If you have a 1930-1931 Model A and use the original style caps with a "bayonet" style cap there is no problem with fitment. We check every one.
Locking motometer caps and quails however can sometimes bind. The problem results from a neck ramp that guides the tab to the stop point tends to bind at the start. The OE necks are 1/32" larger ID with a slight;y more gentle ramp effect as it rises to the "stop" tang. There are three solutions:
1. Use an OE style cap.
2. Purchase a The Brassworks Radiator made after March 2020 when we began opening the neck ramp to address the Taiwanese cap quails and motometers.
3. If you own another companies radiator you'll need to dremel or file a slight radius on the cast tab holder per the image below (courtesy of a Snyders online .pdf). Model A radiators sold by Brattons, Snyders and Macs etc which are not made by Brassworks will require this modification.