Frank,

I think you answered your own question, actually. If the performance issue is just cold compression for starting and ease of handling, then it is the fit. Your current example appears to be the OS 46SF which should have a single, standard ring. This is a tension ring which is part of the key to success. Other engines, perhaps your HPs(?) have a Dykes ring which is a different animal, and may have poor compression if flipped by hand but still run great. I've got a few K&Beast 40s with Dykes ring that can fool you.


Here are some thoughts:

Some experts are more expert than other experts. Proof of expertise is in the results. Everything else is just talk....

If you don't run an engine after you set it up, then you have to be really good at it to know that it will work fine. Lesser experts and more cautious engine builders who value their reputation would run it to make sure it will seat and not bind, and that the ring has acceptable temper and doesn't go "flat" after getting it hot. I got a couple of ringed engines from Frank Bowman, and he ran both of them after he completed his work. So he could be very, very confident that the setup was right.

If all the parts are really good--like factory new--and really to tolerance--and now you have to fit a ring to the engine, that takes skill but is not hard to succeed if you can make a ring to fit your parts. That means lapping it to width as needed and gapping it for leakage and to prevent seizing. Taking one out of a package and expecting it to work great is probably a little optimistic.

If you tell someone "to just change the ring" that can handcuff them. For example, if the installed liner is not round, then you can screw with the ring all day and be disappointed in the results. If the liner is ok, but the case bore is not round or is otherwise damaged, then the liner will conform to that and again, ruin your chances of a good fit. Engines that hit the ground hard may no longer have straight cases. I "repaired" an Enya that was like that.

If the metallurgy on the ring is not right, it's not going to be a good performer. On a tension ring (I think the SF falls in this category, but I don't own one...) the ring has to have a larger diameter than the piston, but not so large that the friction is too high, etc. I don't know how Frank was making his rings, but the ones that I made years ago were machined to a very specific larger diameter before they were slit. That gave me an initial tension setting. Making a ring the ID of the liner and then "opening it up to tension it" will almost certainly make it oval shaped. This doesn't work well in my experience. It is the last resort of the desperate, the night before a contest, with no spare rings in the drawer....

I suspect that a lot of engines are using fine-grained cast iron rings. (Meehanite)   Maybe there is something more modern being used.

Excessively rough finishes on the ring or on the cylinder will prevent it from ever seating. Not prepping the cylinder before putting in the new ring seems like a waste. At best, the results would likely be hit and miss.

I found that pinned rings are harder to set up. Harder to measure the gap accurately.

Trying to run a chromed ring on a chromed cylinder used to be a fool's errand unless someone has invented some magic. For example, if the fit was so perfect that it did not need to wear at all to seat--that would be magic!

You probably know all this already, being a ringed engine man. And I didn't really help you. But we can always commiserate, right? That doesn't cost extra....  And that is good therapy.

Dave


PS--My last "project engine" was a friend's K&B 61. The last engine man abused it for sure, but probably the one before that did some fundamental damage to the case bore and the liner. I did what I could but after getting rid of the pliers marks on the liner flange and the huge burrs on the inside of the case from a broken ring (improper disassembly) I will probably have to buy and fit a new ring. If the liner fully springs back to round after heat cycling it there is a chance....  I won't know until I run it. But right now, the compression is poor.

      Hello Dave and Randy:

      Thanks so very much for taking the time to make very comprehensive replies. 
 
       I was apprehensive concerning getting replies similar to yours as I imagined that I was opening Pandora's box when ringed engines were discussed.  The best option that I think that I have is to rid myself of all of the ringed engines that do not please me and replace them with engines that I will be happy to use.

      My flying mates and I have a few older ST ringed engines that run great out of the box.  These include G21-.46, G-.49 Sportsman, G- .51 and G -.56.  These are all box stock and could not run any better despite being old school design.  For some reason, the quality of these engines were/are great.  We also run some older ST .15, .19, .23 and .35 engines that still run perfectly even though they are of lapped design.  I have been told that the "newer" engines are much better and if that is true, we have come a long way in engine materials, fits and finishes.

                                                                                                                                             Be well,

                                                                                                                                             Frank McCune

     Hello Again:

     Just a few thoughts re. the above posts.
 
     I remember reading an engine report about  the new 1972 Rossi .15 engine.  It was the king of the hill for .15 engines in speed and expense.  The article took the reader through each manufacturing step for each part and the emphasis was placed on every FIT! Of course this cost more money to do this but the results were well worth the extra expense.

     In the pre Foxberg Racing, there was something like a 1 oz. Goodyear Race in the Wichita area that required a Fox .35 Stunt and a Goldberg profile stunt kit.  When our club tried this I was all in.  I spoke to a friend and he said, "Mail me your engine." When I received the engine it looked untouched inside and out but it would pull my porky Shoestring at 22 seconds for 7 laps.  Most other Fox engines were running 27 seconds for 7 laps and winning.  I called my friend and asked what he did to the engine and he replied," I just squared it up." He would no give any details! He later mentioned that having the piston and crank at 90 degrees made a huge improvement. How would one change this relationship in the engine?  This engine smoked everything it raced but after winning the feature one day,it puked the shaft and destroyed the engine.  Since then I have never seen a Foxberg racer that would do much better than 27/7 laps.

      Anybody have any idea as to what was done to this Fox .35 Stunt that was well used before he touched it to enable it to be so fast?  Fits?
 

                                                                                                                                     Be well,

                                                                                                                                     Frank McCune