If the other bolts are still holding the head on how did the broken bolts experience tensile failure? Did the head deflect just a bit in that area to allow tension? Aluminum threads were fine and the broken studs came out without a problem.
The fractured bolt failed due to fatigue. Fatigue failures that have this specific type of appearance (crack initiation in the thread root, flat propagation area, final fracture due to tensile overload) are usually due to preload variation, fastener misalignment, or a manufacturing defect (thread lap in the root, etc.).
Over or under tightening, torque figures are not good for critical applications, if you used torque to set the bolt preload they can be subject to 305 error
My best guess is uneven torque on the bolts. These are head bolts, and should have a prescribed tightening sequence, probably torqued in steps, too (like 30%, 60%, then 100% of proper torque). If the head was replaced without the proper procedure, then the bolts would not be loaded evenly, when you add the cycling of the high pressure pump, the highest torqued bolt will receive the majority of the alternating load. Once that one goes, then next one, and on and on until it starts leaking so bad the operator has to turn it off.
Don't jump to conclusions. The bolt in the picture has clearly experienced fatigue failure. Fatigue requires cyclic loading, which usually indicates insufficient preload in a fastener. I think redpicker has described a very likely scenario, and Tmoose asks some pertinent questions. Always compare fatigue fracture appearance with the matrix of sketches given in the ASM Handbook, which helps categorize the stress conditions.
The bolt also appears corroded, and this possible factor should be investigated. Cyclic loads + corrosive environment turns fatigue into corrosion fatigue, which requires lower initiation and propagation stresses.
I have no doubt the entire bolting SYSTEM (all components and the torquing procedure) needs a thorough review.
The bolt is not corroded, typical black oxide socket cap screw. This isnt a car so I doubt its a multiple torque sequence. Its a chinese Briggs and Stratton pump. The socket cap screws are in a tight location, hence the style used and....why there was no torque spec. I believe they were hand tightened at the factory with an allen wrench. Interesting that all 3 bolts broke off flush at the joining surfaces. These are 1/4" bolts so I was thinking of torquing them down to 4 lbs/ft and possibly using a loctite compound like sleeve retainer.
What is the piston diameter and working pressure? The sum of the bolt clamping force needs to be more than that to avid fatigue. Need thread engagment of a couple of diameters to allow developing full strength of bolt. And to prevent embedment the pump head material needs to have a yield strength sufficient to withstand the contact pressure of the socket head cap screw listed in the real shcs books, and not be undermined by oversized hole or excessively large chamfer
I'm sorry, but there are corrosion products on the bolt.
Whether or not it is a car has nothing to do with a multiple torque sequence. Any well-deigned manifold will have a prescribed torque sequence. Whether or not there are multiple steps is debatable, but even installing all the bolts "hand-tight" then applying the prescribed torque is a multiple torque sequence.
Whether or not the product was produced in China has absolutely nothing to do with, well, anything. If it bears the Briggs & Stratton name, then B & S engineering signed off on it as meeting their design requirements. I do have to say that I don't think I've every seen a 1/4-20 bolt on anything coming from China, but the last B&S pressure washer I worked on was built in the USA.
There is a 100% certainty that there are torque specs for those bolts. Every Briggs & Stratton service manual I've read had torque specs for every fastener on the product.
If the bolts are difficult to get to, then there is actually a much higher chance that the manufacturer torqued them properly. The manufacturer can afford special tools designed to specifically tighten those bolts properly, and it just makes good business sense to use the proper tools.
All three broke at the mating surfaces because that is where the bending stress in concentrated. Not really that surprising. The ratchet marks and beach marks on the fracture surface are clear evidence that bending fatigue was the mechanism. And, brimstoner is probably correct that corrosion-fatigue (corrosion-assisted fatigue, if you prefer) was an active mechanism.
If you've already decided on your remedial actions, this is probably a waste of time, but I think you'd be much better off getting a service manual from the manufacturer and following their procedures.
I would guess if aluminum tapped threads, the torque would be less than those values shown for steel bolts and steel threads.
Briggs and Stratton probably had specified the torque value
in some manual. I would guess the bolts to be grade 2 equivalent
if used with aluminum parts.
Before I did anything else I would have the bolts sent to a lab for analysis.
Further when a bolted joint is made the proportion of the external load seen by the bolt over and above its pre-load is determined by the stiffness of the joint material and the bolt itself.
Imagine a joint with a single bolt carrying 100N external load but the bolt is only preloaded to say 80N, the joint material then separates leaving the bolt to withstand the full external load, a bolt that is tightened above 100N pre-load will not separate and will only carry a portion of the external load dependant on the joint stiffness, therefore the bolt tightened to the highest torque does not see the highest portion of the external load which as been suggested in an earlier post.
I called B&S and they told me that there are no techs there to provide information. Another sign that all of their stuff is chinese made. Ted , where did you come up with 10-11 lb-ft for a 1/4-20. I believe Dinjin when he said most likely a #2 grade. I looked up some socket cap screws on mcmaster carr and fastenal and thats a pretty common fastener. I could not even find a grade 8 in that style of bolt. The torque charts I found online list a grade 2 at around 4 lb-ft and a grade 8 (hex head) at 8 lb-ft. Does it matter on the internal thread material?
At 4 lbs on a 1/4-20 the claming force looks to be 960 lbs x 5 = 4800. pressure at head is 2600. 3 pistons, swash plate design.
This fracture started at the root of the thread and propagated from a sharp notch site. Hence the beach marks, correctly pointed out by DesertFox and HydTool. The beach marks are made as the notch temporarily stops prior to starting again from the loading application.
The tear on the far side is that due to tensile loading, probably the pre-load in the bolt. It is possible the bolts have an issue with them, you would need to see the manifesto for location, maybe plant number or machine lot.
I've seen bolting fail like this as the result of improper tooling when cutting the threads. May or may not be the issue here. Could also be chemistry for example, sulfide stringers embedded in the carbon steel alloy base or even some grain issues incurred during the manufacturing process.
But I agree with DesertFox and HydTool on their observations as well.
