what type of bolts

Rogue Wave

Senior member
Mar 12, 2004
Cruising area
Boat name
Dyna Mo Humm
Boat make
Avon Searider 8.4mmmm
Cat 4216 450hp
I have just returned from a very confusing visit to the Bolt shop.

I wanted to buy some High tensile bolts to connect Billys drive plate to the propshaft, cos he's nearly fkkn ready for a little swim.

The man in the shop told me I needed 316 stainless for that job and I said no i don't I want high tensile bolts cos Stainless is brittle after a while.

Is stainless steel also high tensile
Can only help with US specs.. but here's my two pence worth..

When we want high tensile bolting for moderate temperature use we specify B7 grade which is CrMo and has a ultimate tensile of 860Mpa
B8M (316) can be "upped" to 760Mpa (UT) if you specify class2...

Kinda means that you can buy "higher strength" S/steel bolting if you can find a supplier....
Just as a matter of interest, B8 (304) is actually "stronger" than B8M(316).....Oh? and if they're strain hardened studs will invariably be "stronger" than bolts...

Now you should be really confused....;p; .. Just ask the FGT for his advice..... he is the font of all knowledge.......:dizzy:
rowg waiv said:
I have just returned from a very confusing visit to the Bolt shop.

At 1.43 am! so the shops either in Scotland, or its a 24 hour bolt shop!...Cool! if the later, I need their number.

My £0.02 worth, which is probably only worth £0.01 is, You've made the correct choice with High tensile, and use a decent (Perect seal) thread sealer/anti corrosion agent.

s/s is nice and indeed nessasary when imersed, but this application is in my opinion, is definately high tensile territory.

PS. If we're talking bolts AND nuts, use fine thread, ie, UNF, and nylock nuts.
..and another thought.. is the fastening going into aluminium alloy?........:down:
Quicksilver Perfect seal'! superb jollop
Johnson said:
Quicksilver Perfect seal'! superb jollop

How does that work then? Does it completely insulate the mating surfaces (thread and clamping face) ? What sort of effect does it have on torque setting?.... and is it cheap?
Thanks guys

Guess what Billy's been doing today?

24 hour boatshop wpould be good, but I guess Screwfix comes in as close as could be to that.
"Perfect seal" is a very sticky, viscous jointing compound that I really believe in. It works extremely well for mating surfaces and bolt threads alike, in time, it dries to a wax like texture and seems to 'stay put' like no other stuff. A good example is the head bolts of a big block chevy, the threaded holes into which the head bolts locate, are not blind, instead they are open at the bottom to the engine waterways, as most engines of this type are raw water cooled, that is, sea water is passed around the engine block cooling system, corrosion is a major issue, especialy when salt water creeps up the threads of the head bolts doing all manner of damage.
Using perfect seal on the head bolt threads seems to totaly stop any water making it up the threads, often, when removing the bolts, you can clearly see where the end of the bolt (last few threads) where sticking into the water way, the thread has been comletely eaten away, leaving little more than a stump, but the bolt, when removed, still unwinds with a nice guey feel, and the portion where the PS has been trapped within the threads is 'as new'.

It's marketed by Mercury marine as "Quicksilver Perfect seal" and is quite pricey, but the exact same stuff, in an identical tin, with the same lid complete with aplication brush, marketed by OMC is a quarter of the price.

I use it pretty much everywhere on the boat to lube/seal threads.

I guess in theory, you would need less torque loading using such a viscous thread lube, but I've never made any such changes.
I'll have some of that Anyone know of an OMC dealer in sarfampton
I'll have a look see at the product name as "OMC"

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