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Thursday, April 19, 2018

Doug's Domain

Doug Vetter, ATP/CFI

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Friday, September 29, 2017

Ring End Gaps Set

With the torque plate installed on the block it was time to set the ring gaps. I started with the oil rails because the gap specification is a minimum, rather than a hard target, so I knew they would tolerate a bit of overzealous filing as I gained experience with my tools. To check the minimum gap of 50 thousandths I taped three feelers together -- 25, 20, and 5. I found it took on average 80 turns of the filer to widen each gap to meet that spec. I would later realize (and my race-engine-building neighbor would confirm) that the oil rails are made from a harder metal than the other rings. This seemed a bit counterintuitive but my neighbor said this is because the compression rings must wear in to conform to the bore in order to seal, while the oil rails just have to scrape excess oil off the bore.

I expected a similar process for the second rings but found most of the gaps wider than expected out of the box, to the point that they required little if any filing. My goal here was 17.6 thousandths and while trying to find the initial gap I found the 18 thousandths feeler entered some of the gaps out of the box. For consistency I widened the remainder to the same extent. Based on my ability to "rock" the feeler a bit in each gap it's fair to say that all are all a touch over 18 thousandths now (perhaps 18.2/18.3).

While I found the initial top ring gaps were a lot tighter than the second rings out of the box (12-13 thousandths feelers were the largest I could fit into the gaps), it didn't take much filing to slowly bring them up to the target of 16.6 thousandths. I rotated the filer handle at most 2 turns initially, tapering down to 1/2 turn for the final iterations. As the top gaps were the most critical I spent a lot of time with each ring -- around 10 minutes each. While I didn't have a feeler gauge to cover that 0.6 thousandth, I was able to grind away just the right amount of material to allow the 16 thousandth feeler to fit without drag but the 17 thousandths feeler to bind. I figure the end result was close enough to 16.6 thousandths that I could run with that.

On a few rings I had some difficulty getting the feeler into the gap, but once there it moved smoothly, or I was able to insert it only part of the way into the gap. This was evidence of a slight bevel or angle to the "shop edge" (the filed edge) with respect to the factory edge. These bevels can be dangerous to the novice because they may trick you into thinking the gap is too tight and this will encourage additional filing when only some minor correction is required. When I finally realized what was happening I took extra care to correct the error, filing the shop edge only as required to remove the angle. To confirm I had corrected the error I brought the ring edges together in front of a bright light to verify they fit before returning the ring to the cylinder for another gap check. Fortunately, these bevels were few and far between, mostly confined to my early attempts before I learned how to properly align the ring on the filer.

Speaking of the filer, I'm happy with the performance of the Summit Racing unit and recommend this tool for DIYers or first timers. It allows filing of the rings in a safe and repeatable fashion. Based on the reviews which complained about the longevity of the abrasive wheel I bought a second wheel as a backup. I did not wind up using it. They may have improved the quality of the abrasive since those reviews were written or my rings did not wear down the abrasive as quickly. In any case I plan to sell the extra abrasive with the unit when the time comes, which will guarantee a similar experience to mine.

Ring Filing Tips

My tips for those attempting to file ring gaps for the first time:

Next Up

The next step is to mount the rings on the pistons so I can insert them into the block for a plastigauge test.