Mr. Lang,
Thanks for the memory joggle. Also, nice picture.
The need for the thermostatically controlled damper does seem to be a matter of degrees on GMC 248, 270, and 302 engines used on larger trucks through at least 1953. The standard (through 1953) 228 GMC engines used an automatic manifold heat control, similar to the damper on Chevrolet engines. However, the larger engines had a manually-set heat control (even on the updraft manifold used on the COEs).
This allowed the owner to manually adjust the control valve from full-off to full-on, depending on the prevailing temperature of that location or the time of year.
I cannot find information about the manually-controlled heat riser valve on A-D trucks (USA and Canadian Shop Manuals) but it is described in the 47-54 USA GMC Maintenance Manual.
Tim
