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Old 08 August 2005, 16:30   #11
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Forgot - Lumiweld is also supposed top be pretty good but never used it myself.

http://www.frost.co.uk/item_Detail.asp?productID=8823

Frost are a great site - all sorts of great stuff for car buffs but much of it usefull for RIBS as well...........
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Old 08 August 2005, 16:31   #12
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For a reasonably non structural component - agreed
For something with a large surface area of contact - agree

But, I think you completely underestimate the loads that are transmitted through the gearbox. That's 200HP! Plus all the shock loads it takes when romping along in the rough stuff.
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Old 08 August 2005, 18:35   #13
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Going to think about this one overnight.

Obviously if I had the choice I'd strip it out and get it welded, however as mentioned the previous repair I found has welded the bearing retaining ring into the casing, so it would have to be welded whilst still assembled, and run the risk of wrecking the bearings. Or a lot of metal would need removing to get the ring out.

I'll let you all know how I get on.

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Old 08 August 2005, 18:45   #14
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Quote:
Originally Posted by MadMat
For a reasonably non structural component - agreed
For something with a large surface area of contact - agree

But, I think you completely underestimate the loads that are transmitted through the gearbox. That's 200HP! Plus all the shock loads it takes when romping along in the rough stuff.
It all depends on where the crack is - without seeing a photo or diagram it is a bit hard to comment.

As to the loads transmitted through the gearbox i would have thought they actually go through the gears and shafts - obviously some boxes are more stressed than others - F1 cars for example use the gearbox as the mounting for the suspension!!!
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Old 08 August 2005, 22:28   #15
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Don't let them grind the weld off the outside, just leave it, its stronger.
Wrong. If it's welded PROPERLY its stronger with the proud weld ground off and polished smooth. Any irregularities (such as a raised weld) have the potential to start a crack-this is why conrods on racing motors are polished. After welding cast aluminium (or any metal) the grain structure of the casting is affected and has the potential to start a crack forming due to the differing grain properties at the boundary of the weld material and the original material.

BTW-I should have read the original post properly-if the bearing retaining ring has been welded up by accident then it's going to be close enough to a stressed area that it WILL need welding-epoxy isn't going to be strong enough and the crack is likely to spread. Besides, if you grind it out to epoxy it then you'll make more work or possibly make it impossible to weld. IF the guy welding it misses one tiny particle of epoxy then it'll blow holes in the weld.
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Old 09 August 2005, 04:15   #16
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Wrong. If it's welded PROPERLY its stronger with the proud weld ground off and polished smooth.
Interesting ... the people that welded my box with the same defect, said in their experience the crack is better left without being ground down. From what I remember they suggested that the thinkness of the metal, brittleness of the metal, and the heat/vibration caused by the grinding process all where factors in them recommending I did not request it to be grinded. Hence I have my weld bead visible. Maybe they just could not be arsed and fobbed me off, I have no idea.

Its made no difference to the performance of the engine. It does not look bad its looks like a good job.
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Old 09 August 2005, 04:37   #17
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Just take a step back and think about it again.

What holds all these very strong shafts, gears and bearings in place, exactly?

Quote:
Originally Posted by codprawn
As to the loads transmitted through the gearbox i would have thought they actually go through the gears and shafts
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Old 09 August 2005, 04:41   #18
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Reaching the limit of my knowledge here, but I think it's to do firstly with the hardness/structure of the metal in and around the weld that's been affected by the heat, secondly by the stress riser in the shape of weld itself, and thirdly from stresses induced in the welding process itself. You often get a welded up component cracking again along the edge of the weld.

Quote:
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Interesting ... the people that welded my box with the same defect, said in their experience the crack is better left without being ground down. From what I remember they suggested that the thinkness of the metal, brittleness of the metal, and the heat/vibration caused by the grinding process all where factors in them recommending I did not request it to be grinded. Hence I have my weld bead visible. Maybe they just could not be arsed and fobbed me off, I have no idea.

Its made no difference to the performance of the engine. It does not look bad its looks like a good job.
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Old 09 August 2005, 04:59   #19
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Iíve had another good look at it this morning before I left for work and decided Iíll try to get it welded. Mainly because itís not very thick, so even if I grind it out to 45 deg the Epoxy will not have a big area to bond to.

I agree by the way that removing the weld bead is the stronger way to go.
The joint between the old surface and the edge of the bead is a stress former. If left the bead, being thicker, is stiffer than the surrounding thinner metal concentrating any minute movements to the joint between the two. Over time the joint may fracture, particularly as the heat from the weld may have upset the metallurgy of the casting in the surrounding areas. Although this can be reduced by cooling slowly.

Iím not having much luck locating a used casting, so the old one is looking more and more precious.

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Old 09 August 2005, 07:10   #20
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Quote:
Originally Posted by MadMat
Just take a step back and think about it again.

What holds all these very strong shafts, gears and bearings in place, exactly?
Obviously the casing - as I said it all depends on the design. Remember - unless a bearing is siezed there shouldn't be much force on it unless it is a thrust bearing.
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