chrishelen
N/A
Hi, what are the best bolts to use to attach my new shiny mounts to a steel bearer? I'm thinking high tension, but might they be a bit to brittle?
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Engine mount bolts
- Thread starter chrishelen
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Talulah
Well-Known Member
We just used M10 A4 stainless steel.
The bearer has a 10mm thick mild steel plate glassed in along its length.
The bearer has a 10mm thick mild steel plate glassed in along its length.
PetiteFleur
Well-Known Member
Use stainless bolts and nylock/locking nuts or you could use standard zinc plated gr 8.8 BUT they WILL RUST! I used 10mm stainless coach screws as I was screwing into wood.
Avocet
Well-Known Member
What are your engine mounts made of? If they're mild steel, I would definitely NOT use stainless bolts unless I could electrically isolate them from the mounts (e.g. with shouldered plastic washers). How heavy / powerful is your engine? If the bearers are like most boats (a flat "shelf" and the mounts sit on top of them), there will be very little load on them. Ordinary grade 8.8 bolts are pretty ductile. Certainly not "brittle".
I will be corrected but the blanket comment and myth that 'high tensile steel is brittle' is a load of rubbish. There are many applications where high tensile is used where its ability to spring back is a significant characteristic. High tensile chains, G80, G100 and G120 (made from 800 MPa, 1,000 MPa and 1200 MPa steels) have a greater ability to elongate than G30, G40 and G70 chain - in fact the min elongation set by American chain makers is 15% for for low strength and 20% for HT chains. Some HT chains are brittle - but they are made for specific application.
SSAB make some very high tensile steels, look at their Domex range, which have an exceptional ability to bend and are certainly not brittle. Armour plating steel is very high tensile - but by no stretch of the imagination would it be described as brittle.
Bisplate 80, of infamous background (nothing wrong with the steel nor company - just one user), will stretch about 20%-25% before failure and most of that elongation is before yield. Bisalloy describe this characteristic as 'toughness' but I'm not sure if this term is adopted generally.
I do not know if 'brittleness' or toughness is a measure (specification) of bolts - but it might merit looking.
If you engine mounts are mild steel, they need to be treated to minimise corrosion anyway, but you should not use stainless (as mentioned above) and galvanised or zinc electro plated even mild steel or Class 8 or Class 10 bolts look sensible. But the loads are negligible and I would prioritise having the mount, bolts and engine compatible and/or isolated from each other rather than worry about strength, or brittleness (on the assumption the bolts are of a sensible size). If you do use Class 8 or 10 bolts, whether zinc plated or not, they will be a different steel composition to the engine and the mount - so you would advantageously isolate the bolts from anything else they contact. Plastic washers, Duralac etc. If you use steel bolts they will rust (whatever they are coated with) and painting them new, over the gal or whatever, will extend their cosmetic appearance, a little.
Jonathan
SSAB make some very high tensile steels, look at their Domex range, which have an exceptional ability to bend and are certainly not brittle. Armour plating steel is very high tensile - but by no stretch of the imagination would it be described as brittle.
Bisplate 80, of infamous background (nothing wrong with the steel nor company - just one user), will stretch about 20%-25% before failure and most of that elongation is before yield. Bisalloy describe this characteristic as 'toughness' but I'm not sure if this term is adopted generally.
I do not know if 'brittleness' or toughness is a measure (specification) of bolts - but it might merit looking.
If you engine mounts are mild steel, they need to be treated to minimise corrosion anyway, but you should not use stainless (as mentioned above) and galvanised or zinc electro plated even mild steel or Class 8 or Class 10 bolts look sensible. But the loads are negligible and I would prioritise having the mount, bolts and engine compatible and/or isolated from each other rather than worry about strength, or brittleness (on the assumption the bolts are of a sensible size). If you do use Class 8 or 10 bolts, whether zinc plated or not, they will be a different steel composition to the engine and the mount - so you would advantageously isolate the bolts from anything else they contact. Plastic washers, Duralac etc. If you use steel bolts they will rust (whatever they are coated with) and painting them new, over the gal or whatever, will extend their cosmetic appearance, a little.
Jonathan
vyv_cox
Well-Known Member
There seem to be several people who expect their engine mountings to be submerged! There is absolutely no problem in having stainless bolts in carbon steel if they remain dry, or even just a bit wet.
My engine mountings sit on a carbon steel plate that is bolted to the grp with carbon steel bolts. I have never checked them but I doubt that they are anything special. 8.8 would be a good choice - the meaning of the designation is that the strength is 800 MPa (a good medium tensile steel) with the proportion of YS to UTS of 80%, which indicates that the ductility is reasonable. As the strength increases the ductility reduces, hence 10.9, 12.9 etc.
Stainless steel bolts come in 5, 7 and 8 strengths but I cannot quote ductilities for them from memory.
My engine mountings sit on a carbon steel plate that is bolted to the grp with carbon steel bolts. I have never checked them but I doubt that they are anything special. 8.8 would be a good choice - the meaning of the designation is that the strength is 800 MPa (a good medium tensile steel) with the proportion of YS to UTS of 80%, which indicates that the ductility is reasonable. As the strength increases the ductility reduces, hence 10.9, 12.9 etc.
