Cracked block? There aren't many two-word combinations that send a shiver up a car guy's spine. In most cases that means the block is DOA, and it's worth its weight in scrap only. Sometimes there is hope, though. It all depends on where the crack is, what caused it, and how extensive the damage. It is just metal after all, and metal can be welded.

We should preface this by saying that it's not always possible or advisable to repair a cracked block. If the specimen is common and affordable, replacing it with a solid block may be the smartest route. Where the effort to repair really makes sense is on the rare, out of production, or original numbers-matching stuff. Maybe a replacement block isn't an option, or maybe it's a date-coded original piece to the car. In that case, it's time to consider the options.

Cast iron is a fickle mistress. It's quite strong, but also quite porous and brittle. With carbon content typically in the 2 to 6 percent range, that's roughly 10 times as much as most steels. The high carbon content of cast iron causes flakes of graphite to form, which does not respond well to uneven heating. That's exactly what happens during welding, though. Whereas mild steel is more fluid and will flex and move with the heat or shrinkage during cooling, when the weld metal shrinks the stresses can easily be enough to crack brittle cast iron. Sometimes you'll hear the dreaded "ping" in an area that's not even connected to the repair.

While the explanation of the operation is pretty simple to understand, it's a delicate operation and is definitely best left to those with healthy amounts of welding experience. That's not us, unfortunately, so we turned to our friends at Miller Electric and asked for expertise. Miller Electric has quite a few welding gurus on staff who have a deep understanding of metallurgy and loads of field experience atop it, so if anyone could make it happen, it's them. For our cast-iron engine block repair, we were put in touch with Josh Sprinkle, Industrial District Manager and expert welder, whom we were told could handle it, if anyone could. Sprinkle was game and made a special order for the welding rod needed.

Sprinkle did mention something to always keep mind with any cast iron repair; even the very best planned ones with all the right materials and knowledge come with zero guarantee of success. But here we sit with a cracked 351 Cleveland that's teetering on the edge of scrappage. What have we got to lose?

Syncrowave 210 TIG/Stick Welder

The Syncrowave 351 shown at work here is a great unit from Miller's past, and shows just how robust Miller gear is, but if you're up for a job like this, you'll want to look at Miller Electric's new Syncrowave 210/TIG/Stick Welder. Its new inverter-based design is lightweight, making it ideal for automotive repair and personal use. An operator-friendly interface is AC/DC TIG and DC stick-capable, with the ability to weld up to 1/4-inch-thick material in a single pass. The Syncrowave 210 is anchored by innovative technologies like the Pro-Set feature, which takes the guesswork out of setting weld parameters by offering preset controls designed by Miller weld engineers. Users simply select either the DIG or Balance feature and adjust until Pro-Set appears on the display. Miller's exclusive Auto-Line technology and multi-voltage plug (MVP) allows for easy hookup to any input voltages within 115 to 230 V with no manual linking, providing reliable input power regardless of welding conditions.


Here's our patient: a virgin bore 1970 date code 4V 351C. It has been machined and fully prepped for a build, so we do have high hopes. Good Cleveland cores can be hard to find since they don't tolerate large overbores well, thanks to core shift during production and thinly cast walls. That translates into .030-.040 overbore for most blocks to minimize the risk of overheating.

We didn't even notice this epoxied crack under the paint and grime. We're not even sure how it could have happened—perhaps extreme over tightening of a bolt. Before Sprinkle can get a clear picture of the extent of the damage and assess the possibility of welding, all the goo needs to be removed.

While the side of the crack crossing into the water neck passed all the way through, we were pleasantly surprised to find that the epoxy was very superficial and easy to clean out with a burr bit. Note the hole drilled at the end of the crack; this is done to “end” the crack and prevent it from spreading further when welding. Without doing this, the crack will spread right in front of the weld pool.

The crack passes all the way across the water neck, so we can't do anything there, but Sprinkle points out the two critical arms of the crack that we have to prevent from worsening to make the repair possible.