Author Topic: How long should it take to get some compression on a G.51 rebuild? (Read 2559 times)

Steve Helmick · « **on:** August 02, 2010, 10:17:31 PM »

Rebuilt by a West Coast stunt tuner of good repute, USA made piston ring, etc. Ten tanks of 10-29 didn't do it. Two more of 10-22 didn't help. Another half gallon of 10-29 hasn't done a thing. I'm getting discouraged. Each tank ran about 4 minutes at the start, and up to about 5 minutes now. It's turning an 11 x 4.5 TT Cyclone at around 9,500-10,000, which is at least 2,000 on the rich side of peaked, it appears. Am I doing something wrong? Are my expectations too high? Ideas of what to do???

Steve

Brett Buck · « **Reply #1 on:** August 02, 2010, 11:57:42 PM »

Quote from: Steve Helmick on August 02, 2010, 10:17:31 PM

Rebuilt by a West Coast stunt tuner of good repute, USA made piston ring, etc. Ten tanks of 10-29 didn't do it. Two more of 10-22 didn't help. Another half gallon of 10-29 hasn't done a thing. I'm getting discouraged. Each tank ran about 4 minutes at the start, and up to about 5 minutes now. It's turning an 11 x 4.5 TT Cyclone at around 9,500-10,000, which is at least 2,000 on the rich side of peaked, it appears. Am I doing something wrong? Are my expectations too high? Ideas of what to do??? Steve

Maybe your expectations are wrong, but running all that castor is likely to glue the ring into the slot and cause lost compression. I would run it on 10/18 for a long time and ti might come back.

The more important question is - how does it run in the air? If it's good, I wouldn't worry too much.

Brett

Balsa Butcher · « **Reply #2 on:** August 03, 2010, 08:24:06 AM »

I am running a ST-51 also "modded". Even with a new american made ring it doesn't have a lot of compression but it turns an Eather 11 1/4 x 5 3 bl CF prop at 10,400 rpm (and that's flying at 5000'msl) in a solid 4 stroke, never misses a beat and starts first flip. The perceived low compression I credit to the head being relieved (hemi?). Fuel is usually 15 % nitro and have used both 11% x 11% and 25% (castor) 3% syn blend. It sounds like it is ready to be put into a plane and like Brett said, if it works, don't worry about it.

Bill Little · « **Reply #3 on:** August 03, 2010, 01:49:20 PM »

Quote from: Pete Cunha on August 03, 2010, 08:24:06 AM

I am running a ST-51 also "modded". Even with a new american made ring it doesn't have a lot of compression but it turns an Eather 11 1/4 x 5 3 bl CF prop at 10,400 rpm (and that's flying at 5000'msl) in a solid 4 stroke, never misses a beat and starts first flip. The perceived low compression I credit to the head being relieved (hemi?). Fuel is usually 15 % nitro and have used both 11% x 11% and 25% (castor) 3% syn blend. It sounds like it is ready to be put into a plane and like Brett said, if it works, don't worry about it.

Hi Pete,

I have used almost the identical set up. An Eather 3 blade at 11 1/4 and 4 3/4 pitch. Tom Lay said run all castor (a 1993 build!) and i have found that in *my* situation, the engine always ran better on all castor with just an ounce or so of syn added per gallon. The engine has never let me down, flip and fly all the way.

Mongo

Steve Helmick · « **Reply #4 on:** August 03, 2010, 07:30:19 PM »

Thank you all for your replies.

To be clear: The compression has not improved at all, since new, after about 25 total runs, and about 1.5 gallons of fuel. If the glowplug was removed, it would not significantly reduce the compression. I have tried hand starting it, and on two occasions got a pop, but no starts. Fizz fizz! Once I apply the electric finger, it fires off quickly. I haven't flown it yet, but am thinking that it wouldn't hurt to load it into the airplane and go fly it some. GMA liked to do lazy 8's to break in a new engine, claiming that it sped the process along. Might do the trick, if I can remember how to do a lazy 8. Anyway, as near as I can tell, the engine is as original, except the cylinder, ring, venturi, and a bunch of engraving. If I toast the cylinder and ring, it doesn't seem like much of a loss, at present...almost good riddance.

