I really like my Stewmac scissor jack. It's my goto choice for a jack that needs to be placed in the lower bout, but since they are a bit pricey I only have one. On long cracks that don't align well that leaves me one or two scissor jacks short of success.
I admit I get satisfaction from repurposing a tool like the Harbor Freight 4" ratcheting bar clamp/spreader, especially when it costs under $3 dollars U.S. I've been using these with crudely made wood "head" and "foot" ends for several years. I've finally gotten around to printing more refined head and foot ends after being reminded how helpful these spreaders were on a 8" long back crack repair on a Martin.
Building one starts with removing the fixed end and the roll pin at the opposite end. The bar is slipped out of the handle and cut to the desired length. Since the working extension range is about 3/4", you may need more than one length. I have a couple that have a 3 1/8" bar, and one that has a 3 3/4" bar for deeper body guitars. The bar is cut as shown to preserve the small hole which is used to secure the new "head". Make sure you file the edges of the bar ends to a slight bevel to aid in reinserting the shorted bar back in the handle assembly. I have about a 50% success rate reinserting the bar into the handle without removing the 6 phillip head screws. If you remove the screws and one of the springs pops out of place, I included a pict. of how they should be positioned.
The magnet in the head is optional, but I included it as an aid in locating the spreader. The screw to secure the head is also optional since a couple drops of CA glue will hold it with no problem. I use a 4-40 x 5/8" screw, but a small sheet metal screw would work just as well. Or you could use the belt and suspenders approach and use both. The foot just slides onto the handle assembly using a friction fit, but it could also be glued. Just make sure the foot is fully seated before committing with the glue or the bar won't be able to recess down into the foot.
When you want to release the spreader tension with the release lever, there is a wrong way to do it which results in great frustration in not getting it to release. I may know a guy that it happened to a couple of times. The pictures with the hand tell the story.
Enjoy!
