Outboard mounting bolts

[mattwarner]

Just wondering am I ok using regular A4 (316) stainless M12 bolts to mount my 90hp to the transom or is there anything special needed?

[Gynner]

Not sure about bolt size but use 3M 4200 to seal holes.

[jonp]

Originally Posted by mattwarner

Just wondering am I ok using regular A4 (316) stainless M12 bolts to mount my 90hp to the transom or is there anything special needed?

I've been advised not to as the proper engine bolts are far stronger than normal stainless which is why they are so expensive.

[breezeblock]

Originally Posted by jonp

I've been advised not to as the proper engine bolts are far stronger than normal stainless which is why they are so expensive.

they are usually a finer thread than bog standard stainless bolts as well I would go for the proper bolts I think you could really do without a set of bolts shearing off and the motor falling into the drink

[beamishken]

I've Seen plenty with standard 316 bolts been on for years with no problems so it wouldnt worry me to use standard bolts with nylocs. A 90hp isnt exactly huge in comparison to some engines hung on the same brackets
The fine thread bolts that come with some engines seem to have nuts made from a fairly soft alloy so i doubt its to do with strength
Id happily use standard m12 316 bolts with your engine

[breezeblock]

the clamping force that a fine thread gives is higher than a metric coarse bolt will give for the same applied torque insufficient torque means less clamping force which could result in things like fretting on the mounting faces 316 is a fairly poop stainless more likely to get galling as well but hey ho its your engine not mines I dare say there are loads of folk that know there onions when it comes to bolts and threads 90hp may not sound like a large engine but I bet the manufacturer will recommend decent hardware ive never seen in an instruction manual anything that reads fit any old crap that you've got lying around the shop
ps use decent washers as well to get a wide clamping area as well

[beamishken]

Originally Posted by breezeblock

the clamping force that a fine thread gives is higher than a metric coarse bolt will give for the same applied torque insufficient torque means less clamping force which could result in things like fretting on the mounting faces 316 is a fairly poop stainless more likely to get galling as well but hey ho its your engine not mines I dare say there are loads of folk that know there onions when it comes to bolts and threads 90hp may not sound like a large engine but I bet the manufacturer will recommend decent hardware ive never seen in an instruction manual anything that reads fit any old crap that you've got lying around the shop
ps use decent washers as well to get a wide clamping area as well

Thats all very true but the fine thread alloy nuts are like chocolate so i doubt the clamping force is the issue.
Its the same size and no of bolts in a 90 as much larger engines
You'd break the engine before the bolts
If your realy bothered use metric fine there easily available
Op doesnt state if hes planning on using 1.75 or 1.5 or 1.25

[mattwarner]

Thanks guys,

To be honest the outboard is a fairly old Tohatsu 90 and not like its a brand new 4 stroke yam, of course it would still ruin my day if it falls off the transom!

Surely the breaking strength on an M12 bolt has gotta be several tons and with four snugged up tight with large washers I cant see it being under massive strain?

My usual experience is that anything with "marine" or an outboard manufacturer on the package means a licence to inflate the price massively

[beamishken]

Originally Posted by mattwarner

Thanks guys,

To be honest the outboard is a fairly old Tohatsu 90 and not like its a brand new 4 stroke yam, of course it would still ruin my day if it falls off the transom!

Surely the breaking strength on an M12 bolt has gotta be several tons and with four snugged up tight with large washers I cant see it being under massive strain?

My usual experience is that anything with "marine" or an outboard manufacturer on the package means a licence to inflate the price massively

Exactly!! the inflated price has nowt to do with the material its made from, its the "marine" premium that it acquires when it goes in a bag marked Yamaha,mercury,honda,suzuki

[Lee1]

I have used standard A4 bolts, but for a 90hp I would probably use the pro bolts, if you do use the A4 bolts use a standard nut A4 nut with a A4 locking nut locked to that nut.
Have a search for A4 bolts/nuts with a closer thread than the standard nuts and bolts, they will be on eBay and half the price of ones listed as 'marine' !!

[Phil M]

Also, if you use A4 bolts, put some copper grease on the threads before you tighten them. 316 steel work hardens very quickly and can destroy the threads before you have even got that tight and will require cutting off. Trust me, I recently wrote off a load of u-bolts as I didn't know this before I fitted them.

Phil M

[Fenlander]

I notice the Suzuki supplied transom bolts that came with my lowly 20hp new in 2016 are a finer thread than standard (as mentioned above) and seemingly corrosion treated/plated rather than stainless steel. Phil makes a good point re tightening stainless bolts where the nuts seize on.

I have no experience of copper grease on stainless but know there are specific anti-seize products for stainless. I usually just put on a dab of waterproof grease as it's what I have to hand.

[jeffstevens763@g]

Bear in mind stainless stretches when over tightened distorting the thread.