Ok, I'm a mechanic by trade for the past 38 years but I do realize that the rules of "modern engines" do not necessarily apply to the T Model Ford. I'm getting ready to rebuild the original engine for my 1915 and I had the block, head, crank and flywheel magnafluxed. All good except the black has a number of 'hairline' cracks. My question is "if the cracks are all external, meaning nothing near the mains or cylinder wall, how critical are they"? There is no typical freeze cracking damage and until the block was magnafluxed it looked fine and my eyesight is still pretty good. I was going to install the head fill, block off the return coolant opening, fill it with water and pressurize it slightly (1 to 2 psi) and look for leaks. Should I bother? As I said earlier I don't see any cracks were I'm concerned something is gonna fall off.
I await wisdom from the vast array of knowledge that is the Model T forum.

The T block is non pressure so drill a small hole at the end of the cracks to stop them from growing, "V" it out a little and fill with something like JB Weld. Or you could have them stitched but part of the stitching process is peeing each plug and I would not trust the thickness of the metal not to crack further.
A good welder of cast iron to also fix it.

Mark, I have welded cast iron before using Tig and aluminum bronze on ac current. Works pretty well but was wondering if I should bother? I'm not concerned about structural strength, I guess just coolant leaks?

You didn't say where the cracks were located, Most likely they're freeze cracks from some time ago . A common place for those is on the left side just below the water inlet to the block running horizontal front to back. They are very easy to address especially if the engine is out of the car. There are several methods of repair as you might know from your background ranging from cheap to very expensive. Usually the degree of difficulty is reflected in the cost. Not always related to success rate.

Some common repair methods are stitching with plugs, welding, brazing, soldering, and patching. I've had the most success with patching which was also by far the cheapest and easiest.

I used a product you should be familiar with called Ultra Black made by Permatex. Basically I glued a piece of fitted brass shim stock over the crack and applied a coat of Ultra Black over the patch then painted over the repair. Been leak free now for years since there is no pressure in the T's cooling system. The patch is flexible and not like a JB Weld product which will not stand up to expansion and contraction.

Think of the TV ads about Flex Seal and the screen door boat bottom.. LOL

Good Luck and let us know what you decide.

YMMV

Lock-n stitch, best way to fix them. If it works on a Cummings Diesal, it will work on a T. I do it all the time. Dan

Gene, I love ultra black, excellent product and I may try the patch method if my welding doesn't work.
Dan, I have had stuff stitched before and maybe I just need to wrap my head around the cost.

Model A blocks will develop hairline cracks in the same place. I use JB Weld on my A and T blocks with no issues. If the engine is out of the vehicle and I can lay in on its side, I use the liquid form of JB Weld as described above by Mark. If engine is in and there is no other reason to pull it, I use JB Weld "Water Weld" - it's a putty, so it can be stuffed along the crack with running down the side of the engine before it sets up. Here is an example of the Water Weld on an AA engine. It sets up in about 10-15 minutes and fully cures within an hour. It dries white, so dust with paint afterwards.

*without running down the side....

There was a small crack in the speedster block just under the water inlet. Vic Terrell found it in 1992, he v-ed it out and filled with JB Weld. It is still there today.

Awesome,thanks fellas. I did one time many years ago repair a damaged keywords on a crankshaft snout with Jb Weld, worked beautifuly. I'll give it a go as the block is out and disassembled.
I use Jb Weld on non critical stuff on my old cars but as you might appreciate I don't use it on my clients vehicles. Lol

Keyway that is, not keywords. When are these Darn phones gonna start to work with us instead of against us??

Jamie:
I would also magnaflux around the valve seats as the two center exhaust seats crack and need repair. Same goes for water jacket cracks.

These two pictures are the repaired areas by metal stitching.

Thanks for the pics and advice. I'll check the block and compare it to what you've posted J

Jamie,

Did you ever tell us where the cracks are? If they're in the coolant jacket area and you can't even see them without mangaflux, I would ignore them completely. Same holds true if they're in some other unstressed area.