Bolt strength & capality
[QUOTE=Bushwacker;
Regarding bolt strength, 6 bolts is rather serious overkill! Although I would defer to the opinion of Capt. Terry, who was much more of a "hard metal" mechanical engineer than me (I specialized in the "hot air" heat transfer/thermodynamics end of the business!), my take on it is that 4 bolts are more than adequate.
The main thing is to just make sure the bolts are torqued evenly and correctly with an accurate torque wrench. QUOTE]
Happy New Year-
Bolts are strongest in pure tension, next in shear and weakest in bending. Properly designed bolted joints avoid bending. If only one bolt at the top of the transom was supporting the outboard, it would be in bending due to engine weight and the prying action of the torque on the engine bracket due to the thrust at the prop. However, with a “good foot print”: between the engine mount and the 4 properly preloaded bolts to the transom, this can be assumed primarily as tension and shear load, but minimal bending in the bolts
In an ideal bolted joint the bolts are torqued to elastically stretch the bolt to nearly the yield limit of the bolt material (exceeding the elastic limit would permanently stretch it). The stack under the bolt is typically aluminum, steel, titanium, etc. which have much higher modulus of elasticity and therefore stiffer stack than one consisting of the engine and fiberglass transom. The objective is the initial or preloaded bolt stress should be the highest stress the bolt incurs. If a bolt were lightly torqued, operating stresses could exceed preload stress resulting in premature failure due to fatigue of the bolt material. Not so if the bolt preload stress exceeds the operating stress. And strange as it may seem the operating stress is not additive to preload stress and is not detrimental unless operating stress exceeds the initial preload stress. Bolt preload is dependent on the applied torque and the thread coefficient of friction. For instance dirty bolt threads will absorb some of the initial torque resulting in lower bolt preload than desired. Hence the reason for cleaning & lubricating head gasket bolts with oil before torqueing.
About a year ago I raised my ETEC. I found a SST torque table online which listed 45 ft lbs for a .500” pitch diameter stainless bolt (assuming some coefficient of friction for the adhesive I was using). I measured 7 – 8 ft lbs to overcome the run-on torque of the nylock self locking nut. Therefore I torqued the bolts to 50-55 ft.lbs. Later I think I saw the ETEC owner’s site recommended 40 ft. lbs. I noticed afterward a bit of compression/deflection in the transom (a bit soft or compression from the initial engine installation?). Thereafter, I added a 2’ long aluminium right angle extruded bar between the lower engine bolts to spread out the load on the transom.
A marine 304 SST .500” diameter bolt, torqued to 90% of the 31,200 psi room temperature yield strength, would have a tensile preload of about 6000#. Any shear or bending would detract from this capability, but 4 bolts should be enough to support the Suzuki. However, if Suzuki recommends six, why depart from that and provide them with anything to void a warranty claim? Perhaps they know of some flexing and cracking of the engine where it attaches to the transom, similar to what I had on a Chevy Astro where the bracket on the front of the block which supported all the accessories cracked due to vibration from an improperly torqued bolt.
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