Major problem with broken intake grate bolts.Help please

[Hal]

This is the second time this year I have checked my intake and found three of the four bolts that hold my intake grate with the heads popped off. I know what would happen if the last one would break and that would mean the boat would make a very sudden turn most likely without me in it. The last time I thought they just worked loose and broke so I put lock nuts a safety wired them.
Apparently that was'nt the answer. I am using 5/16" SS bolts. Should I go to 9/16"? I did notice that the plate that bolts to the pump is not flat. It looks like it warped when it was welded. If I lay the plate on a flat surface I have about a 1/8" gap on each side. I guess that could be part of the problem but I realy don't know. I appreciate any help you can give me on this.
http://free.***boat.net/gallery/Reader_Rides/Jet_Boats/intake2.JPG

[flat broke]

The problem isn't the diameter so much as the material. Switch out to a good grade 8 black oxide coated fastener and see what happens. You'll need to get under there every so often and see how they're fairing against rust, but they should be more than strong enough. Stainless just doesn't posses the shear strength you need.
Good luck
Chris

[SilyDevil]

First i would get rid of that grate. They have much better ones now, and drill and tap the bolts threw and add nut the bolts on the inside. Thats how its done at the track.
wink

[MikeC]

I had this problem also. I would take the advice and thru bolt as many as you can, if you can. The other thing I did, was, is the bottom end that goes into the intake does not rest on the bottom of the suction piece, weld an ear or 2 so it hits/rest on it. This way the presure can't pull it down.
Later
MikeC

[Boater Bill]

Uh, not to be a smart ass but I think you answered your own question. 5/16" bolts should be fine, flat broke's suggestion is a good one but fyi I've been running stainless steel hdwe for last five years with no prob, set up the way SilyDevil described. Invest in a new loader grate.

[Johnwithjm]

flat broke:
The problem isn't the diameter so much as the material. Switch out to a good grade 8 black oxide coated fastener and see what happens. You'll need to get under there every so often and see how they're fairing against rust, but they should be more than strong enough. Stainless just doesn't posses the shear strength you need.
Good luck
ChrisFlatbroke has the right idea. I thru bolted mine on my race boat. I check mine after every run. Every so often will should be fine on a lake boat.

[Hal]

Flat Broke.. They did shear off thats for sure so I will get the harder bolts this time. Thanks..
Sily Devil.. The bolts were threaded all the way with loc nuts on top. The grate is a copy of the original that came with the pump and intake when the boat was build back in 77. The frist grate broke at the welds on the plate when I was doing a little speed run with my buddy in 78. I did'nt think we would ever stop skimming across the water after the boat made that quick turn. But we did and luckly we both came out of it with just a few bad bruises. I garaged the boat after that untill last year. The pump and intake were by Hallcraft and alpha marine. I guess the grate could be outdated sense it is 25 years old. It has run 104 with that grate though. How would a newer design help? More out of the hole, More top end? Will it help hold the boat down at high speed? What would be the benifits of a new grate?
Thanks for your help....Hal

[Hal]

Mike C.. Very good idea. I had'nt thought of that but I can see the benifit of bracing the back of the grate to keep the stress off the bolts. Thanks...
I guess I'll go see Greg about a new intake also.. Thanks for all the help everyone.. Hal

[LVjetboy]

Yep, Chris's right. Some SS poor quality, even better SS maybe not as strong as black oxide. Had my SS break, switched to oxide...same diameter. No more problems.

[superV]

[QUOTE]Originally posted by Hal:
[QB]Flat Broke.. They did shear off thats for sure so I will get the harder bolts this time. Thanks..
Harder bolts will have even less shear value than a softer bolt. Harder= britile Softer=More elastisity(sp?)
Just my .02

[Hal]

SuperV.. I'am not an engineer but I think SAE grade 8 bolt has about 2 1/2 times the shear strength of SAE grade 1 low carbon steel bolt. At least in the 5/16" size.

[superV]

Hal:
SuperV.. I'am not an engineer but I think SAE grade 8 bolt has about 2 1/2 times the shear strength of SAE grade 1 low carbon steel bolt. At least in the 5/16" size.I agree but in this case the bolt are in tension and shear and i was referring going from Stainless to carbon you can get a high shear but you will give up some yield and tensile.Grade 8 bolts might be 30 percent stronger, but the grade 5 probably absorb
30 percent more energy before failure. The engineer terms here are ductile
vs brittle. A material having less than 5 percent elongation at fracture is
brittle and one having more is said to be ductile.
Grade 8 would be used when loads are high but predictable and perhaps
cyclical. Grade 8 bolts can be tightened close to their yield point to
lessen fatigue effects. Piston rod bolts and airplane wings are good
examples of grade 8 applications.
Since grade 5 bolts absorb energy in the plastic range (stretching), they
provide an extra safety factor for large and unpredictable loads. Winches,
bumpers, and seat anchors are things that might be attached with grade 5
fasteners. Also, particularly in the case of roll bars, the direction and
type of force (twisting, bending, tension, etc) is impossible to predict.
Keep in mind that after an extreme load, these bolts would be discarded.
SAE J429 specifies the strength of grade 5 bolts at least 120,000 psi and
elongation of at least 14 percent. Grade 8 numbers are 150,000 psi and 12
percent. SAE J1701M gives metric strength 830 MPa for Class 8.8 and 1040
MPa for metric Class 10.9. I'll let someone else convert these, I'm too
tired. Metric elongation for these bolts are 12 and 9 percent respectively.
All that said, you can see that the higher grade fasteners are indeed closer
to brittle than the lower grades, but they all fall in the ductile range.By the way, in metric fasteners, the number in front of the decimal
represents one-hundredth of the minimum tensile strength in MPa and the
decimal part of the designation approximates the ratio of the minimum yield
stress to the minimum tensile stress, the flatness of the stress/strain
curve in the plastic region. Ref: Machinery's Handbook.