Replacing steel bolts for stainless

[Allye]

Afternoon all,

It’s clear a fair part of my tohatsu has been apart before as most of the factory paint is no longer present on the head bolts, water jacket etc.

Is it safe to swap this for stainless? I’ve rebuilt a fair few cars in my time and know to never use stainless in load bearing situations, I believe there is the train of thought that stainless in steel is a bad idea regardless.

There’s a tohatsu zapcat race engine on eBay at the moment and I notice most of the bolts have been changed.

[Nasher]

You should be fine, although I'm sure there are some who'll say otherwise.
I ran several 2-stroke bikes with Stainless 'Vanity' head bolts without an issue, and they had increased compression etc.

But make sure the bolts you buy have decent quality formed threads. They are rolled in rather than cut, and I've seen some cheap stainless bolts with badly formed threads.

The aim of making sure they are formed properly is two-fold.
First to ensure the soft Aluminium thread in the block isn't damaged by them.
Second to ensure you get an accurate torque reading when you torque them down.

On the second point I'd change them all so you don't get uneven torque from different bolts.

Nasher.

[Allye]

Sweet! I’ll start measuring up and get some ordered. Tart it up a bit.

[GuyC]

Do you run the risk of introducing galvanic corrosion if you swap from carbon steel to stainless? There's not really an electrolyte to make the circuit but sea air being what it is may cause some corrosion in the longer term.

The resistance to stretch in a stainless bolt is different to that in a carbon steel bolt. There is only 10% or so in it so I guess it is probably working within its inherent factor of safety. I'm in the camp of not swapping out head bolts but I'm probably over cautious when it comes to things like that.

[mikehhogg]

what makes you think that the bolts are not a grade of stainless.

[Allye]

Originally Posted by mikehhogg

what makes you think that the bolts are not a grade of stainless.

I can assure you they are not!

[mikehhogg]

I just did a head gasket on a tohatsu and noted that the studs opened easily and without ringing. That would explain things as stainless reacts with aluminium if its not isolated resulting in seized studs. Your opening a can of worms.

[phantom 19]

I replaced some water jacket bolts with stainless many years ago. Each one dribbled water as the manufacturers flanged headed bolts to actually provide a watertight seal.

[jeffstevens763@g]

is there not a reason why a manufacturer uses a certain bolt material i'm not up on the outboard bolt uses, but in my young days when i had an hillman imp [ali engine] all the bolts were steel with helicoil threads into the ali. i've just gone over my mercury 3.3 with a magnet and there's a mix one has a steel bolt with a stainless nut the drive shaft has a good magnetic pull so its stainless with a high steel content for strength i would think, most of the others have no magnetic pull i cant get to the head bolts without strip down. interesting topic though

[Allye]

I can only assume it’s a mix of cost and tensile requirements. No stainless bolt will have the tensile properties of a high grade steel bolt. But as we know steel will slowly corrode moreover with dissimilar metals. Compounded in many cases by heads and water jackets being removed several times.

Having said all that I’m in no doubt there are plenty of stainless bolts that have the torque requirements of outboards well within their limitations.

[Nasher]

It's not a Supercharged Methanol burning V8 doing 4 second quarters in a Top Fuel Rail

The Aluminium thread in the block will give up before an M10 Stainless bolt.

Nasher.

[Milgrip]

A combination of aluminum and stainless steel causes galvanic corrosion.
You shouldn’t use stainless steel and aluminium together. If you do, at least use Duralac or similar when using stainless bolts into aluminium.