This is part 5 of many in Limitless’ engine build. – be sure to check out the rest of the posts there. A ring filing tool is critical for, well, filing rings.
Intro – What is Ring Filing, and Why do I Care?
Today we’re going to cover filing our piston rings until they’re the right size. Like all of the other components in the engine, it’s really important that your clearances are “just right”. Too small, and very bad things can happen…like catastrophic engine failure. In the specific case of the piston ring, it’s a runaway failure that involves ripping the top of your piston off. Not fun. But too big, and your engine will lose power as combustion gases escape past the ring, and burn oil as oil seeps past the rings into the combustion chamber. Neither are good, but they’re much better than ripping the top of your piston off. So in this case, it’s definitely better to err on the side of too much – rather than too little – ring gap.
Now, some rings come “pre-gapped”. But in a performance, blue printed engine like this one, the proper choice is to check and set all of the ring gaps yourself. That’s because you want to get things just right, and pre-gapped rings won’t do that. In my case, because it’s a marine engine, we need notably larger than spec’d ring gap… and I’d rather burn a bit of oil and lose a bit of power than destroy my engine. So I ended up with a set of Total Seal conventional racing rings, 0.040″ over, which are the perfect fit for my pistons.
Step 1 – Where to Measure the Gap?
There are a few things you need to ensure when you’re filing your rings. The first is that they’re the right distance down the bore – in this case, 1″ exactly. The second, and far more important, is that the ring is perfectly square in the bore – any deviation will cause the gap to appear bigger than it is, which is bad! In this case, I 3D printed a jig to make it easy to both space the ring, and ensure it’s square. It worked perfectly!
Step 2 – What Gap do I Need?
Oh boy…what a can of worms. It really depends on who you ask. The included guide for both my pistons AND rings claimed that, for my bore size of 4.290″, I would need ~18-19 thou for my end gap (0.018″). The problem is…neither of them spec’d anything for marine usage. If my pistons needed extra clearance from my cylinder walls because it’s a marine application, and my mains and rod bearings needed extra clearance from my cylinder walls because it’s a marine application…logic follows that the piston rings will also need extra clearance from my cylinder walls, given the marine application.
I called Total Seal and, after speaking with them on the phone, they said anywhere from .022 to .026″ overall would be a better fit. I thought that was a pretty wide range, and when I called them back to give more specifics (like how I’m using a thermostat for my water and oil, what load and RPM the engine would run at, etc), the tech said that a thermostat would need a bigger end gap – which just isn’t true. I called them a third time and again got told somewhere around .025″. Ultimately I decided to go with 0.024″ for a couple of reasons. First and foremost, it’s right in the middle of their initial recommendation, and safely above the initial 0.018″ originally required. The other reason was that my feeler gauges only go to 0.025″, and to stop myself from going over, I couldn’t go any higher than 24 thou. This is not a suggestion to take shortcuts because of your tools – but in this case since it was within the correct range anyways, I felt it would be OK.
There’s a lot of debate on whether or not the bottom ring should have a larger, or smaller, gap than the top. In theory it’s further from the combustion, so it heats up less, and doesn’t need as big of a gap. But you also want to ensure there’s never any pressure on the bottom of the top ring, as that can cause other issues…so a lot of people say to gap the second ring even more than the top. In my case, Total Seal called for equal gaps on both – and to me, that makes sense. As the engine heats up, the top ring will naturally expand more than the bottom one, ensuring that no pressure ever builds below the top ring, while minimizing the gap in the second ring to reduce oil usage.
Oh yeah, speaking of oil usage, don’t forget to check the gap on your oil rings. Mine were right on spec and I gave them a little extra filing just to be safe.
Step 3 – Filing a Bit at a Time Until it’s Perfect!
Now that you know your needed gap, and you know where it needs to be measured, it’s time to get filing – and measuring. One random piece of advice I came across only once, that I’m extremely glad for, is to file the lower ring first. It’s a much softer material than the first one, so it will file much faster. If you get the hang of how much material is removed per pass on the top ring, and then adapt to the lower rings, you’ll over file them for sure! My lower rings filed away almost four times faster than my top rings…so it was very helpful advice.
Now the actual filing begins…a slow and patient process. Be sure to do one cylinder at a time, and keep the rings with their respective cylinders. In other words, file a unique top and bottom ring for cylinder 1, and make sure those rings end up on the piston that goes into cylinder 1. Filing the rings is pretty simple and there’s tons of tutorials online…but using a hand filer like in the title photo, take a little bit of material off each side of the ring, and recheck your gap every few passes to monitor your progress. To measure the gap, re-square the ring in the cylinder (being sure to remove any burs or flakes!), and use your feeler gauges. If the gauge fits, the gap is bigger than that gauge. If it just fits, it’s probably that size – and if it doesn’t fit, well, your gap is smaller than that gauge. My gauge set went in one thou increments from 1 thou through 25…perfect for what I needed.
The actual filing process is pretty simple. It takes a bit of practice to hole the ring edge square to the wheel, with just the right amount of pressure, without the ring moving. The way my ring filer worked was “backwards”…making it much more difficult. If I mounted the ring onto the base of the ring filer, and then spun so that the ring was always filed “inwards”, it would unscrew…so I had to mount the ring backwards, and the filer didn’t have enough space to hold the ring on that side. Nevertheless, I figured it out after too much trouble. Otherwise the procedure was pretty simple. File, clean, square, measure…repeat. I had to take about 6 thou off mine, which took a while. All in all it took me about 80 minutes to file everything – that’s about 10 minutes per cylinder. The first cylinder I worked very slowly, checking my progress with the feeler gauges quite frequently. By the last cylinder, I’m proud to say I got the correct dimension on the very first measurement – not bad for a first timer.
Ultimately, ring filing was pretty fun, and a nice break between all of the high stress work that came before (crank) and after (literally everything else until the long block is done). It’s a nice low stress therapeutic process and there’s no need to rush. Just remember that you can always file a little more off, but you can never add any back!
