Question on hi-comp heads and gaskets

VUgearhead Thu Sep 19, 2013 10:59 am

Yesterday evening, my son and I went outside to start up the old medallion engine we had cleaned up a while back.

Anyway, I wouldn't call it a success as I could not get it to run much longer than about 30 seconds. I have a high-compression Cox glow head installed. I started with one head gasket but couldn't get it to start. I finally added 2 more (for 3 gaskets total), and was able to get the short 30-sec run. The needle was very unresponsive. I could give it a half turn without any appreciable change in the way the engine ran. I know these engines can be finicky when run with a hi-comp head. Glow head was working fine (never disconnected the starter battery).

My question to you guys is; When using a high-comp head on an engine that normally came with a standard glow head, how many gaskets do you typically use? I may have to drop back an punt (go back to a standard glow head) if I can't figure this out.

I still think there is something wrong with this engine, in terms of friction/rubbing. There was some discoloration in the oil residue (aluminum) but I couldn't tell if it was from the inside of the engine or from it rubbing against Bernie's test stand (it worked it's way loose while running). Also, Every time it stopped, it was stuck (not seized). I can't tell if the cylinder is out of round, or if the crank pin is rubbing against the backplate or what. I'll have to take it apart to inspect. Not looking forward to that.

Ken Cook Thu Sep 19, 2013 6:00 pm

That question might be quite difficult to answer. I can only add my experiences. I use high nitro only. High nitro requires a lower compression ratio hence more gaskets. In addition, the temperature your running the engine is going to make a large difference. Cooler temps won't require high nitro to attain the same power in hot and humid conditions. When the air is cooler, props are more efficient and the engine is making more power. Engine condition is another factor to look at. If your running nitro in the 25%-35% I would start with a minimum of 3 gaskets. I have no idea of the current head shim dimensions that are being offered. I have shims that are .002" -.010. The main issue is that if your engine is scrambling plugs, add a additional shim until you stop blowing plugs. A new engine is much more likely to take out a plug as well as props that aren't balanced correctly. I have engines that have as many as 5-6 gaskets within them. Some engines only require one. The Medallion is a moderately powered engine and the high comp head should yield a few hundred rpm's over the standard Cox plug. I would be more concerned about tearing up a $9 plug so I would use 3 gaskets and try it out. If more power is needed or starting is becoming complicated, remove one and try again. Ken

pkrankow Thu Sep 19, 2013 6:48 pm

If you haven't run the engine in forever it needs cleaned. Take the back or cylinder off and wash it out with denatured alcohol or fuel. If there are baked on residues then soak it overnight in denatured alcohol or fuel.

I will assume you are using fresh good quality 25% nitro fuel such as Sig Champion 25% or equivalent.

Overnight should not hurt the plastic intake, but don't go more than 24 hours of soak without removing plastic to be safe.

Check the head while it is off and make sure it is still good. Make sure you are using fresh batteries, or an adequately charged ignitor.

Now that the engine is clean, make sure it rotates properly. Put the cylinder on without any head and see if it flips easily. There should be no binding since you just cleaned everything out. IF it is binding STOP and figure out what the source is. Take the cylinder off and check for binding again. The cylinder might be damaged, but they get crushed by pliers all the time and function properly so they are quite durable. If it is binding with the cylinder off then you need to take the engine all the way down and make sure it is clean everywhere.

OK, so no binding? This is a lightly run engine? Put 5 gaskets in (Per Bernie and high performance reed engine break in) Is this a well used engine? Put 3 gaskets in.

Make sure to put fuel on the drive plate to lube since this got cleaned out. Prime as usual, flip a few(to distribute oil), hook the ignitor up and go for it.

