Hi All:

          I replaced the rings in an od new stock McCoy .29 today that had low compression with custom made rings.  I checked the ring gap and it looked to be correct.  The new rings did not improve the compression.  Another engine in the same condition, nos had better compression.  Both engines had never been run prior to today.  The engines were difficult to start as the compression was very low in each engine  They both ran with very little power and the needle valve was very critical.  I ran the engine with the new rings for about 20 minutes.  Will more running time improve the compression?  Someone suggested an hour is needed to seat the rings.

        Does this mean that the cylinder is eccentric preventing the rings to seat? Perhaps I should have sent the p&c out to be honed prior to running them.  Will the compression ever improve?

       I have had rings fitted by "experts" and out of the four times that the experts did the work, one was totally useless and had to be sent out to another expert to have the ring properly fitted. Now, all of the engines run perfectly!

      What did I do wrong?  How can this problem be solved? Who would be good person to contact about this problem?  I still have a new set of rings to be installed in another McCoy .29. This engine too had never been run but has very little compression.  Will more running time properly seat the rings?  I also have a couple of nos McCoy .19 engines from the same era that have the same problem.

                                                                                                                                                 Tia,

                                                                                                                                                 Frank McCune

   

When installing a new ring  in an old cylinder sometimes you don't get a good ring seal. and the engine will need another sleeve or a very tight ring after more honing,
Is your 28 a standard or Dykes ring engine?
How used is the sleeve ?
What is the gap, new  on the  ring?

     Hello Randy:

     The rings are the standard style rings that were made by McCoy circa 1948.  There are two rings on the piston.

   
                                                                                                                 Have a great Holiday,

                                                                                                                 Frank McCune

       Hi Randy:

       I checked the ring gaps as you have described near the top and near the bottom and they were the same. This does not guarantee that the cylinder is concentric.

       Today, the engine started with one  flip starts for first 4 runs.  After that, there was not enough compression to hand start the engine. The engine also runs  erratically with a wide fluctuation in speed.  There are no leaks in the engine.  Perhaps it will improve after a bit of more running.

       I did "rough up the cylinder bore with a bit of 600 wd paper to help the rings seat faster.  Do you remember back in the Dark Ages, people used brake cylinder hones on model airplane cylinders to help seat the rings.  Of course this was on steel cylinders.  Perhaps I should have tried this trick.  I have not honed a wheel cylinder since Ike was in the Whitehouse. Lol

                                                                                                                             Thanks for taking the time to share your expertise,

                                                                                                                             Frank

     Hello Bob:

     If you send me your address, I will be happy to take you up on your kind offer.

     Both of these engines have the rear venturm turned up at about a 45 degree angle and are positioned  in the rear of the case.  The front of the case is removable.  I also have a couple of .19 engines configured the same way.  These too have no compression. If you want to look at them, I will also include them in the same package.

                                                                                                                                   Have a great Holiday Season,

                                                                                                                                   Frank McCune

What fuel are you using in these engines?

Frank, this is just my $0.02 as an aircraft mechanic. Low compression during ring seating/break-in is not uncommon. To get the rings to seat correctly, your "Dark Ages" methodology is still used in modern air-cooled aircraft engines and will help quite a bit. Another thing is to avoid any synthetic oil during break-in. It inhibits the break-in process. Last, you normally need to develop at least 75% power to break in a ring. The reason is that the combustion gasses flow down the sides of the piston into the ring groove and exert an outward force on the ring.

Good luck!