316 or 18.8 stainless steel bolts

[isis]

I want to replace the 5/16" x3 1/4" bolts in the lower unit.What the better bolt to use and the different?Also what are the stock bolts?

 

[Frank Acampora]

Re: 316 or 18.8 stainless steel bolts

316 is one of the most corrosion resistant common stainless steel. But usually, the ones you buy at Fastenal, for example, are 304 unless specified 316 which is more expensive. In your case, it really doesn't matter because the stainless sets up a galvanic potential with the aluminum and the aluminum preferentially corrodes.

SO: no matter what stainless bolts you use, coat them liberally with anti-seize and if used in salt wated remove them once every year.

This also applies to the spool inside the gearcase behind the prop. If not serviced every season, corrosion will build up and likely split the gearcase. Even if it doesn't split the case, it will be a bear to get out.

 

[isis]

Re: 316 or 18.8 stainless steel bolts

Fastenal wants .90 cents for 18-8 and $1.37 for 316. Big differents.

 

[RRitt]

Re: 316 or 18.8 stainless steel bolts

316 is a very hard and very strong metal. However, it is also insanely aggressive towards aluminum. Common stainless is not quite as hard, not quite as strong, and significantly less aggressive. What this means:

316 is stronger
316 causes more corrosion to aluminum
316 is more likely to seize (bolt seizure is caused by aluminum oxide)
316 is less likely to break off (if allowed to seize)
316 is almost impossible to drill out without damaging aluminum

304 is weaker
304 causes less corrosion to aluminum
304 is less likely to seize
304 is more likely to break off (if allowed to seize)
304 is relatively easy to drill out


I see horrible corrosion on trim systems all the time. The Force trim systems have far too much stainless for saltwater use. When used in saltwater the galvanic corrosion eats them up from the inside out. Theoretically the ideal thing is 316 SS shafts just like the original along with religous maintenance. Realistically 304 is the better choice because even the best maintenance doesn't stop galvanic corrosion on saltwater boats. So my off the cuff advice is 316 for freshwater and never give them another thought. But 304 for saltwater and retorque them every 2 years (back out 1-2 turns and then retighten to prevent seizure).