I'm aware of the possible effect of all castor fuel in a ringed engine, but I'm trying to follow the tuner's instructions. I'll probably be running it on 10-22, once my stock of 10-29 is gone. Until I don't have any more 10-22, that is. After that, I dunno what fuel we'll be able to get 'round these parts. Wildcat 10-23 ducted fan fuel seems likely, with a few oz. of Randy Aero Snake Oil added. I'll have to cross that bridge in a year or two...

Steve

Edit: Currently, the compression IS pretty equal, whether you crank it forward or backwards...

Martin Quartim · « **Reply #5 on:** August 04, 2010, 07:03:57 PM »

wow if removing the glow plug makes no difference I think this engine has a serious problem. When some one say has poor compression is hard for us to know exactly what it means...but as you described there is no compression at all. I'd send it back to whoever worked on your engine. Some ain't right with it. There could be various problems, damaged ring groove, too loose piston fit, a huge ring gap, deep scratches on the liner or a deformed liner.

I have seen some pretty bad ST G51 piston with many castings flaws, so bad I would not be surprise if there are open flaws from one side to the other of the piston.

If it doesn't start even when hard flipping, most likely this engine will not run well in the plane and I'd be scared to go inverted and get a dead engine flame out.

Good luck,

Martin

Steve Helmick · « **Reply #6 on:** August 04, 2010, 09:05:31 PM »

No, not no compression. Even without the glowplug in any 2 stroke, there will be the "base" compression, or vacuum, as the engine is turned over without the glowplug. This one is almost that bad with the glowplug. Maybe I should shove a new plug in it, just to be sure the plug's seal hasn't done blowed up (NASCAR lingo, dontcha know). It's in the truck...maybe I'll go run it again tomorrow after work, then stick it in the plane tomorrow night and start flying it Saturday. Leaving a week from today for "Perry Fire", in Edmonton. Grass field, so if it flames "oot", it won't hurt a lot! $:-\$ Steve

Steve Helmick · « **Reply #7 on:** August 07, 2010, 04:06:50 PM »

Flew the G.51 today. Had a couple flights on it, when Mike Haverly arrived. Flipping the prop, he said "It feels like an electric!"

Not a bad description, actually.

I put on a 12.25 x 3.75 APC (thanks for the tip, Dan McEntee!) and launched first flight at 9.9k. A little fast for the line length, but longer lines will help that. I can't recall if these are 62' or 64'. I stayed with the 10/29, but will be changing to 10/22 or 15/20 in Edmonton. It doesn't start worth beans inverted, electric finger and all. Seems like it doesn't have enough fuel draw to suck fuel up into the venturi, but with the plane upside down, it's ok. I'll just do that for now. After the two contests on the next two weekends, I'll arrange for the parts to go see Frank Bowman. That's all for now!

Steve

Brett Buck · « **Reply #8 on:** August 07, 2010, 04:54:38 PM »

Quote from: Steve Helmick on August 07, 2010, 04:06:50 PM

Flew the G.51 today. Had a couple flights on it, when Mike Haverly arrived. Flipping the prop, he said "It feels like an electric!" Not a bad description, actually.

I put on a 12.25 x 3.75 APC (thanks for the tip, Dan McEntee!) and launched first flight at 9.9k. A little fast for the line length, but longer lines will help that. I can't recall if these are 62' or 64'. I stayed with the 10/29, but will be changing to 10/22 or 15/20 in Edmonton. It doesn't start worth beans inverted, electric finger and all. Seems like it doesn't have enough fuel draw to suck fuel up into the venturi, but with the plane upside down, it's ok. I'll just do that for now. After the two contests on the next two weekends, I'll arrange for the parts to go see Frank Bowman. That's all for now! Steve

   Before that, try this. Get some 10/18, or better yet, SIG Syn-Power, put engine on bench. Put on a 9-6 propellor, and about a 16 ounce tank. Start it, peak the engine out, back off just enough to tell that it's not too lean. I would guess it will be going about 14,000+ rpm. Let it run out the tank, then cool off, then check the compression. This is about what we used to do with ST46s to break the ring free without disassembling and cleaning it (and maybe damaging the ring groove in the process). Except I used an 8-4.

