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A transmission flange to coupler bolt sheared on Crows Nest. I’m guessing that the bolts have been on there at least 18 years and are pretty corroded. They’re not stainless. Can I use stainless and locknuts for replacements? Or are the bolts a specific hardness? Should I get them from my local prop/shaft shop or the hardware store? Any idea on proper torque? Crows Nest has 300 hp Detroit diesels. Thanks.

Use Grade 8- fine thread hardware for coupling bolts. Do not reduce the nominal size. A 7/16 hole in couplings should have 7/16-20 hardware, 1/2" hole, 1/2-20 hardware etc. I mention as fine thread hardware is not as readily available in hardware stores and some go , "eh, it's close enough".

Do not use Stainless steel hardware for the couplings.


Can be found at Quality hardware stores, and prop/marine machine shops. Or online - **McMaster Carr, Fastenal among others.

Thanks Carl. Just what I needed to know. I’ll just go to my prop/shaft guy and have him set me up.

McMaster Carr will also have them.

Could have sworn mine bolts were metric. Inspect the gearbox coupling "centering ring??" prior to reassembly. When my bolts sheared, the centering ring got a small ding in it that prevented me from aligning the shaft. Took me forever to figure out the issue. Had to file the defect down.

I went to Kastel Brothers in St Michaels. They did the shaft/coupler work for me a few years ago on the port side. They miked up the bolt I brought with me, set me up, I bolted it back together, and ran her yesterday. She’s fine. Thanks for the help.