Phil

66 Malibu Thu Sep 19, 2013 7:33 pm

I have had the same symptoms on newly acquired Medallions and TD's and the first thing after a thorough cleaning is to reset the piston and rod to zero lash.Piston slop makes compression go away and timing go haywire. My guess is a good cleaning , piston reset, some low viscosity oil, hi-comp 1702 head with 2 gaskets and it will sing at about 16-18K.
FWIW

VUgearhead Fri Sep 20, 2013 7:42 am

Thanks for the advice guys. This engine got a thorough break-down cleaning just a short while ago. I asked about cleaning methods for it here in a previous thread.

https://www.coxengineforum.com/t5272-this-okay-for-engine-cleaning

After cleaning it got a good lube with after run oil. This is just the first chance we have had to try and start it. Anyway, to answer some of the questions from you, it's south Florida here, so weather is hot and muggy. For fuel I'm using some O'donnel R2R 30% that I goosed with some extra castor, so total oil is around 21% (half being castor) and nitro is around 26-27%. It's fresh (as in I bought it less than 2 months ago). Starter batteries were fresh, glow head was working well. During the short run, I could see a marvelous orange glow off the top of the cylinder throught the exhaust ports.

As for the friction/rubbing issue. When it was first clean and lubed it felt fine and had good compression. But then it developed an unusual resistance after TDC. Usually after TDC, the compression pushes the cylinder back down to open the exhaust ports, right? Well, on this it hangs on something. I'm just not sure what is causing it. I have some other piston/cylinder pairs, so I may sub one of them in to see what happens. That will at least tell me if its the top or bottom end where the problem is at.

Anyway, if you have any other suggestions, I'm all ears. Keep 'em coming!

pkrankow Fri Sep 20, 2013 8:15 am

If the piston bleeds down it can have vacuum holding it above the exhaust ports. If this is the case the engine is fairly worn and had too much varnish removed.

I have a couple bees that are like this. Once started they are fine, but they require very energetic propping to get to start the first time. They restart just fine.

Humidity might be part of the blame since it can cause castor to form varnish quicker.

http://coxengines.ca/engine-and-fuel-guides.html

Try this starting procedure...

set your needle to 4 1/2 turns open. (or per instructions)
choke and prime.
flip a dozen times without ignition (careful, sometimes they do start without an ignitor)
hook up ignitor
pull carefully and quickly though till you feel the detonation. Hold the prop while doing this.
flip to start

This is the procedure I follow for all my engines, especially bigger ones.

Phil

looperdude Fri Sep 20, 2013 3:09 pm

a glow engine starting without the glow driver attached, how does that happen?

pkrankow Fri Sep 20, 2013 4:39 pm

looperdude wrote:a glow engine starting without the glow driver attached, how does that happen?

A perfect combination of temperature an compression. It happens often enough on big engines in hot weather (Fox stunt 35's are known for it). Any time the engine is primed and ready...

It's not like sticking your fingers in a running .049 will do grievous injury like braking bones and removing finger joints, but it really does hurt.

No, I have never had a .049 start without an ignitor. I have seen it happen to my Dad's McCoy's a couple times. The weather was HOT every time.

Phil

looperdude Fri Sep 20, 2013 7:43 pm

oh okay, so it only happens in hot weather

batjac Sat Sep 21, 2013 1:00 am

pkrankow wrote:
looperdude wrote:a glow engine starting without the glow driver attached, how does that happen?
A perfect combination of temperature an compression. It happens often enough on big engines in hot weather (Fox stunt 35's are known for it). Any time the engine is primed and ready...

I don't know about the temperature or humidity, but once when some of us kids were at one boys house, back around '71 or '72, he brought out the PT-19 he had got as a present. I was about 9 years old, and had played with a PT-19 before. He had started it up with his dad, but he didn't like the engine. He was afraid of it. Being the stupid kid I was, I was flipping the prop with the spring starter while holding it out towards him. I knew there was no chance of it starting, since I didn't have a glow plug battery attached. Next thing I know, the engine is running! Affraid or WOW!

That I'll NEVER forget! So, yes, it can happen. This was my second most interesting thing to happen as kid with Cox engines. The first most exciting thing was when I was starting a Cox engine in my driveway to show the neighbor kid how they work, and my right hand which had been covered in glow fuel from over-zealous priming caught fire. Since the fuel burns invisible, I couldn't figure out why my had got so cold all of a sudden. Then, when I noticed the hand had a little blue corona over it, I figured it out....

The Surprised Little Boy Mark