   Alternately - diassemble the engine, down to removing the ring from the piston, and put all the metal parts in the crock pot with anti-freeze, and clean everything until it's back to bare metal. Use a piece of Ringmaster-grade 1/16th balsa to be sure and clean the groove out without taking any chance of damaging it. Then tape a piece of 400 wet/dry to a piece of glass. Use this to *very gently* lap the bottom surface of the ring using water. You just want to get a consistent gray surface, just break the glaze, not take off any significant metal. Clean it to remove any grit. Take a brake cylinder hone and do the same to the liner surface - turn very slowly by hand or with a lowest speed possible on a drill, with a lot of oil - same thing, you just want it scratched uniformly. What you are liable to find is that there are circumferential high spots in the bore. They may already be shiny or visible from running. Clean everything, reassemble, and compression should be pretty good. If not, you can try the above procedure, but you are probably shafted.

   Let me ask this question - when you look in the exhaust, what does the ring look like? Light gray or silver? Or brownish? I bet light gray or silver. That means you are probably shafted, and the ring material is the problem. That happened all the time - for a supposed engineering material, meehanite varies all over the place from batch to batch,.

   All this sort of stuff was pretty commonplace back in the dark ages with ringed engines. That's why as soon as I could, I got an ABC. Also bear in mind that this is all based on very-hard-won experience with ST46s, but the compression/ring seal part should be the same.

   Brett

Steve Helmick · « **Reply #9 on:** August 07, 2010, 06:56:06 PM »

Brett...Thanks for the ideas. About a week and a half-ago, I pulled the engine apart, to check the ring for being stuck or not. It's completely loose in the piston...and very brown. It's a Bowman ring, or at least, is supposed to be. The cylinder bore had no sign of crosshatching, but had some very shiny spots, mostly around various ports. I went after it with my brake cylinder hone (about 30 years old) until I got an even crosshatch on the whole bore.

I put it back together and ran about 8-10 more (5 oz) tanks of 10-29 through it, with the 11 x 4.5 TT prop. No change worth mentioning. Fizz-fizz...waaaahhhh! Still, once running, it's not all that bad. It starts well, upright, with the electric finger. Not ever, inverted, regardless. Previously, I don't think I ever turned it over to start, but the Bruline filter no doubt helped.

The longer lines appeared in today's mail. Jim Snelson is a good dude! Off to Edmonton at 4:30 am Wednesday for "Perry Fire". C'mon up. Bruce is providing airplanes for all...

Steve

Bill Little · « **Reply #10 on:** August 07, 2010, 09:01:19 PM »

HI Steve,

Why the hesitation to return it to the engine builder? That's the first thing I would do, for sure. I have had to do it one time in the past, but it was worth it. problem was fixed and the engine was one of the best I have ever had (an OS .40VF).

Bill

RandySmith · « **Reply #11 on:** August 08, 2010, 10:35:39 AM »

Quote from: Steve Helmick on August 07, 2010, 06:56:06 PM

Brett...Thanks for the ideas. About a week and a half-ago, I pulled the engine apart, to check the ring for being stuck or not. It's completely loose in the piston...and very brown. It's a Bowman ring, or at least, is supposed to be. The cylinder bore had no sign of crosshatching, but had some very shiny spots, mostly around various ports. I went after it with my brake cylinder hone (about 30 years old) until I got an even crosshatch on the whole bore.

I put it back together and ran about 8-10 more (5 oz) tanks of 10-29 through it, with the 11 x 4.5 TT prop. No change worth mentioning. Fizz-fizz...waaaahhhh! Still, once running, it's not all that bad. It starts well, upright, with the electric finger. Not ever, inverted, regardless. Previously, I don't think I ever turned it over to start, but the Bruline filter no doubt helped.

