Project Firebolt continues apace, dear readers. This past holiday weekend, Todd and I tore into the old BSA Hornet mill I picked up to do a full inspection and major cleaning. One of the pieces most desperately in need of sprucing up were the cylinders. They were gross and, in their state, completely unusable. I had a plan, though. A clever plan that involved a drill, some oil, and about a thousand tiny silicon carbide balls.
You all have heard of the Flex-Hone, right? Made by Brush Research Manufacturing, the Flex-Hone is a super simple, easy-to-use smoothing/deburring/honing tool used to clean up cylinders that need a little cleanup but don't need to be fully rebored. The tool itself is super simple; just a twist of heavy wire with nylon strands woven into it and abrasive "globules" attached to each nylon strand. You typically use one when doing something simple in your top end like replacing your piston rings. As I alluded to earlier, I used a Flex-Hone to clean out the Beezer's cylinders, and while I did, I took some pictures to put together this handy how-to reference guide. So, without further ado, let's get to it.
Step 1: Choose Your Weapon
Flex-Hones come in a dizzying array of sizes, styles, and grits. The globules are made from all sorts of materials—silicon carbide, aluminum oxide, boron carbide, tungsten carbide, etc.—and the combinations are nearly endless. Figuring out which tool is right for your job can be a little intimidating, but thankfully, BRM offers tech support by phone or email. Very handy. Since the A65 engine's cylinders are un-plated cast iron, I chose a basic, middle-of-the-road, 3.25-inch tool with 240 grit silicon carbide globules. I grabbed mine on Amazon, but you can find Flex-Hones at various tool and hardware stores. It was just rough enough to scrub off the surface rust and other junk without really scoring the bores.
Step 2: Mise en Place
Now that you have your fancy new Flex-Hone, get all your stuff together. You'll need a stout drill, the part you're honing, the Flex-Hone itself, some shims (just in case), and some lubricant. You can totally use motor oil, but I went ahead and bought BRM's special honing oil because I'm a sucker for fancy-pants ad speak. I highly recommend using a corded drill for this job instead of a battery-powered one. Something like this old, 80s-era Milwaukee Magnum has the grunt and speed you want, grabs the tool more tightly with its manual chuck, and its speed is more easily controlled. Your mileage, of course, may vary.
Step 3: Prep the Part
Project Firebolt's cylinders were nasty. They had a nice coating of surface rust and were noticeably scuffed and dinged. I wanted to get them cleaned and honed before even trying to measure them. Getting an accurate reading on your bore gauge is a lot easier with a smooth, clean bore than it is on a bumpy, rusty one. Prepping the cylinders was pretty simple. Once the piece was cinched up in my vise, I applied a liberal coating of the special honing oil to the cylinder and sprinkled a little on the tool for good measure. With the Flex-Hone mounted on the drill and the cylinder thoroughly lubed, I was ready for the next step.
Step 4: HOOOOOOOOOOOOOOOOONE
Sadly, there are no pictures of the actual honing here since my hands were kinda full so you'll have to use your imagination. To hone the cylinder, run the drill up to about three-quarter speed and plunge it into the oiled bore. You want to work the Flex-Hone in and out of the bore for one minute, making sure it comes half-way out each side of the cylinder. Keep a steady speed for 55 seconds, then for the last five seconds increase both the drill speed and the in-out speed. Once the minute is up, remove the Flex-Hone still spinning and power down. That last part is important. Never stop a Flex-Hone in your cylinder or insert one that's not spinning.
Step 5: Bath Time!
Once your bores are thoroughly honed, they'll be covered in a slurry of oil, metal, and grit from the tool. The best way to deal with this is a nice, warm bath. Using a mix of hot water and simple dish soap, I lathered up the cylinders and washed them out with a cloth. Now, I did this in my bathtub which made an unholy mess, so be warned. Maybe do yours in your basement slop sink or something (mine was full with another, not-bike-related project). Just make sure you get all the gunk off and dry the part really well before returning to the shop.
Step 6: Inspection and Oiling
There you go! Now you have some freshly honed cylinders and it's time to finish up. Take a good look at your handywork. You're looking for shiny bores and a nice, tight, cross-hatch pattern in there. That cross-hatch pattern helps the rings grab on to the cylinder walls and makes sure that oil gets to where it's needed and stays there. If you see any major remaining imperfections, or if your cross-hatching isn't quite the thing, you might have to go in for another pass. If everything looks good, coat the bores in a nice, thin layer of motor oil to protect your work and ward off flash rusting.
All told, this project took me about an hour start to finish once I had everything together. It was easy to do with the basic mechanic tools I had lying around (aside from the Flex-Hone itself, of course, which I had to order) and cleaned up those nasty cylinders a treat. Now, fair warning, the actual honing part of this process, the part with the drill and the oil and the spinning tool, is extremely messy. It flings gunky, hard to clean honing sludge all over your work area. So, you know, a couple drop cloths over your bench and tools is a good idea.
If you're even a mildly competent wrench comfortable with the insides of engines you can totally do this. Just choose your tool wisely, follow this guide, and check out some of BRM's videos and you'll be set. Good luck!