Hairline Crack in LM7 5.3 Block
 

About a year ago I got this junkyard LM7 5.3 motor swapped into my truck. I did maintenance work on just about every component, but the bottom end is untouched. It has been running great for a year now and I've not had any problems with it. Then, yesterday I was underneath the truck doing some suspension work and noticed this strange wet spot on the side of the block. It was coolant but it had no source, as if it emanated from the side of the block! After cleaning it up, the spot remained dry. It wasn't until running the engine up to operating temp that I discovered there was indeed a hairline crack in the side of the block and coolant was slowly seeping out. So little that it would evaporate and dry before ever dripping on the floor. The crack is about 1" long and wraps around the vertical ridge on the block. I've drained the oil and there is no coolant in the oil, so I'm not too worried that this crack has propagated internally.

After reviewing photos from when I bought the engine, I realized that this has been leaking since before I acquired the motor as you can see a stain on the block from the leaking coolant.

At this point, I'm trying to decide what to do. I know the correct solution would be to start looking for a new block, but I'm wondering if there's anything I can do to try and stop this leak for now. I'm already in this far with this motor so what would it hurt to at least try something? Any recommendations or suggestions from the collective wisdom of this forum would be greatly appreciated!

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Re: Hairline Crack in LM7 5.3 Block
 

Grab yourself some of these and add to the cooling system....they will work on your tiny weep...

GM (12378255) Available fro Amazon...or GM)


5 pills per pack....follow package instructions...

:chevy::chevy::chevy:

Re: Hairline Crack in LM7 5.3 Block
 

All kinds of block sealant products at the local parts store. More than worth a shot instead of replacing the block.
Per my old time engine builder every chev that came off the production line had some type of coolant sealant installed at the factory.

Re: Hairline Crack in LM7 5.3 Block
 

it looks like someone installed a head bolt with water or oil in the hole. the bolt pushing down on the water/oil caused a hydraulic pressure spike and cracked the block.

Re: Hairline Crack in LM7 5.3 Block
 

I would clean it up and try some JB weld. Failing that I would grind it open a little and weld it.

Re: Hairline Crack in LM7 5.3 Block
 

went through a similar scenario years ago, Had a pro welder try to weld a hairline. After hours of attempts we never got it sealed. Naturally we cleaned it several times before hand. Tried jb over the welds- nope. For the additives some swear they will partially clog up radiator & other components. It has to go somewhere right? In the end drove it till it died & a buddy bought it & dropped in a diesel.

Re: Hairline Crack in LM7 5.3 Block
 

Thanks for the suggestions everyone. I have heard that it is difficult to weld a crack like this, so I probably won't try that.

I bought some of those tablets mentioned, but have not dropped them in yet. I had someone else mention that they could gum up spark plugs if there is an internal leak. My thinking is if that's true, then at least I'll know it's leaking internally as well and I should really start looking for a new block.

If I drop the tabs in, I'll probably also slap some sort of epoxy on the outside as well. I'll update once I try a few things and let you all know what happens!

Re: Hairline Crack in LM7 5.3 Block
 

The epoxy is a waist of time imho. I had a 6.9 Ford diesel delivery truck and a 2001 Duramax that both developed coolant leaks due to failing head gaskets or?
I used the Bar"s Leaks product and it fixed both and did not plug up the radiator or cause any cooling problems.
The main thing is you need to follow the instructions on the bottle to a T.

Re: Hairline Crack in LM7 5.3 Block
 

https://www.summitracing.com/parts/m...iABEgInZPD_BwE

I used this on a 350 Vortec that had cracked heads, it worked great. I was told to try that by a machine shop owner, he used it on a 400 SBC dirt track engine that he ran for three years before he over revved it and put a rod through the side of the block!

Re: Hairline Crack in LM7 5.3 Block
 

Says it's not compatible with anti-freeze

Re: Hairline Crack in LM7 5.3 Block
 

You drain the system, use water only and run according to the instructions, after that go back to the anti-freeze solution.

Re: Hairline Crack in LM7 5.3 Block
 

The GM tablets I listed are compatible with antifreeze and will not clog the cooling system.

They are used by GM when tiny cracks develop in castings etc...they work !

They wont impact spark plugs, the flame front in the chamber will actually set the material harder...

:chevy::chevy::chevy:

Re: Hairline Crack in LM7 5.3 Block
 

I wanted to update this thread to add a resolution to this issue. I decided to go with the GM tablets as a first attempt. I dropped 3 tabs in the radiator and let my truck idle up to temp. Directions call for adding the tabs with new coolant. I just put new coolant in it two weeks ago when I swapped the radiator, so I figured it was still "new". After warming up, I saw only the slightest hint of moisture coming from the crack. Next, I went for a 30 minute drive and then let the truck cool down. After subsequent drives I have not seen any coolant whatsoever. It appears that these tablets have done the trick, at least for now!

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Re: Hairline Crack in LM7 5.3 Block
 

Very cool.

Re: Hairline Crack in LM7 5.3 Block
 

Glad for the progress....

I use these tablets in both of my LS powered rides and have had zero issues.

I believe they were originally formulated to seal small cracks in the alloy heads, but they work on cast iron very well...

I add new tablets about every six months to ensure the continuing protection.

:chevy::chevy::chevy: