Bolts for Engine Mounts

[nigel1]

About to replace the original 22 year old engine mounts on my Bene 473 fitted with a Volvo Penta TMD22.


The bolts on the two aft mounts, the ones securing the mounts to the bearers are quite corroded, may need cutting off the heads and pushing the bolt down.
(Corrision occured prior to me owning the boat due to a leaky raw water pump)



Question really concerns the replacement bolts, what material should they be, stainless would prevent any future corrosion, but might be too brittle for this application, mild steel wont be brittle, but will corrode if given the chance.
Any suggestions?


By the way, not going to be easy to remove and fit these bolts. Beneteau's method of fixing the mounts is by use of nut and bolts, with access to the nuts via a small cut out on the inboard side of the bearers.

[ItDepends]

The engine mounts themselves are steel, so you are not going to gain much using stainless bolts--assuming you don't let the water pump">raw water pump leak.

In any event, I would be surprised if these are "mild steel" bolts. Look carefully at the heads and see what grade markings are on there, and duplicate those. I would guess they are Grade 5.

https://www.aftfasteners.com/grades-...trength-chart/

And, as I am sure you already know, 22 years is way past time for new mounts!

[tkeithlu]

Ditto on ItDepends. The rubber in the mounts is way past its expected life. They are not damping vibration as they should. The question of bolts will be solved by what comes installed in the mounts.

You do have the opportunity for chasing down the right mounts at a reasonable price. Volvo is going to charge a bunch for them, but Volvo didn't manufacture them. If you can find the source, or a non-yacht application, they will be cheaper.

[nigel1]

Originally Posted by tkeithlu

Ditto on ItDepends. The rubber in the mounts is way past its expected life. They are not damping vibration as they should. The question of bolts will be solved by what comes installed in the mounts.

You do have the opportunity for chasing down the right mounts at a reasonable price. Volvo is going to charge a bunch for them, but Volvo didn't manufacture them. If you can find the source, or a non-yacht application, they will be cheaper.

Got the replacement mounts from Parts 4 Engines (in the UK), the four mounts came to £100.
However, they do not include the bolts for securing to the bearers, only the studs and nuts for adjusting the engine height.

[boatpoker]

Originally Posted by nigel1

Got the replacement mounts from Parts 4 Engines (in the UK), the four mounts came to £100.
However, they do not include the bolts for securing to the bearers, only the studs and nuts for adjusting the engine height.

Think twice about stainless steel. Constant vibration and salt moisture (You know there's gonna be some) will lead to corrosion when that moisture turns stagnant depriving the ss of oxygen.

[bil56]

8.8 steel bolts/nuts/screws are good for this job ...

[stormalong]

Count the hash marks on the existing bolt head and add 2. That is the grade of bolts you have. ie. Two hashes is a grade 4 bolt.

For such an important bolt I would do grade 8 - the strongest you can get. They can't be too strong.

[tkeithlu]

They can't include the bolts for the bed, because they don't know what you are attaching the engine to - it could be wooden stringers. So, yes, go with good bolts. Then look up the torque value for that bolt and comply.

It was only fairly recently that I learned that bolts should be torqued independent of what you are bolting - you don't tighten it more because it's important or big. When you torque a bolt you are stretching it into its elastic range, and that's a characteristic of the bolt, not the boltee. That makes the bolt a spring holding things together. Over-torquing puts the bolt into its plastic range, and is a total no-no. I learned that after some human yo-yo/previous owner decided that since he was putting oversized tires on his (now my) Jeep, he should torque them to 200 ft-lbs, not the expected 100 ft-lbs.

[c.K.]

Originally Posted by nigel1

Got the replacement mounts from Parts 4 Engines (in the UK), the four mounts came to £100.
However, they do not include the bolts for securing to the bearers, only the studs and nuts for adjusting the engine height.

most bolts you buy at the marine stores are 308 not 316 stainless steel,yet very good quality and a lot cheaper then 316 Stainless st

[Searles]

As these bolts are in shear and probably 3/8 or 10 mill hot dipped galv is as far as I would go,replace every ten years when you replace your keel bolts ,I hope .⚓️⛵️

[FPNC]

Were talking about 8 bolts. Don't mess about. Use 12.9 alloy steel bolts. These are as strong as you can get and while they will rust, they are very resistant to corrosion. Use blue Locktite to help seal the threads from water and you will not have to worry about heads popping off. If you can't O'er the price, 10.9 will work too. Also, use extra thick washers.