Port chamfering is the act of filing or sanding the edges of your cylinder's ports in order create a slight bevel or radius. A less dramatic operation than Port reshaping or Port matching, chamfering helps to smooth the passage of the piston rings through the cylinder, reducing wear on the rings and increasing the engine's reliability and lifespan.
From the factory, most new cylinders come with the ports having jagged / sharp edges or burrs. Chamfering these sharp edges into a smooth radius is widely considered to be required before installation. Most new cylinders / kits are made of aluminum, with the cylinder itself being treated with a very hard nickel/silicon "Nikasil" coating to prevent premature cylinder wear. While excellent for its purpose, the Nikasil treatment can extend over/into the ports somewhat, creating the above-mentioned sharp edges or burrs. This tough material will resist your attempts to file or shape it -- A blessing, and a curse. While it can take a good while to properly & safely chamfer your ports, the amount of time it takes to remove material will make it more difficult to screw up and accidentally alter the timing of your cylinder.
The goal is to sand / file / mill the edge of each port until it feels buttery smooth, with nothing at all catching the skin of your finger. Aim for 45 degrees or to simply round it off. As mentioned, going too far / removing too much material can change the timing of the cylinder, turning your expensive, new upgrade kit into your expensive, new boat anchor. To avoid this, it is recommended to use the gentlest possible chamfer tool for a given situation.
Tools
There are three main tools used in a chamfering operation: The chopstick, the diamond file, and the dremel bit.
The chopstick
The sandpaper-wrapped chopstick is a tried-and-true chamfering tool that is gentle on the port and can produce buttery smooth chamfers. To make the tool, wrap and affix some 200-grit sandpaper to the end of a chopstick. A small round chopstick will be excellent for the tighter corners, while a square chopstick can work more area at once and is great for wider parts of the port.
Chopsticks are at their best for deep ports such as exhaust and intake, where they can get a nice full stroke to work at maximum efficiency. They can be trickier / less efficient for smaller ports like transfers, where the lesser 'height' of the port will limit the length of stroke you can make. In these cases, it may take time, but eventually, the chopstick will do the job.
Slow to work and slow to screw up, the chopstick is the recommended tool for first timers to learn the chamfering operation. However, the position of a port or the construction of the cylinder might keep the chopstick from fitting, and you may have to use another tool.
The diamond file
More controllable than the full-size file or dremel bit, and quicker/more abrasive than the chopstick, the diamond file is the tool you want when there is a significant amount of material to remove. They are cheap, readily available, and come in a variety of shapes to ensure you can get into any strange little corners. Additionally, diamond files are known as a 'non-grabbing' abrasive, when compared to milling cutters or spinning sanding drums, which helps keep the Nikasil safe from chipping.
Large, stubborn Nikasil burrs can be removed with less time & effort than with a chopstick. Also, many times your ports may have small metal inclusions that are, realistically, too large to simply remove without a major Port reshaping operation, yet their oddly-shaped edges must still be chamfered. You may appreciate how a round or triangular-shaped diamond file fits into those nonstandard corners, getting a smooth radius where a chopstick simply would not fit. You will thus be able to smooth your ports and have a serviceable kit without undertaking a potentially fatal reshaping job.
Diamond files are extremely hard and thus well suited to working Nikasil, but this hardness can also be a liability. If you lose control, it is far easier to put a nice scratch in your shiny new cylinder, leaving a weak point where carbon deposits can build and inviting a host of potential issues down the road. For this reason, keep the following tips in mind:
- Apply gentle pressure on the down stroke, and lift the file on the up stroke.
- Avoid 'sawing' quickly to get the operation over with, it is easy to lose control this way and pressure on the upstroke can chip Nikasil if you're unlucky.
- Check your work often. They are slow to work compared to big files, but work faster than the chopstick. Though a relatively safe chamfering tool, it can be easy to zone out and take off a bit too much.
The dremel bit
Powerful and dangerous, the dremel bit is recommended for experienced users only. At their best, they can make chamfering operations a breeze, letting you finish the job with time left to hit the taco stand. At their worst, they can easily chip Nikasil or jump right out of the port and carve a crazy new design on the inside of your shiny new cylinder. Dremel tools are generally much larger and tougher to handle than a diamond file or chopstick, and this awkwardness can reduce control and make mistakes easy when trying to reach those odd areas.
Dremel bits can take off large amounts of material very quickly. Their abrasive power comes from the high RPM at which the tool spins. This spinning action also makes them difficult to control, leaving them prone to leaping out of the port and into your cylinder. If you're okay with probably ruining your cylinder, or if you want to practice on some junk cylinders you have laying around, here are some tips for using Dremel bits:
- Use only diamond-coated bits. Regular milling cutters are prone to catching the Nikasil as they spin, chipping your cylinder instantly.
- Steady the cylinder in a vise or similar holder. Keep your eyes on your work and use both hands to steady the Dremel so you can keep it where the work is.
- To reduce the chance of 'catching' the Nikasil coating, use a high RPM setting.
If possible, use Dremel bits only to "spot-clean" large burrs or inclusions that would take far too long with a file or chopstick. If you're thinking about using a Dremel to remove decently-sized casting inclusions, you're honestly looking more for the Port reshaping article.
Notes
It's recommended to chamfer all the ports in your cylinder - Intake port, exhaust port, and side transfers.
The most important port to smooth out is almost always the exhaust port. Piston ring gaps are often found on the underside of the piston where the exhaust port could potentially catch them, although piston manufacturers generally try to keep ring gaps safely away from them. Intake and transfer ports are important too, but it can't hurt to spend the most time on your exhaust port.
Keep in mind that your chamfers to not have to be wide or even easily visible. Although any chamfering action is going to 'widen' the port very very slightly, this shouldn't be enough to alter the engine's timing. For standard moped kits of 50cc-70cc, all that's required is for a once-sharp edge to become smooth. If your finger doesn't catch and the metal feels smooth as you circle the port, you've done enough. Larger bikes or kits may benefit from a very short 10 degree taper, which can be quite difficult to achieve: For more details, see The Two Stroke Tuner's Handbook.
