Next up are the liners. I should mention upfront that the path I took detoured a bit as things progressed, so the pictures might seem a bit out of sequence without some elaboration. When the CI liners were lapped to final bore & heat shrunk into the aluminum cylinders, the cylinders squeezed back under cooling. The bores reduced to the extent that they needed to be lapped all over again to return them to target dimension. I figured any shrinkage would be quite small, like within a few tenths, thus only requiring a quick lapping correction tweak. But they reduced more like 0.0005-0.0015” & also non-linearly down length of liner. Likely a function of the cylinder’s tapered shape squeezing it differently on the top vs. bottom of liner. Anyways the bottom line is my careful bore finishing work before mating into the cylinders kind of went out the window. I could have opted for something more like a slip fit, but I started to read articles suggesting some liner interference is required to achieve proper heat transfer. And as the engine warms up under running condition, a sliding fit will only get looser yet as the aluminum cylinder expands more than CI liner.
My initial workflow was to first establish the bores of the aluminum cylinders to a consistent diameter & finish, which was brought about by a reamer. It was an imperial size next closest to the nominal metric size specified on the plans. I was already modifying the cylinder barrels as mentioned earlier, so this change was incorporated. This resulted in a slightly thicker liner wall which I thought was fine, maybe even desirable. With the cylinder ID established, I would finish the liner OD to whatever dimension was required for the correct slight interference fit. The interference amount was driven by being able to place the mated liner/cylinder assembly into an oven at moderate soak temperature so that they would release from one another based on the different thermal expansion of the two materials. I’ve done this operation many times on RC engines with my small toaster oven to replace liners.
But let me back up a step. This is my first engine & I was intimidated by making good quality piston rings. This topic has been beaten to death in many other posts, so let’s just say I found myself at the same fork in the road I suspect others have arrived. I was aware of the Trimble ring method documented in Strictly IC magazine. There are also some excellent build posts on this forum where others followed Trimble’s procedures with great results. I find Terry Mayhugh’s (Mayhugh1) build posts to be particularly informative. Making the ring fixtures represents some work, but didn’t seem too onerous. But I didn’t have access to heat treating equipment or related experience which seemed pretty important to success. I wanted this engine to run & rings are crucial to success. So, to my thinking, there are 2 main paths:
(A) Bring all liner bores to ‘whatever’ diameter they arrive at, as long as each are identical to one another & appropriate final finish. Using that resultant measured bore as an input value, all of the dimensions to make the ring blanks & heat set fixtures can be determined using Trimble’s equations. The advantage here is that all the liners can proceed along together somewhat as a group. They receive the same tool setup treatment one after another, especially up to the latter stage of finishing where it counts most.
(B) Purchase commercial rings, assuming they can be reliably sourced. This solves the ring making issue. But you need to make the bores exactly the same as the liners they were intended to run in. The O5 is a nominal 24mm bore which happens to be the same as an OS-56 4 stroke engine. Therefore, it seemed like a good idea at the time in my case to purchase 5 rings, including spares for unforeseen replacement. So, I somewhat naively, went down this path. Although it seems like a good plan, in reality it’s actually more work & higher potential for mess up. At least for multi-cylinder engines where the count increases. The issue is the dimensional target – trying to stay within say 0.0001” bore target and simultaneously arriving at that target with the appropriate finish. If the bore is inadvertently exceeded for whatever reason, that’s kind of the end of the trail as it will no longer match the commercial ring. Next engine I will likely go the Trimble route.
When the rings arrived, I measured the cross section against the Trimble values & they were quite closely which is assuring. I also got a new OS-56 liner to closely examine for fit, finish & use as a dedicated glorified bore gage. And added a piston to obtaining corresponding dimensions like ring groove, crown & skirt OD’s etc. to replicate for my pistons. Thus, the shopping list expanded but I figured I could sell the piston & liner as spares one day & recoup some costs. As of today, several years later, they are still sitting in my box. :/
