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why would an intake manifold stud have both coarse and fine threads?

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twas_brillig

Jedi Knight
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Okay: I'm trying to convert a 1972 BMW 2002 from carbs to EFI, using gutted Weber DCOEs as throttle bodies and a dual side draft intake manifold out of the USofA.
The car is metric; the carbs are metric; the studs connecting the Webers to the manifold are coarse 5/16" x 18 TPI and their other half connecting to the carbs are fine thread 5/16" x24 TPI.
I'm assuming that using the coarse threads into the manifold maybe are for manufacturing ease (but would appreciate comments from someone more knowledgeable than I), but why in heck switch over to fine threads for the nuts holding the webers to the manifold? Fine threads are stronger, but why switch? Any wise persons out there?
I guess I can just be thankful that they aren't British Standard Whitworth combined with goodness knows what.
Thanks,
Doug
 
Coarse threads are used in softer metals such as aluminum. They won’t strip like fine threads will.
 
other than the course/fine - in terms of Metric/Imperial - would someone have replaced the stud with the wrong one?
 
I have seen that many times, with metric, American and British threads. You use a coarse thread into the block, manifold, or whatever, particularly if aluminium. Fine thread is used for the attachment of the piece, where final tightening is done.
Just bot led the exhaust manifold (header) to a Lotus twin Cam head, same arrangement. Good practice!
 
Thanks guys! The "coarse thread into soft metal" makes a heck of a lot of sense; I'm not sure why switch to the fine thread for the nut to bolt the carb to the manifold though.
We'll have to work up something as I'm adding about a 1/2" spacer to the existing 1/2" spacer so the studs won't reach. We'll either use coarse threaded rod throughout or maybe see if a bolt can be used (coarse again) to get a stronger end on the carb side. I recognize fine threads are stronger than coarse , but am confused as to why it would be necessary to go to fine for the carb attachment - we'll sure as heck be betting that it's not critical.
I should also apologies for my grumpy comment about a metric car with a metric carb and a manifold using old Imperial threads - when the manifold were designed, North America was a long way from being metric, and all the tooling would be for Imperial/SAE. Thanks again, Doug
 
Fine threads are not only stronger, they can be torqued much more precisely, and don't shake apart as readily. You'd always prefer them on threads used for final assembly.
 
The reason for combination coarse/fine threads on studs is simple. The coarse thread end goes into the permanent part then it is usually easy to tighten and remove the fine threaded end without disturbing the coarse threads. It's pretty common on most double-ended studs.
 
Chatted with my brother (retired auto mechanic, mostly Fords) last night and he mentioned that Ford used the coarse/fine thread stud combination for attaching carbs to iron manifolds; GM used bolts rather than studs. Just an FYI. Doug
 
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