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Author Topic: 340 rear main cap damage  (Read 367 times)

Offline Beekeeper

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340 rear main cap damage
« on: April 19, 2017 - 11:07:42 PM »
Dusted off a spare 340 I had in the corner of the garage which I was going to sell to a guy for $800. He asked to look inside and we found this crack on the corner of the rear main cap where the oil pump drive shaft connects. He decided not to buy which is fine but I am really curious what would cause such a crack. It appears only at the hole hole that the shaft passes through. Anyone seen anything like this? Doesn't seem like a high stress area to me. Is replacement of the cap required or can it be run as is? If replace, then obviously I'd have to find a cap and Align hone all the mains.




Offline 1 Wild R/T

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Re: 340 rear main cap damage
« Reply #1 on: April 19, 2017 - 11:22:54 PM »
Someone put a serious sideways pull on the oil pump... That didn't happen from normal engine forces.. I'd run it without worry...

Crank has a couple slugs of mallory metal clearly it wasn't a cheap balance job...
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Offline jimynick

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Re: 340 rear main cap damage
« Reply #2 on: April 19, 2017 - 11:24:15 PM »
My guess is somebody gave the oil pump a whack to dislodge it and it cracked the cap. If it were mine, I'd boil it clean, dremel V-cut the crack and take an oxy/actyl torch and heat the crack red and then hit it with a mig, trying to localize the heat so as to not induce any warpage. The cap is pretty big and it'd take a lot of heat to warp it in my opinion. What have you got to lose anyway? Worst case, you do what you mentioned; best case, you clean it up and bolt it back on. Just my  :2cents:


Offline Beekeeper

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Re: 340 rear main cap damage
« Reply #3 on: April 20, 2017 - 12:00:27 AM »
Ok, that makes sense. This engine is an unknown to me. The guy I bought it from has lots of high dollars toys. He had someone yank this original motor from his 69 Cuda to replace with a big stroker motor. This engine had only a few hundred miles on a fresh rebuild. I grabbed it because he was tired of it sitting in his side yard and the price was just too good to pass on.

Funny thing is someone opened the motor up and stole the oil pump along and timing chain set. Under the wind age tray, it is beautiful. The rod caps were rounded off during balancing and even polished for some reason.

The thing is so clean inside, im tempted to rebuild and sell it for more. I could throw in some new bearings just for good measure.

Offline 734406pk

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Re: 340 rear main cap damage
« Reply #4 on: April 20, 2017 - 10:50:53 PM »
My guess is somebody gave the oil pump a whack to dislodge it and it cracked the cap. If it were mine, I'd boil it clean, dremel V-cut the crack and take an oxy/actyl torch and heat the crack red and then hit it with a mig, trying to localize the heat so as to not induce any warpage. The cap is pretty big and it'd take a lot of heat to warp it in my opinion. What have you got to lose anyway? Worst case, you do what you mentioned; best case, you clean it up and bolt it back on. Just my  :2cents:

 :iagree: I would weld it carefully and reinstall. Do you have a weld shop that you can trust near by?
1973 Challenger 440 6 pack auto 3.91 rear
2012 Dodge Ram 3500 dually 6.7 Cummins Fleece EFI Live
1973 Challenger 318 2bbl auto 2.73 rear 22.5 mpg RIP
1970 Challenger TA 340 4bbl auto-Sold and sad
1999 Dodge Ram 3500 dually 5.9 Cummins Fleece tuned VGT-sold
1995 Kawasaki ZX1100E & still alive

Offline Beekeeper

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Re: 340 rear main cap damage
« Reply #5 on: April 20, 2017 - 11:29:43 PM »
There's one I know of that's decent. Before I bought my own welder, I had them do a few things and I was happy. I have never asked them if they weld cast iron. I understand it is difficult to weld since cast iron is porous and lower quality metal. I'm thinking it should be easy for an experienced professional.

I kinda liked Jimynick's idea of heating with a torch and then using my mig on it. Problem is I don't have a torch.

