Palmers book

Hi, i'm a bit confused:
i'm searching information about 45 cilinderheads Iron & Aluminum and compression markings.
in Palmers book: on page 201
he refers to 3 types of iron heads marked 4.5 for high compression,
marked 5.5 for medium and 6.8 for low compression
however i own 3 couple iron heads, 1 pair with narrow sparkplug hole ( should be Servi-car: later years and low compression and 2 pair with the 18 mm sparkplug hole from Ex-police motorcycles
all are marked 6.0 (for howfar i can deciver the corrosion)
also some people around here (well known with old Harley's) are talking about : 6.0 for high compression, 5.0 for medium and 4.5 for low compression.

concerning the aluminum heads: medium compression are marked 5.0 (as i found on some military heads and also the numbers : 119-39 F and 119-391 R)
and marked 6.0 for high compression (which i don't have)
but my confusion is about these Iron heads
please: let me see clear.
Thanks
JP

On the older iron heads, both for 45 and Big Twin, the number in the bottom surface was the combustion chamber VOLUME, not the compression ratio. So the higher the number, the lower the ratio.On the Aluminum heads the numbers are the ratio, not the volume! And on your later head it is once again the volume, so those #6 iron heads are low compression for commercial use, most came on Servi-Cars or special units like the Police bikes you are talking about. All very confusing but two different systems of measurement!
Robbie

The iron heads numbered 4.5, 5.5, and 6.8 were used on '36-'39 models only.
(different ones for W, WL, and WLD models)
In 1940 the W model was dropped, and the WLD got the high compression aluminum heads.
So there was only one type iron head left, and these were now marked 6.0.
It was stock on all WL and Servi-cars '40-'73.

Cast Iron high compression heads for the 37,38,& early 39 WLD are 3 bar heads with 18mm plugs & 3/8" mounting holes for studs using nuts.Pretty easy to spot,not easily found.More rears about but no many fronts.