with one being the lowest and three the highest.
Robbie

[BigLakeBob]

Thanks Robbie, I understand and agree. So that would make the 6.0 med compression (2 bars), why are they marked 6.0 and not 5.5? Parts books state 'marked' 5.5, that is where my confusion lies. Bob

[dane]

What do the numbers on the right hand side of the head on the outermost fin designate. I have a 48 wl with a 6 stamped in that location.

[Rubone]

Dane,
The alloy heads are usually marked either 5 or 6, with the 5 being medium compression (used on all military models) and the 6 high compression used on civilian models. Some early alloy heads ('40 and maybe early '41) are marked with ribs and no number

[dane]

Thanks Robbie for the clarification

[len dowe]

Bob,
I also have a 45" iron head with 2 bars and 6.0. Although I have misplaced my camera at this moment and the little trolls have not brought it back at this time.