RTV or Gasket for Cylinder/Crankcase?

[ben_beyer]

I have a Brillelli 366 (60cc) motor that I'm going to be rebuilding. I was having problems with it and discovered the ring was worn out. I'm going to be getting a Bowman ring for it and probably replacing the cylinder as well. Should I also get some gasket material for the cylinder/crankcase or is there an RTV that I can use for that area to seal it good?

A gasket or RTV was not used before and the engine was purchased new in 2008.

 

[Tired Old Man]

It's my personal belief that every cylinder should have a gasket or some other means to prevent leakage between the cylinder and the crankcase. Some Japanese engine makers perform such superb machine work that gaskets are not needed in some areas of an engine but those are typically automotive products, but even those use some type of head gasket. We should not expect such a high level of machine tolerance with our model engines.

I do not like a regular RTV for cylinder gaskets for a couple of reasons, one of which is what I call the "ooze factor". RTV is quite dense and can dry relatively hard. If globs break off after oozing to the inside of a cylinder they can make it to the top of the cylinder and foul a plug. Alternatively, a glob can drop into the reed cage and keep a reed open. It is also impossible to alter a squish band or raise port timing if needed using any kind of RTV sealant. If the base of the cylinder was cast too thin the port height might be considerably lower than it needs to be. This is where gaskets can play an important part in engine performance by changing thickness.

OTH, if a gasket cannot be used a sealant that will not cause issues from minor oozing can be used. One of those is Permatex Anaerobic Sealant. In direct exposure with gasoline it remains quite soft and will somewhat emulsify, permitting it to depart the engine with minimal impact on the engine. It also will not harden unless it is no longer being contacted by air. The product should be used quite sparingly, only a few thousands of an inch thick. If the thickness is so deep it is no longer "see through" you used too much. The surface prep catalyst that is sold separately is not generally necessary.

 

[Bunky f. knuckle]

Spackle compound??

 

[ben_beyer]

I will see if I can find some thin gasket material to use!

 

[Tired Old Man]

Spackle compound??

Hmmm, that might just fill all those pits in the cheap cylinders, lol.

 

[ben_beyer]

Is this something even worth rebuilding?

I will probably keep it though I don't know what I would put it in or I might try to sell the motor.

 

[Tired Old Man]

If bearings aren't worn out, the crank, rod, and crankcase are good, it's worth re-building. OTH, if newer engines in similar size with great reliability can be had for less $$ the decision becomes one of desire over performance. Sometimes it's more fun do do what you want to do instead of what makes the most sense. Rebuilding engines is fun for some people, and the reward of good performance obtained with your own hands far outweighs expedience and economics.

 

[ben_beyer]

I can get the parts for a reasonable price so I will probably just rebuild it, hit the parts with some ATF, and mothball it (so to speak) in case I ever need it. I don't really want to get another 50cc plane, but it would be nice to have it should I ever destroy my DA 60 and not have the $$$ to get another one.

 

[Tired Old Man]

Good call.

 

[ben_beyer]

I haven't bought any parts for my motor and it was never excessively lean so I decide to look at the ring again. It was difficult to remove and I think it's really ring sticking to the piston that was causing my problems. I put the ring in the cylinder to see how much gap was there and there isn't much. I cleaned out the ring groove and now I have to compress the ring to get it back in the cylinder. I'm probably going to still buy a Bowman ring for it but I'm glad I thought of this.

 

[Tired Old Man]

Look where the ring ends normally ride in the cylinder. It will be easy to see as a straight vertical line up the cylinder. Insert the ring in the cylinder the same way it would sit if it was on the piston aligned with that line. Slide the ring up the cylinder until it sits just at the top of the exhaust port. If you have the piston off the rod slide the piston up under the ring to square it in the cylinder. measure the end gap with the ring at the top of the exhaust port. That will be a more accurate dimension because the bottom of the cylinder is not worn as much.

You probably have a lot of life left in the engine in any case.

 

[red7fifty]

TOM (Pat) is ring sticking likely caused by rich oil/ needle settings?
And, if so, would switching to a quality synth, (say Redline at 40:1) promote cleaning such build up that is causing the ring to stick? Or, would a tear down be the most effective way to clear up a sticking ring problem?

 

[Tired Old Man]

Once a ring has stuck there's only one way to get to the issue, and that's to remove the top. The ring has to come off to clean the carbon that has built up on top of, and at the inside of the ring. Changing oil at this point isn't going to help. One also has to determine if the ring became hot enough or run for so long in that state it lost it's tension. One should, as is being done in this instance, check the ring end gap for excessive wear to determine if a new ring is needed.

Ring sticking can happen with any oil type if everything else is done wrong. Too much of any oil won't cause the problem but continuous hot running and lean tuning are sure bets for it to happen eventually.

 

[ben_beyer]

What's an acceptable ring gap on a single piston ring engine?

Also, what's good for cleaning up and carbon deposited in the ring groove?

I removed most of it last night with a small screwdriver (being careful and not forceful) but there is some left in the groove.

 

[red7fifty]

Have you tried a tooth pick, or edge of card stock? Or, is it harder than that?

 

[Tired Old Man]

An end gap over ~0.018" to 0.020" might be viewed as end of ring life on a small engine. There's really no set number for these things but once they wear enough they just don't make a good seal.

I use an old broken piston ring to clean the ring grooves. Make a 45* bevel on one of the broken ends and carefully drag it around the circumference of the groove after dripping some cutting oil in the groove. I haven't tried it but some spray type carb cleaner might help out. I'm not keen on using a blade where the angle can be changed a lot because pistons are pretty soft, and enlarging the ring groove is not a good thing to do. I'm against using a screw driver for cleaning anything. An alternate would be to modify an Exacto blade by rounding the end to prevent a point from scoring piston at the back of the groove. Go slow, use care, there's no reason to race through the process. Spotless isn't necessary. In any event, you're better off not cleaning the ring groove at all if you suspect you might end up gouging on the metal.

The bottom of a ring pretty much never needs to be cleaned, the crud is always on top and on the inside edge. Given a choice between cleaning the inside edge of the ring and the inside of the ring groove, do the edge and omit the ring groove. You can scrape, carefully, the inside of the ring with a sharp blade, but vastly better is to use some semi worn out green Scotchbrite and some oil to remove the carbon. When doing the top of the ring lay it flat on a hard surface to support the ring. Don't use the Scotchbrite to hold the ring while scrubbing top and bottom at the same time. You do not want to round the corners of the ring, which happens if you wrap the cleaning pad around the ring.

If you've done it a lot you can cheat and do things really quick, but only if you've practiced a bit. 1,200 to 2,000 grit wet or dry polishing paper (auto paint store) laid on a very flat surface like glass. Apply a few drops of cutting oil to the paper, lay the dirty side of the ring in the oil, apply light and equal pressure to the ring and move it in an orbital manner on the paper just enough to clean the carbon, not polish the metal. Polishing the metal means you just made it thinner. 3 in 1 oil, ATF, Marvel, or similar can be used if you don't have cutting oil. Heck, even your 2 stroke mix is better than nothing.