JAP engine threads

[vhgmcbuddy]

Hi everyone
As some of you are aware I am in the proses of rebuilding my late fathers trusty but have come up against a problem. One of the inlet and exhaust studs have snapped of and the other treads are in a bit of a mess, I have managed to clean them out but I am not sure of the sizes. A UNF bolt fits in but is very loose, and the nut from the other stud does not fit the bolt so not sure if it might be a cycle thread, the engine is 1944, any help would be appreciated, thanks Brian

[charlie]

The threads will most likely be Whit and or BSF, not UNF, UNC or metric. UNC is close to Whit and UNC nuts may go on a Whit threaded bolt or stud but they will not be a correct fit.

[wristpin]

As has been said the threads are most likely to be BSW or BSF but I recently worked on a JAP two-stroke that what at first sight appeared to be BSF were actually Cycle Thread. I was able to obtain a selection of cycle thread nuts from an Internet trader amusingly named Nooky’s Nuts!

[trusty220]

All of the JAP 5 engines that I have worked on have been cycle thread which is also known as CEI. The most recognisable feature is that CEI is always 26 threads per inch (TPI) no matter what the diameter, so if you have a thread guage you should be able to recognise it quite easily. The hexagon heads seem to vary from early British sizes (BSW or BSF) on the early ones, transferring to the American sizes (AF) later on, but no matter what the head size is the thread is always CEI.
I hope that helps- Happy New Year!

[Author]

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