Lambert with cracked head tube, Worthing, UK

I offer this theory based on the limited dataset of one example: my 24K with the original V.1 death fork and original headset. After I removed the original crown race and measured the interior diameter, I discovered it was 27mm - characteristic of lower-price European and Japanese headsets. A 27mm crown race fits bearing cups with 30mm insert diameter, which fit a thicker headset tube: Either a lower-grade tubing, or a higher-grade tubing that hadn't been reamed out to the larger 30.2mm diameter (fits a 26.4 or 26.5mm crown race).

When I transferred the 27mm crown race to my first replacement fork (a 1990 Tange chrome fork that turned out to be a long reach 27"; I'm about to swap a 1988 700c fork in its place) and attempted to tighten the original headset, both the upper and lower bearing cups pulled out of the head tube. I don't install headsets often, but this seemed very wrong. This shocked me enough that I had to open up Sutherland's Handbook for Bicycle Mechanics 4th Edition, just to make sure headset's aren't supposed to work this way (they're not).

The next time I was at my local shop, I asked the senior mechanic what was going on. He said that a lot of factory bicycles from the 70s-80s often have mis-sized original headsets, either because the suppliers sent the wrong item and nobody checked, because the employees were paid a piecework rate to assemble bicycles whether the gear fit properly or not, because variations in tubing size/shape may have made slapping a differently-sized headset in easier than reaming out the head tube to a consistent size to fit the proper headset.

In my case, it's possible that a little distortion in the head tube might have led the assembler to put the 27/30 headset in, because it was easier to fit than the 26.4/30.2, and it fit well enough to work. I didn't notice any rocking at the headset before removing the death fork. But if the deviation was slightly greater, and there was rocking in the headset that the rider ignored, I can easily imagine the head tube cracking as a result of the stresses at the bottom bearing.

For others who've messed around with their death forks and headsets: Have you measured the crown race sizes and/or the inner diameter of the head tubes? Have you measured the insert diameter of the bearing cups? I suppose it's possible that Lambert might have used 27/30mm headsets for the less-expensive bikes in their lineup, and 26.4/30.2 headsets for the Grand Prixs.