motor mounting bolts sealant... the best to use

[MID FL CAP]

Looking to remount motor after rebuilding the til/trim unit, had to loosen bracket,now I would like to use the proper material to make sure the bolts are water tight. Thanks for your input!

[panasonic]

3m 4200....

[DennisG01]

There's really no such thing as "best" as there's multiple manufacturer's that make quality products. The 3M4200 is good stuff (DON'T use 5200)... It's not UV stable, but given the location, it's not really a big deal. BoatLife makes great products, too... either the LifeSeal or LifeCaulk (longer lasting, but takes longer to set). Bostik and Sikoflex are also excellent, although I can't quite remember the exact variant in those two companies. Personally, I've only been using the BoatLife products for the past 15 years as this is pretty much all they do. But that, in and of itself, doesn't make them 'the best', either.

[FabricGATOR]

Organic silicon and aluminum do not play well together. Something in the organic silicon sealant, be it acetic acid or ammonia will cause aluminum corrosion.
I am told that an inorganic silicon is fine. As DennisG01 said, Sikoflex, I was told that was inorganic silicon sealant.

[DennisG01]

Organic silicon and aluminum do not play well together. Something in the organic silicon sealant, be it acetic acid or ammonia will cause aluminum corrosion.
I am told that an inorganic silicon is fine. As DennisG01 said, Sikoflex, I was told that was inorganic silicon sealant.

Do you know if this is a 'for real' thing... as it really happens in real world situations? Or is more of a here-say thing or possibly just in a laboratory under very extreme conditions like much higher concentrations of 'acid'? The reason I ask is that I have never seen evidence of this happening (including the marina I work at part time) with either the BoatLife or 3M products. But then, I don't know their full composition, either. But then, I was never worried about it since it always worked fine

[pstephens46]

Organic silicon and aluminum do not play well together. Something in the organic silicon sealant, be it acetic acid or ammonia will cause aluminum corrosion.
I am told that an inorganic silicon is fine. As DennisG01 said, Sikoflex, I was told that was inorganic silicon sealant.

Silicon or silicone?

[99yam40]

Do you know if this is a 'for real' thing... as it really happens in real world situations? Or is more of a here-say thing or possibly just in a laboratory under very extreme conditions like much higher concentrations of 'acid'? The reason I ask is that I have never seen evidence of this happening (including the marina I work at part time) with either the BoatLife or 3M products. But then, I don't know their full composition, either. But then, I was never worried about it since it always worked fine

I do remember many years ago when people would use silicone sealant to caulk around there windshield in cars when they had a leak and cause those areas to rust out even faster, but that was not dealing with aluminum

[DennisG01]

I do remember many years ago when people would use silicone sealant to caulk around there windshield in cars when they had a leak and cause those areas to rust out even faster, but that was not dealing with aluminum

I wonder, also, if that was a totally different formulation of silicone used back then? Not just the organic/inorganic theory, but the chemical makeup. And then, passed down over the years, it becomes "fact", even though things have changed... sort of like an old wive's tale. Same as it's generally been thought, and often promoted on forums (probably from simply reading it on other forums), that you can't apply polyester gelcoat over epoxy resin - but that's been completely disproven for quite some time.

[Elwood22]

I used LifeCaulk on mine last year. So far, so good,

[FabricGATOR]

Do you know if this is a 'for real' thing... as it really happens in real world situations? Or is more of a here-say thing or possibly just in a laboratory under very extreme conditions like much higher concentrations of 'acid'? The reason I ask is that I have never seen evidence of this happening (including the marina I work at part time) with either the BoatLife or 3M products. But then, I don't know their full composition, either. But then, I was never worried about it since it always worked fine

I recently dismounted a pair of twins, Yamaha 2005 V6, that the installer or perhaps the owner had filled all the unused mounting holes on the bracket including the remainder of the oval slots with silicone sealant.
Upon removing the sealant I noted a extra amount of corrosion that was abnormal. I remembered that back in the day, silicone sealant was used in aircraft window installation until it was found that the acetic acid formed when it was curing and if used on aluminum, it accelerated corrosion.

Some silicone sealants have been reformulated but what ever was used to seal these engine mounting hole was a poor material selection imho.
When I asked a learned colleague before posting my notes, that is where I gleaned the organic / inorganic information.

[FabricGATOR]

Silicon or silicone?

Nice catch,

silicone

[DennisG01]

I recently dismounted a pair of twins, Yamaha 2005 V6, that the installer or perhaps the owner had filled all the unused mounting holes on the bracket including the remainder of the oval slots with silicone sealant.
Upon removing the sealant I noted a extra amount of corrosion that was abnormal. I remembered that back in the day, silicone sealant was used in aircraft window installation until it was found that the acetic acid formed when it was curing and if used on aluminum, it accelerated corrosion.

Some silicone sealants have been reformulated but what ever was used to seal these engine mounting hole was a poor material selection imho.
When I asked a learned colleague before posting my notes, that is where I gleaned the organic / inorganic information.

Ah, gotcha. Maybe whatever that installer used was that 100% pure silicone from Home Depot? I've only ever used the BoatLife stuff and 3M and have never seen any issues with those (including at the shop). But I suppose you would expect (hope) that those products would be fine since they're marketed to the marine world...