Need help from "bolt experts"

[Cadence]

I can't help it. Kryptonite bolts might work. Grade 8s and insure your alignment is good.

[blueazimuth]

Zstine is right: the breaking of 1 bolt, let alone 3, is the symptom, not the cause. Replacing the bolt with one of greater strength or diameter is a temporary fix, at best, not the solution.
The cause may be your custom built bracket. Custom does not necessarily mean it's engineered right nor built right. I'd start there. Find the cause, fix it, and you'll have happy bolts.
S

[Hudsonsailor]

Check Belt alignment before you break casting by over tightening.

[seabreez]

Originally Posted by Eigenvector

Without seeing the layout of the bracket or the fracture surface of the bolt, here’s what I can tell you……….assuming the axis of the bolts are parallel to the alternator shaft.

If the bolts are tight (roughly 80% of yield), the clamping friction during makup of the bolts carries the entire load from the bracket to the block. If a bolt gets loose, now you’ve possibly introduced one or both of the following: shock loading due to a shear clearance / cocking of the bracket which introduces a bending stress.

Vibration will loosen bolts. Short bolts and studs are more problematic than longer ones. If you think of the stretch of the bolt under torque as a spring, the longer the bolt the more stretch at a given torque. Loosening 0.001 inches axially could eliminate 100% of the preload in a short bolt but only 10% in a long one. In high vibration environments, spacers with longer bolts/studs a typically used.

For a proper bolted joint as assumed, the bolt would see a constant stress equal to the initial tightening stress. It will not fluctuate.

If all three broke at the same time you’ve got a displacement driven problem.

Check to make sure the surfaces are parallel. No non-parallel surfaces will cock the bolt during installation, introducing an addition bending stress.

I’d repair, replace, torque, run……..and then check the torque again. Locktite is also an option.

As for stronger grade bolts: The stronger the material, the greater it’s fatigue resistance. The S-N curves for fatigue life are based on percentage of yield of the material.

However, SERIOUS CAUTION here. I see it all the time where people are breaking bolts and replace with Grade 8 only to now start forcing the failure into the Very expensive components they’re bolting together. Best to always find out what the problem is, rather than throwing stronger bolts at it.

My 3 peso’s of advice………

Very well said... I will add is check the hole in your bracket and make sure they have not become oversize and check belt alignment.

[Cheechako]

No need to worry about corrosion on engine bolts, get better bolts. Some of the high carbon bolts on engines are already high strength and no one worries about corrosion.
Interestingly... I had vibration cracking problems on a little 2 cyl Kubota I had. Wonder what's going on there...

[buzzstar]

Just read this. Mostly good advice. Whatever the problem, it needs correction ASAP. My first thought after reading your OP was an alignment problem. Grade 8 bolts with proper torquing would help avoid or delay the failures, but not solve the problem. If it is vibration, why? Also you may want different motor mounts. But most of this needs to wait until, as you say, to get to a place with, er, more skilled mechanical services than wherever you are. I am definitely not an expert, but then I doubt it is just a bolt problem.

[Oceanride007]

Another solution that may help you now with lower quality bolt supply, is increase its resililience. This is quite simply getting a bolt say double the length and use decent packers (some use belleville washers), this will increase the fatigue life as the stress/strain has been effectively halved.

[Symphony]

I've seen engines sit on their mounts even though half the bolts were missing. If your engine is breaking these bolts I would consider Eigenvector's advice to find the problem before you transfer the breakage to things more difficult and expensive. Good Luck and let us know what solves the problem

[senzafine]

custom design......maybe bolt is undersized?

[leftbrainstuff]

You want a proper bolt for your mounts. Grade 5 or above. They are marked on the heads. Nothing from home depot or from a fence.

If you are breaking bolts you may have misaligned mounts or an over constrained mount layout. You may also be overloading your bolts. Some pics or sizes and quantities will help us diagnose.

One simple check is to loosen all mount bolts. None should be bound or hard to remove. You should also tighten them when they are all installed. You will often see slots to allow for tolerance stackup due to manufacturing variances.

Are the bolts breaking cleanly (shearing off) or do they have a cupped or pointed break? Are yhey coming loose? Some pics will help us diagnose.

Ia this a standard install or something cobbled together?

[leftbrainstuff]

Originally Posted by Oceanride007

Another solution that may help you now with lower quality bolt supply, is increase its resililience. This is quite simply getting a bolt say double the length and use decent packers (some use belleville washers), this will increase the fatigue life as the stress/strain has been effectively halved.

This will do nothing but add weight and fill your bilge with dropped washers.

For a tension or compression spring adding more coils will lower the spring rate. Properly torqued bolts do not exhibit this behaviour. Resilency is not a property of a bolt. Elasticity and surface hardness gives toughness which is what you want in a bolt.

Bad advice and technically wrong unless you sell fasteners. Then I would applaud your marketing prowess.

[Dan_K]

Without knowing the specific mode of failure, a solution to the fracturing is not available, but here are some bolt details for fasteners used in a fatigue application. Essentially all higher strength consumer fasteners are heat treated after thread rolling, so all the benefits of the thread rolling operation are lost. Aerospace fasteners designed for a fatigue application have the threads rolled after heat treatment (which leaves the thread root in a compression residual stress state, good for fatigue resistance). In addition to that, they have controlled root radius threads (a nice rounded shape to reduce stress concentrations in the root of the thread). One other feature of fatigue resistant bolts is the rolling of the shank to head fillet, putting the surface in residual compression helping improve the fatigue resistance. Using a consumer type Grade 8 bolt may help a little, but if the failure is fatigue due to cyclic stress, then a bolt designed for fatigue resistance is needed (assuming the suggestions discussed about are addressed).

[Midnight Son]

STOP THE PRESSES, Thunderbird had the right of it. All the blather about Grade 5 or 8 . In a Beta engine????? Hello Kubota 'metric'. 8.8 or whatever, but try looking at how far away from the engine the alternator (or whatever) is from the engine too far equals too great a distance so that engine vibrations can get the alternator resonating at a frequency and fatiguing the bolt. So two things to look at bolt spec., and possible better mount for the accessory.

Cheers all wish I was out there with you

[ArmyDaveNY]

In addition to all the good comments, LeftBrainStuff had a good point. Be sure to buy quality bolts from a reputable source. I know of a few people who bought cheap stuff from Communist China. The manufacturer just used cheap steel and copied the bolt heads. The buyers were excited by the price but didn't realize the bolts were not the quality the heads indicated. Correction, they didn't realize the bolts were substandard until they failed!