Using Zinc Coated bolts on shaft/trans coupling

[swbca]

Does anyone know the recommended torque on the shaft coupling bolts ?

After replacing the damper plate on my boat, I transported the boat before aligning the engine, and left the 7/16 x 1.5"  20-tpi bolts far away by mistake.  I picked up the same size bolts/nuts at a hardware store, but they may not be the highest grade.  This is for temporary use until I get replacement OEM bolts.

extending the concept of friction from clamp force created by automotive wheel lug bolts, when torqued properly there is never any shear on automotive lug bolts with any type of driving including racing.  I am certain the same engineering concept applies to the shaft coupling. 

Is there a recommended torque value here ?

 

Edited May 5, 2023 by swbca
cant type

[teammalibu]

My dad started his boat repair

career in the mid 1950s and used to say crank em down till they snap and back em off a quarter turn. Hope that helps!

[jhughes]

Shearing force is really really tough so I wouldn't really worry about the bolts shearing and I would also not worry about torquing them down too much, maybe 35 ft/lbs? It's not a huge fastener so maybe even less.

[skihacker]

Tight is right too tight is broke. 

[BraceMaker]

Oh gosh run em.  What I would do is consider going to the hardware and pick up a split clamp.  Most "real" boats have something be it in front of the strut or in front of the shaft log that clamps the shaft.  This is so if anything breaks or comes lose the shaft cannot slip back and sink the boat from the shaft falling out. 

Imagine the shaft breaks at the coupler and the prop and shaft rip out leaving a 1.125" hole in you hull you sink.  NBD on a ski lake huge deal on the ocean. 

[RichardDoane]

25 ft. lbs. is plenty. Use nylock nuts. 9/16 end wrenches are all you need. 

[Hucklefin]

If you can't find a service manual, just reference any torque spec sheet online and apply torque based on fastener size and bolt grade.  Get higher grade fasteners if you're concerned.

[HMan66]

Grade 8 bolts/washers and aero nuts is all you need. Firm/tight and you’re good to go.