Are there any difference in the radiator cap threads of the 09 thru 27 model T's? I just purchased a winged cap and motor meter from Langs and it slips down into the neck without engaging the threads. It mics about .022 less than the original cap when measured on the outside of the threads. I have a 26 coupe with what looks like the original radiator. The present cap is probably a repo since it is chrome with the four short ears like the original and it fits perfectly.
The threads in the radiator are the same over the years but repro parts has been known to vary in quality
the problems with caps for moto-meters used to be the opposite, they were too large. Perhaps some kind of misguided overcorrection resulted in a batch with too small caps?
See this 2012 thread about too large threads:
There must be a standard for this type of thread with minimum and maximum tolerances..
Bob, you should call Lang's and ask them to test their remaining caps in a radiator. If they find a working cap they should send it to you, the rest should be sent back to the producer.
Here's another older thread where the problem is like you have - the cap drops down:
If your radiator filler pipe isn't widened or cracked in the threads, then maybe the threads on the repro cap has been destroyed by careless buffing as Bill Harper suggested in 2007?
Thanks for the input. Langs is going to find me a good one. I sent the defective one back.