The longer lines appeared in today's mail. Jim Snelson is a good dude! Off to Edmonton at 4:30 am Wednesday for "Perry Fire". C'mon up. Bruce is providing airplanes for all... Steve

Steve

When you re ring the motor you need to first check to see that the sleeve is ..round, and has not worn into a bell shape at the top..
clean everything put the ring into the sleeve and use the top of the piston to push the ring up too the top were it would go if in the engine mounted in the piston,
look for light around the sides of the ring, if any the ring is worn or warped, or the sleeve is worn or warped.
Also check the ring gap in the middle of the sleeve and also near the top of the sleeve, if it is open more at the top you need a new sleeve, of course check for gouges or cuts.
Caution when sliding the ring into the sleeve be very careful to NEVER let the end gaps slide past on open port, arrange the ring so it always slide on continuous metal
Check the rod for any excessive wear at the wrist pin and crankpin, check the crank pin, and check the pin boss in the piston for any wear, if you have excessive wear there the piston will rock and you will never maintain a good seal.
If all is OK and you have honed the sleeve lightly use a new tight ring, put it up and recheck just as you did with the current ring, set the gap at between .001 and .003 depending on the grit hone you used and how aggressive you got with it,
reclean everything and make sure the ring groove in the piston is clean and has NO damage or burned on Castor.
install the ring and put the rod and piston on the shaft
The ST 51 ring has a notch cut into the inside of the ring to fit with the roll pin that is pressed into the top of the piston...CRITICAL,,set the notch to match and be extremely careful when you slide the sleeve back into the case,
I use a liberal amount of an assembly lube, or use grease or use Castor
You may have to wiggle the sleeve a ting tiny amount getting it to compress the ring when you do the install...This is where most damage occurs in re ringing a ST 51
I have setup over a 1000 ST motors and over 100 of them have been ST-51s, this procedure is for the 51.
You should have good compression within 5 runs....make SURE you keep the engine rich and cool during the first couple of runs and NEVER let it get hot and lean.

Randy

Steve Helmick · « **Reply #12 on:** August 08, 2010, 12:41:55 PM »

Thank you for your input, Randy. The engine is Italian production, FWIW. The new cylinder could be Italian or Chinese, replacing the retimed R/C cylinder that actually ran pretty well. It was also supposed to be honed round and the new Bowman ring fitted properly. I fully expected decent compression after the first dozen runs. After the crosshatching and 8-10 more runs, I again expected a significant improvement. I was (relatively) elated yesterday, when I actually got a "bump" when pulling the prop through, and it almost started without the electric finger...that was a first, since the rebuild. It's not getting worse, anyway...

One detail that is bothering me...the sleeve is a shrink fit in the case. It isn't coming out or going in, without a heat gun being applied. Is this a correct fit? I think it's a good thing for heat transfer, but it does make assembly more challenging. Plus, I've got to wonder about how true and round the two are. I've been very careful with the ring/piston/sleeve reassembly, as you explained. Without any spare rings, I can't take this one off and re-use it, so it is what it is, for a couple more weeks. If it improves, fine, but if it doesn't, I'll have a decision to make.

Steve

RandySmith · « **Reply #13 on:** August 08, 2010, 01:41:01 PM »

Quote from: Steve Helmick on August 08, 2010, 12:41:55 PM

Thank you for your input, Randy. The engine is Italian production, FWIW. The new cylinder could be Italian or Chinese, replacing the retimed R/C cylinder that actually ran pretty well. It was also supposed to be honed round and the new Bowman ring fitted properly. I fully expected decent compression after the first dozen runs. After the crosshatching and 8-10 more runs, I again expected a significant improvement. I was (relatively) elated yesterday, when I actually got a "bump" when pulling the prop through, and it almost started without the electric finger...that was a first, since the rebuild. It's not getting worse, anyway...

