Thread sizing - aluminum mast

Posted By: Dave
Date: 7/18/07 8:09p.m.

Report: outside forum guidelines

Hi, I am wondering if there are any machinists out there who can advise me on this repair: The spreader bases on my 82 SF 26 are attached to the mast with two 1/4-20 SS machine screws in addition to a large bolt that also captures the lower shroud tangs. Some past stress event pulled the port spreader base away from the mast and in the process stripped the 1/4-20 screws. I must now re-thread the holes for the next larger size fastening, which is 5/16. The two common tap sizes available are 5/16-18 and 5/16-24. Which should I use on an aluminum mast, 18 threads per inch or 24? It seems to me that the 24 will provide more contact surface in the thin wall section, but the threads will be finer and I don't know if this is appropriate for soft aluminum. Thanks, Dave

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