Stainless steel bolts come in 5, 7 and 8 strengths but I cannot quote ductilities for them from memory.
jakeroyd
Well-Known Member
I would echo the comments above.
they are no engine mtg bolt applications on yachts that require anything more than a grade 8.8 or grade 8 bolt
they are no engine mtg bolt applications on yachts that require anything more than a grade 8.8 or grade 8 bolt
Lon nan Gruagach
Well-Known Member
The biggest that will fit...
I would be slightly wary of Avocet's comment about little load on them.. Bolts are to be used ONLY in tension, never ever shear. To prevent shear the bolt must be tighend enough that friction between the 2 components being bolted together is sufficient to prevent slippage. So, even when the engine is mounted nice and flat you must ensure that it will not dance about.
I would be slightly wary of Avocet's comment about little load on them.. Bolts are to be used ONLY in tension, never ever shear. To prevent shear the bolt must be tighend enough that friction between the 2 components being bolted together is sufficient to prevent slippage. So, even when the engine is mounted nice and flat you must ensure that it will not dance about.
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chrishelen
N/A
8.8 it is then....might have a job getting the old ones off, they are nicely rusted on.
Tranona
Well-Known Member
The load on engine mounts is taken by the mounts themselves - that is what they are for. Very little load on the bolts that hold them to the beds. They are only there to stop the the mounts from moving and this is achieved by the clamping pressure of the bolts, particularly if locknuts are used, onto the beds. As suggested rust is a bigger problem than galvanic action and I would use stainless rather than mild steel. The corrosion problem is usually the threads which are potentially in a damper environment that the heads, making them difficult to undo when the time comes for replacement.
rogerthebodger
Well-Known Member
The thrust created by the prop into the prop shaft is often transmitted to the hull through the engine mounting bolts which will be ,in this case, in shear.
I am in the process of changing mine to stainless steel due to the leaks in the raw water pump and exhaust injection elbow (both now fixed) dripping onto the engine mounting and causing the mild steel bolts to rust away.
I am in the process of changing mine to stainless steel due to the leaks in the raw water pump and exhaust injection elbow (both now fixed) dripping onto the engine mounting and causing the mild steel bolts to rust away.
Lon nan Gruagach
Well-Known Member
Indeed the prop thrust is often transmitted to the hull through the engine mounts, but the engine mounting bolts (sticking up from the mount) experience compression due to engine weight and bending, not shear, due to thrust and engine vibration. The bolts used to secure the mounts to the hull should only be in tension, with friction between the mount and the hull preventing shear.
rogerthebodger
Well-Known Member
Only if the torque applied to the bolt is high enough to create sufficient friction.
When I was designing structures we would always consider the bolt in shear as the factor to size the bolt/bolts in any structural joint as there is no way the designer could know the tension of the bolt on assembly and during the life of the structure.
In the case of an engine mounting where there is considerable vibration not knowing what kind of nut is fitted could loosen if a non self locking is used.
The engine mounting bolts (sticking up from the mount) experience compression due to engine weight, bending and shear but the bending moment is the highest stressing force so in sizing of the bolt I would mainly consider the bending stresses and a bolt in that application sized for those bending forces would be much larger than needed to handle the shear of compression loads IMHO.
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chrishelen
N/A
All done, I used m8 galvanised, the same that came out. I was pleasantly surprised how easy the old bolts came out considering how rusty they seemed. And I was also surprised that the packing under the forward mounts was plastic, the sort they use for chopping boards I think.I assumed it was steel, and I replaced them with steel one I had already made up.
Avocet
Well-Known Member
I think it's pretty safe to assume the OP intended to tighten the bolts when fitting them!! You don't need very much torque in an application like that to generate PLENTY of friction. Worth having a look at how the wheels are held on to your car, in fact!
Tranona
Well-Known Member
Somebody cleverer than me could probably do the sums, but pretty sure that 8*M8 bolts in shear would more than cope with the maximum thrust that a 30hp motor can generate.
Lon nan Gruagach
Well-Known Member
Ok, in simple terms, if ****s up the thread and the hole the bolt is in
Avocet
Well-Known Member
They would cope with consummate ease (assuming of course, they had nuts on and were tightened more than just finger tight)! Consider that your car wheels are held by bolts (or studs) in shear and so, therefore, are the brake discs. They'll happily stop a ton-and-a-half of car, braking at 1G all day and the bolts will barely touch the sides of the holes. In fact, you could machine all the bolt holes out of the brake discs and the clamping force and friction alone would be plenty.
Lon nan Gruagach
Well-Known Member
Car: Ford focus
weight: 1,300 kg
Wheel nuts:108 * 5 * 12mm
wheel nut torque: 95 ft lbs
bolt clamping force = 41kN
per bolt break torque = 900Nm
all wheel brake torque = 18,000Nm
if that can be applied to the road, braking force = 75000N = 7.5 tonnes
seems like Ford design wheel studs to be tension only
Avocet
Well-Known Member
So... provided the OP tightens his M8s, you reckon there will be enough friction to stop his engine getting shoved into the middle of the saloon by the mighty thrust of a 30 horse motor then?
...I certainly do!
...I certainly do!
