I recently needed to compression test my 2003 3.0L Vulcan and ran into what seems to be a common problem with cheaper compression test gauge kits, like the one I picked up from Harbor Freight for around $35, and found a solution to get an appropriate seal in the narrow clearance for spark plugs in the 3.0 block.

The 14mm spark plug thread adapter that came with my compression tester would not properly fit or seal in the work plug holes, because the threads were too short, and the fitting on the 14mm was too wide to fit in the narrow channels in the block where the spark plugs seat.

I searched all over for a compression adapter that would fit, with no luck until I stumbled on anti foulers. I used Dorman Spark Plug Non-Fouler 42004 from Autozone.

They come in a two pack, you could use one, but threading them into each other gives you a longer extension.

Use teflon thread tape on one, and thread it into the other, very tightly with a wrench.

Then, use vise grips around the adapter for your compression gauge, to very tightly thread it into the "doubled up" spark plug anti foulers. Mine had an O ring that sealed, but the compression tester fitting only engaged one or two rows of threads. If your compression tester doesn't have an O ring and is only sealing on a few threads, apply liquid or tape thread sealant to the threads so you get a strong airtight seal, not just a thread seal. Tighten it up as hard as you can, so that it does not loosen when you are putting it in the block.

Then, use your compression tester as normal, the spark plug anti foulers seat properly in the block and should seal.