Engine mounting bolts - use stainless, or no?

justsomeguy · Mar 16, 2018

I would think it's okay, using something like Tefgel to prevent corrosion. Did the warning include a reason?
Got any huge pictures?

Benny17441 · Mar 16, 2018

A chain is just as strong as its weakest link so to pair stainless steel bolts to a plain steel base will not make the attachment any stronger.

JimInPB · Mar 16, 2018

Most grades of stainless have poor resistance to repetitive stress fatigue. That might be one property to consider in a boat that sways back & forth constantly while sitting on a mooring.

capta · Mar 16, 2018

I can't think of any good reason to use stainless when the mounts and engine are all steel. Use at least grade 5 or 8 steel bolts, not galvanized, neverseize on the threads and of course, lock washers.

jibes138 · Mar 16, 2018

Stainless in the grades typically used on boats is not particularly strong. A standard bolt will give you a lot more strength. Yield strength of 304 stainless is only 31,200 PSI with 316 stainless only 42,100 psi. By comparison the yield strength of 6061 - T6 aluminum is 40,000 psi. A grade 5 bolt has a yield strength of 92,000 psi for up to a 1 inch diameter. Never seize will be good and keep it clean and spray with clear coat or paint it.

JimInPB · Mar 17, 2018

capta said:
I can't think of any good reason to use stainless when the mounts and engine are all steel. Use at least grade 5 or 8 steel bolts, not galvanized, neverseize on the threads and of course, lock washers.

I'm curious why you say not galvanized.

Jacktar · Mar 17, 2018

Guess so or they would be called unlock washers?

seadaddler · Mar 17, 2018

All my engine mount bolts are stainless and stainless washers as is from Hunter factory.
Nick

capta · Mar 17, 2018

JimInPB said:
I'm curious why you say not galvanized.

Are there any galvanized bolts on your diesel engine? J

seadaddler said:
All my engine mount bolts are stainless and stainless washers as is from Hunter factory.
Nick

Care to tell us what grade of SS?

JimInPB · Mar 17, 2018

Gene Neill said:
Okay, WOW. Yes, the strength difference is night and day. The fasteners I need are metric M10 x 1.5, 50mm long. In stainless steel, they show a tensile strength of 70,000 psi. In alloy steel, they have 170,000.

So, anti-sieze, huh? And here I was thinking threadlock, the exact opposite concept! I guess the idea is to just rely on the lock washers to do their job?

Metric bolts come in different grades too - https://www.boltdepot.com/fastener-information/materials-and-grades/bolt-grade-chart.aspx For comparison, 1MPa is roughly 150psi.

There are different coatings available for "alloy" bolts. Plain steel gives the least corrosion protection. Cadmium plating is a common surface treatment that gives a little protection. Galvanizing gives better protection. Galvanizing is not normally used on grades above SAE 5 or metric 8.8 for fear of hydrogen embrittlement in the alloys that the higher grade bolts use. I have used SAE grade 5 galvanized bolts with good results.

Your choice will depend on the mode of failure that you expect is most likely. Do you think that the bolt might not be strong enough? Do you think that it might suffer from fatigue & break after time? Do you think that it is likely to corrode & fail from the resulting reduced diameter?

The choice of locktite or anti-sieze is similar. Do you fear it coming apart or do you fear not being able to get it apart when the time comes? In my experience, anti-sieze or grease with a lock washer & proper torque is usually a good combination. In extreme cases, I might also drill & tap a small threaded hole next to a hex head so that I can put in a set screw along a flat to avoid the possibility of the bolt backing out. Some people bend up the side of a flat washer & then tack weld it in place for the same purpose.

JimInPB · Mar 17, 2018

If you decide to go with stainless, the grade is not the only thing to consider. In a marine environment, surface passivization can be equally important. Stainless bolts from a marine supplier should already be properly passiveated & as long as you don't nick them up, you should be able to use them as it. If you get SS hardware from a non-marine source, or if the surface gets marred, then you may need to treat them with "pickling paste" or acid to remove the free ferrous ions from the surface layers & prevent a type of creeping internal corrosion that can result. If the boat will not see salt water, then don't worry about this.

kloudie1 · Mar 17, 2018

Engine mount bolts are not highly stressed and have rubber to ease shock loads.. Strength wise, you can use pretty much whatever hard metal you want to (not brass, aluminum, lead, etc).. stainless would stay prettier in salt water environments. Corrosion is an issue to consider.. but unless you have a lot of salt water spraying about from pump leaks or packing, even that is a non issue.. Anti-seize is always a good idea be that TefGel for stainless or nickel based for steel. Looking at my old Yanmar engine mounts, they are carbon steel and after 28 years were easy to remove and replace.

Daveinet · Mar 17, 2018

Do your engine mounts receive that much stress? Probably not. If the bolts are tight, there should not be motion induced in the bolt, so it should not be subject to fatigue. There are places where I would not use stainless, but the engine mounts don't take that much load. Because the bolt is tight, any vibration is passed through, rather than applied to the bolt.

JamesG161 · Mar 17, 2018

Daveinet said:
Do your engine mounts receive that much stress?

If your 13 ton boat keel gets stuck in Mississippi mud, 50 HP in reverse, can put some stress on them.

______

JimInPB said:
I'm curious why you say not galvanized.

How often do you change you sacrificial Zinc on you shaft?

Galvanized stuff, even anchor chain, is for the show room. Just a slight time delay to corrosion.
The best would be Alloy 20 [Aquamet kinda] or Titanium bolts, if you can find them.

The nearest would be the same metal as your engine. Monitor them and maintain them.
Jim...

Daveinet · Mar 17, 2018

JamesG161 said:
If your 13 ton boat keel gets stuck in Mississippi mud, 50 HP in reverse, can put some stress on them.
..

I realize what you are trying to say, but even a 50 hp diesel is only about 81 ft-lbs of torque. Assuming the mount is a foot away from the crank, that means if there is only one bolt, that bolt would need to hold 81 lbs. If there is a transmission, that may double the torque to 160 lbs. We are still no where near the 35,000 lbs rating. Most likely there are at least 2 bolts and maybe up to 4, depending on how it is built. Not even remotely close to the bolt rating. Even a typical 5 times fudge factor is not even close.

JimInPB · Mar 17, 2018

JamesG161 said:
The best would be Alloy 20 [Aquamet kinda] or Titanium bolts, if you can find them.

Alloy 20 = 35ksi yield strength https://www.corrosionmaterials.com/documents/dataSheet/alloy20DataSheet.pdf It's corrosion resistance is rated for caustics & bases. I don't see a listing for salts. Also, I don't know where to buy bolts that are made out of that material. Have you actually used bolts that were made out of that material before? If so, where did you find them?

Titanium = very expensive & hard to find. There was actually a worldwide shortage when last I tried to purchase some bar stock.

JamesG161 · Mar 17, 2018

JimInPB said:
Alloy 20

It is the only Stress Corrosion resistant Stainless Steel for Salt water use.
Jim...

JimInPB · Mar 17, 2018

Further food for thought -

An M10 x 1.5 pitch bolt has a specified minimum minor diameter of 8.3mm https://www.engineersedge.com/hardware/metric-external-thread-sizes1.htm

KPSI is thousands of pounds per square inch of cross section.

8.3mm diameter = .327”.
π x R squared = area
.327 diameter = .163 Radius
3.14 x .163 x .163 = .0835 square inches of cross section for a 8.3mm diameter
If the material is rated 35 Kpsi, then
.0835 x 35,000 = just shy of 3,000 pounds of yield strength assuming zero concentration factor (which ain’t real world, but looks good on paper).