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How to Bore and Hone Cylinders

Whether you are punching your 883 out to a 1200, or rebuilding a clapped-out Shovelhead, you are going to need your cylinders properly bored and honed. On the surface, the concept of boring and honing cylinders is pretty straightforward, but things get tricky when it comes to the actual execution. To do the job right, you need a variety of expensive machines, precision measuring devices and the knowledge and experience to correctly use them all. Even though it is unlikely that most of you reading this will ever bore and hone your own cylinders, it’s always important to know the proper procedure, so you can ask the right questions when you are looking for a reputable machine shop and be able to give your machinist the right information to do their job. For this article, I traveled down to Bill’s Cycle Salvage in Jacksonville, NC, and let their lead mechanic Dave take me through the entire process.

 

To start off, Dave bolted torque plates to the top and bottom of the cylinder, which not only provides additional support during boring, but also provides two parallel surfaces to keep the cylinder square when mounted in the boring bar stand. Once the torque plates were bolted up, Dave took several measurements of the diameter of the cylinder to check for any unusual wear and to determine the initial settings for the boring bar.

A telescoping gauge allows for quick, repeatable measurements of the inside diameter of the cylinder.

Next, Dave moved on to what is the most critical measurement, the diameter of the piston. Getting this measurement made precisely is important because the final bore of the cylinder is based entirely off the piston’s diameter plus the piston manufacturer’s recommended clearance. To measure a piston correctly, you must use a micrometer (which reads to 0.0001”) and you must also know the gauge point. The gauge point is the widest point on the piston and in this case was 0.4” from the bottom of the piston skirt. Both the clearance and the gauge point can be different depending on the brand and the design of the piston, so it is important to keep any documentation that came with your pistons and pass it on to your machinist, so he can make sure he bores the cylinder for the proper fit. Although these pistons turned out to both be exactly the same size, that is not always the case and when your cylinder clearance is .0025” (it can be even less) it’s very important that each piston is matched exactly to the cylinder it will be installed in.

It's key to measure the piston diameter in the correct location, as pistons do not have a uniform diameter.

Bill’s Cycle Salvage bores their cylinders using a Kwik Way boring bar, which has been the standard for most bike shops since WWII. It uses an indexable single point cutter that rotates counterclockwise and slowly feeds down through the cylinder. Since it’s much harder to add material back to the inside of the cylinder once it has been removed, Dave did multiple passes with the boring bar, measuring in between each pass, until he got within a few thousandths of the final diameter and had a clean cut all the way around the inside of the cylinder.

Getting the cylinder aligned properly before boring is an absolute must.

The last few thousandths of the cylinder wall were cut away using a Sunnen hone. This is an important step because the surface left from boring is not optimal for a good piston ring seal, and the cross-hatching left from honing creates fine grooves that hold oil for proper cylinder wall lubrication. The hone is made up of four abrasive stones spaced evenly apart and mounted to a mandrel in such a way that they can move in and out. When the hone is turning, the stones are forced out to put pressure on the cylinder wall, which allows them to precisely grind away material. The cylinder is moved back and forth over the hone while it spins, which creates the cross-hatch pattern.

A constant stream of lubricant is directed into the cylinder during the honing process.

As with boring, the diameter was checked multiple times during the honing process, although at this point the piston could be inserted and the clearance checked with a feeler gauge. Once the feeler gauge indicated that the clearance was correct, the cylinder was carefully measured using a dial bore gauge to verify the diameter.

A long feeler gauge is used to quickly check the clearance between the piston and the cylinder wall.

With the front cylinder completed, the rear cylinder got the same treatment and after some cleanup they were both ready to be installed. An easy job for a well-practiced machinist and well worth the cost. It’s tempting to think you can save some money by honing your cylinders yourself with a three stone hone and a power drill, but if you don’t ruin your cylinders, you’ll just find yourself doing another top end job in the near future. This is one of those jobs best left to the professionals, no matter how much you like doing everything yourself…

A finished cylinder, bored to the correct size and with the proper cross-hatching.

Till next time,

Panhead Jim / @panhead_jim

Bill’s Cycle Salvage


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Commented on 8-14-2022 At 11:24 am
 

Good write up and seems like these guys know there stuff. I’ll likely never bore a cylinder myself due to machinery cost etc, it’s always cool to read up on the process.

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