Lotus Elan

Hairline crack in diff case

PostPost by: seniorchristo » Wed Oct 23, 2019 3:22 pm

After rebuilding my 3.77 diff I noticed a hairline crack in the aluminum case. :( I wonder if this can be safely welded without dismantling the unit. I haven't added oil as yet. You can see the crack all the way across the bottom of the ear.
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PostPost by: tedtaylor » Wed Oct 23, 2019 3:53 pm

OUCH!, Chris, did that appear "after" coming back from rebuilder? looks like the bolt hole is also buggered up.
how'd you discover this? :(
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PostPost by: seniorchristo » Wed Oct 23, 2019 4:16 pm

Ted
Some of the mounting holes are slightly enlarged. I rebuilt myself and only noticed the crack after the rebuild. :(
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PostPost by: prezoom » Wed Oct 23, 2019 4:26 pm

You would be taking a chance. If the crack goes through the case, there will be gear oil that has migrated into the crack. Tig welding will cause the oil to boil up into the weld, contaminating the weld. Ideally, you would separate the two parts, and have the case vapor degreased. But, finding that process today is near impossible with the restriction on carbon tet virtually outlawed. The next best thing is to grind out the crack with a carbide burr, degrease the surrounding area with acetone and then weld.
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PostPost by: seniorchristo » Wed Oct 23, 2019 5:00 pm

Thanks Prezoom
Is there another type of aluminum welding besides TIG that is possible? Purpose of the weld would be to seal the crack and prevent it from spreading. It wouldn't be under pressure or subject to significant structural loads.
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PostPost by: nmauduit » Wed Oct 23, 2019 8:47 pm

seniorchristo wrote:Thanks Prezoom
Is there another type of aluminum welding besides TIG that is possible? Purpose of the weld would be to seal the crack and prevent it from spreading. It wouldn't be under pressure or subject to significant structural loads.

one could use MIG I guess, but TIG will be easier to manage a solid repair.

I would not bet on this not being a stress crack to some extent (unless the car is for display only, it does take some load there), but once properly repaired you can get a diff brace that would take up some of the load.
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PostPost by: bitsobrits » Thu Oct 24, 2019 12:43 am

seniorchristo wrote:Thanks Prezoom
Is there another type of aluminum welding besides TIG that is possible? Purpose of the weld would be to seal the crack and prevent it from spreading. It wouldn't be under pressure or subject to significant structural loads.


Those diff case mounting 'ears' are under tremendous loads when accelerating or decelerating. Thats why they crack and why Lotus went to the beefier design later case and eventually the steel brace on the Sprints.
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PostPost by: Elan45 » Thu Oct 24, 2019 4:13 am

Hi Chris,

I agree that TIG is the proper method for this repair. Its good that you found the damage before installing it in the car. I agree you should separate the diff punkin from the alloy rear case. Good luck.

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PostPost by: StressCraxx » Thu Oct 24, 2019 4:48 am

prezoom wrote:You would be taking a chance. If the crack goes through the case, there will be gear oil that has migrated into the crack. Tig welding will cause the oil to boil up into the weld, contaminating the weld. Ideally, you would separate the two parts, and have the case vapor degreased. But, finding that process today is near impossible with the restriction on carbon tet virtually outlawed. The next best thing is to grind out the crack with a carbide burr, degrease the surrounding area with acetone and then weld.


Rob, you are correct about vapor degreasing here in the Peoples Republik of Kalifornia. However other states are not nearly as strict. The housing should be inspected with Zyglo or dyepen to determine the full extent of the crack. If the end of the crack can be found, drill a 3/16" hole that point to stop the crack from propagating further. The case can be baked at 250F to get any further oil residue out and then degrease again or use MEK. The cracked area can be ground out and then TIG welded. A shop experienced in doing weld repairs on aircraft components can do it, or a competent race prep shop. I have seen some amazing repairs done on vintage aircraft and race cars.
That said, the cost of a good repair may approach the cost of a new housing.
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PostPost by: rgh0 » Thu Oct 24, 2019 7:39 am

I had a cracked diff when a crown wheel bolt failed and punched a hole in the alloy. It was in the back in a non structural area but the repair welding cost from my local aluminium welding expert exceeded the cost of a new alloy housing. i have kept the cracked housing and may try repair welding it myself one day as it just needs to be good enough to keep the oil in :lol:

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PostPost by: Hawksfield » Thu Oct 24, 2019 8:52 am

Rohan
The same happened to me when a planet gear wheel broke in half and exited through the rear case. I was lucky someone in industry welded it up and it's been good for 25 years. I don't know the weld procedure as the mechanic who rebuilt it got the repair done.
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PostPost by: alan.barker » Thu Oct 24, 2019 10:16 am

Strip to alloy case and TIG Weld no problem. After check to see if the tops of the mounting lugs are at same level and flat.. Lightly mill so they are perfectly in line. Fit Diff Brace. Fit heavy duty Diff Mounts.
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PostPost by: Certified Lotus » Thu Oct 24, 2019 10:47 am

The cost of a machine shop to repair this is probably close to the price of another one. I would opt for another one. Ted has a very nice one for sale.....
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PostPost by: nmauduit » Thu Oct 24, 2019 1:19 pm

Bringing a cleaned part to a welder for repair should make economical sense I would think, I would say less than a hundred based on what I get repaired (what I send out is usually magnesium parts that I consider critical or original worth saving, or alloys with too much magnesium for me to handle reliably).
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PostPost by: seniorchristo » Thu Oct 24, 2019 1:22 pm

Thanks for your replies and suggestions. My next step will be to strip down the case, take a closer look at the crack and see if repair is practical. :)
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