Replacing cap bolts- bolt strength?

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I posted a thread about a tractor transmission that failed because all the cap bolts broke.

The original bolts are around 5/16ish, medium thread (didn't get to the store to measure them). They don't have any strength markings on them. They may have failed because things loosened up. It is cast aluminum case. I'm using locktite when I reassemble.

I want to replace with grade 8 (8.8) but I worry that is too much for the case. Any opinions on this?
There is no service manual for this product (it is throwaway). I'm wondering what the torque would be on these bolts. I know for a car diff it is very high, and I know some diffs (like the mark 8) are made from aluminum. From the poking around I see 15-20ft/lbs for similar tractor units.

http://www.bobistheoilguy.com/forums/ubb...24c#Post4106576
 
Watch out, you're terminology is contradictory. Bolts with head markings reading "8.8" are metric bolts that have the equivalent strength of SAE grade 5. Are you working with metric or English bolts?

"10.9" is the metric bolt strength class equivalent to SAE grade 8.
 
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Grade 5 should easily suffice. Grade 8 will work, but its overkill in an aluminum case like that. The case will fail before the bolts will, probably even at grade 5 bolts. Its not a question of "being too much for the case" unless you overtorque them.

The torque spec won't be nearly as high as a car differential, but without looking it up I'd assume your 20 ft-lb guess is pretty good. I'd probably throw some loctite on there for good measure.

Two different oils? Is this a hydro unit? Most Peerless tractor transaxles I know of are packed full of Bentonite grease.
 
There are bolts and then there are bolts. The grade 8 SAE bolts will have five slash marks on the head. Bolt torque values can be found with a Google search.Keep in mind that there is a lot of fastener junk around so do some fastener due diligence. I would think grade five(three slash marks) would be adequate in that application.
 
I believe it is an M8 bolt but I didn't measure it yet. It is 7.76 mm Diameter. It has no markings whatsoever, so that implies it is made to assemble a table.

Thanks for clarifying, I did misuse the strength spec.
 
Originally Posted By: 440Magnum
Grade 5 should easily suffice. Grade 8 will work, but its overkill in an aluminum case like that. The case will fail before the bolts will, probably even at grade 5 bolts. Its not a question of "being too much for the case" unless you overtorque them.

The torque spec won't be nearly as high as a car differential, but without looking it up I'd assume your 20 ft-lb guess is pretty good. I'd probably throw some loctite on there for good measure.

Two different oils? Is this a hydro unit? Most Peerless tractor transaxles I know of are packed full of Bentonite grease.

Yes, 80w90 + some expensive peerless mystery oil in a quart bottle. A smelly gear oil came out, but they likely mixed when I split the case.

In another thread xW-50 was suggested for the pump.
 
Originally Posted By: ford46guy
I believe it is an M8 bolt but I didn't measure it yet. It is 7.76 mm Diameter. It has no markings whatsoever, so that implies it is made to assemble a table.

Thanks for clarifying, I did misuse the strength spec.


Heck, since Peerless is American.. it can't be M8.
7.76 mm should then be 5/16 inch. Pitch isn't the same.

Edit:
http://www.americanfastener.com/astm-sae-and-iso-grade-markings-for-steel-fasteners/
 
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