I guess I could take the cap into that shop and ask what they think. If they were confident of an easy fix with no warp issues, then great. The more I think on this, the more I'm convinced it could probably be run as is without any issues. If the weld shop doesn't give me good odds, I might just do that.

Good thing is the guy didn't want to buy it with this crack and since I don't need this engine for a project anytime soon, time is on my side.

Offline 734406pk

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Re: 340 rear main cap damage
« Reply #6 on: April 22, 2017 - 10:32:48 PM »
This should be an easy repair IMO. I'm thinking TIG weld with a Ni-Cast rod? then grind it smooth with a dremel. Warpage shouldn't be a problem where this crack is located. The goal is being to attach this piece so it doesn't break off and damage something. There isn't any load on this area of the main cap so that's a plus!. Good Luck!
1973 Challenger 440 6 pack auto 3.91 rear
2012 Dodge Ram 3500 dually 6.7 Cummins Fleece EFI Live
1973 Challenger 318 2bbl auto 2.73 rear 22.5 mpg RIP
1970 Challenger TA 340 4bbl auto-Sold and sad
1999 Dodge Ram 3500 dually 5.9 Cummins Fleece tuned VGT-sold
1995 Kawasaki ZX1100E & still alive

Offline jimynick

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Re: 340 rear main cap damage
« Reply #7 on: April 24, 2017 - 11:29:39 PM »
We welded a broken out chunk of the cast grate in a BBQ the way I described and reheated the area back to red hot after welding and let it air cool. It's been through dozens of 500'F heat/cool cycles since and hasn't even twitched. Talk to the local welding shop. This shouldn't be a big or expensive repair and you can save yourself a couple of bucks by slipping the cap into the dish washer when the old gal isn't watching and then I'd put it in a can with some lacquer thinner to totally wash out any grease/oil in that crack. Good luck.  :cheers:

Offline 734406pk

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Re: 340 rear main cap damage
« Reply #8 on: April 25, 2017 - 08:13:18 PM »
We welded a broken out chunk of the cast grate in a BBQ the way I described and reheated the area back to red hot after welding and let it air cool. It's been through dozens of 500'F heat/cool cycles since and hasn't even twitched. Talk to the local welding shop. This shouldn't be a big or expensive repair and you can save yourself a couple of bucks by slipping the cap into the dish washer when the old gal isn't watching and then I'd put it in a can with some lacquer thinner to totally wash out any grease/oil in that crack. Good luck.  :cheers:

That will work too! Good luck!
1973 Challenger 440 6 pack auto 3.91 rear
2012 Dodge Ram 3500 dually 6.7 Cummins Fleece EFI Live
1973 Challenger 318 2bbl auto 2.73 rear 22.5 mpg RIP
1970 Challenger TA 340 4bbl auto-Sold and sad
1999 Dodge Ram 3500 dually 5.9 Cummins Fleece tuned VGT-sold
1995 Kawasaki ZX1100E & still alive

Offline Beekeeper

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Re: 340 rear main cap damage
« Reply #9 on: April 25, 2017 - 10:41:25 PM »
So I was feeling adventurous and decided to give it a shot myself. I cleaned the whole cap really good especially the crack. I was nervous about V notching it so instead, I just gently rapped on the corner and moved it back to where it should be. I could barely see the crack after that. I forgot I had this heat soak welding paste so I packed it all around before welding. Hit it with the mig and put in a bead on each side with pretty good heat. When I removed the paste, the cap was room temperature. First time I used that stuff. Now that the cap is off, I can see how that area was never a high stress area. I'm confident this should be good.


Offline jimynick

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Re: 340 rear main cap damage
« Reply #10 on: April 26, 2017 - 11:14:59 PM »
See, that wasn't too bad, was it? The only issue with cast and heat is the possibility of it cracking when it cools due to heat variations. Take a magnifying glass and using the MkI eyeball, give it a good look over. It shouldn't see a lot of stress as the pump bolts should carry that.  :thumbsup:

Offline Beekeeper

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Re: 340 rear main cap damage
« Reply #11 on: April 26, 2017 - 11:21:20 PM »
Will do!