One detail that is bothering me...the sleeve is a shrink fit in the case. It isn't coming out or going in, without a heat gun being applied. Is this a correct fit? I think it's a good thing for heat transfer, but it does make assembly more challenging. Plus, I've got to wonder about how true and round the two are. I've been very careful with the ring/piston/sleeve reassembly, as you explained. Without any spare rings, I can't take this one off and re-use it, so it is what it is, for a couple more weeks. If it improves, fine, but if it doesn't, I'll have a decision to make. Steve

The G-51 in both Italian and China versions is a press fit into the case, the case bore should be very smooth with zero defects..scratches, gouges etc
The sleeve has the bottom turned down so you can put the bottom edge of the sleeve into the case and get the ring started ....before...you heat the case and press it all the way down.
I never would reuse a ring, but It does help to check the old ring if you are having problems
Randy

Randy

Steve Helmick · « **Reply #14 on:** August 08, 2010, 10:14:34 PM »

Quote from: Bill Little on August 07, 2010, 09:01:19 PM

HI Steve,

Why the hesitation to return it to the engine builder? That's the first thing I would do, for sure. I have had to do it one time in the past, but it was worth it. problem was fixed and the engine was one of the best I have ever had (an OS .40VF).

Bill

Bill...After the first 12 runs, I contacted the tuner by email and told the story...10 runs on 10-29, very rich, at about 10k on an 11 x 4.5 TT prop, and after some discussions with a more experienced user of ringed engines (who suggested that the ring might be stuck in after-run/assembly oil), ran two tanks of 10-22. Etc. He didn't want to hear about it, and stated that "sometimes, G.51's just don't work out, and you need to buy a new engine." That's why I am not likely to send it back to the tuner. Plus, based on 36 years of machining experience, I don't believe the cylinder was ever honed. And it took more than a year to get it back! This is the 5th engine this guy has 'done' for me, but the first one I've run. I think he was PO'd that I ran the 10-22 in it, or maybe just had a bad day. But I don't subject myself to that sort of crap more than once...

Steve

Brett Buck · « **Reply #15 on:** August 09, 2010, 12:04:07 PM »

Quote from: Steve Helmick on August 08, 2010, 10:14:34 PM

He didn't want to hear about it, and stated that "sometimes, G.51's just don't work out, and you need to buy a new engine." That's why I am not likely to send it back to the tuner. Plus, based on 36 years of machining experience, I don't believe the cylinder was ever honed.

And, for the record (and I know that you know this, but not necessarily everyone else), if it needs any significant honing, my brake cylinder hone trick *is not acceptable*. That's just for roughing it up, if you remove any significant metal it will probably screw it up. You need a real machine intended for the purpose.

If this liner is a shrink-to-fit affair, Randy might be on to something. Obviously if they internal bore of the case is not correct, when you shrink it over the liner, any imperfections in the case will be transferred to the liner. Say , out-of-round, or a high spot. It should be obvious from examining the bore after running it - any lumps should show up easily. I can't think of any easy way to fix it - if it's not right, and you try to hand-sand out the high spots, chances are you will create low spot, endlessly until you give up and by a new case. Or a PA51.

I still think you should try my run like an R/Cist plan. It will either work, or ruin it. If it ruins it, your solutions are exactly the same as they are now - replace the cylinder (and probably the piston, too) with stock units. Or get a PA51.

Brett

Bill Little · « **Reply #16 on:** August 09, 2010, 12:23:39 PM »

Quote from: Brett Buck on August 09, 2010, 12:04:07 PM

(snip) endlessly until you give up and by a new case. Or a PA51.

I still think you should try my run like an R/Cist plan. It will either work, or ruin it. If it ruins it, your solutions are exactly the same as they are now - replace the cylinder (and probably the piston, too) with stock units. Or get a PA51.

Brett

Hi Brett,

Or maybe a PA .51?

Big Bear

Steve Helmick · « **Reply #17 on:** August 09, 2010, 05:53:04 PM »

The engine ran fine with the original cylinder, so I expect the case bore is pretty smooth & round, tho I really didn't look. Generally, I notice stuff that's whacked. When I did inspection, I'd reach into a box of 100 parts and pull out one part...and it was the one bad part. Never figured out why. Maybe it happened again?

Maybe I should slot out the front bolt holes of one of the other G.51's I've already had "done" by the same tuner?

I think I'll bench run them before I slot the lugs. More $ on eBay with nice pristine bolt holes...

Steve

Author Topic: How long should it take to get some compression on a G.51 rebuild? (Read 2559 times)

Steve Helmick

How long should it take to get some compression on a G.51 rebuild?

Brett Buck

Re: How long should it take to get some compression on a G.51 rebuild?

Balsa Butcher

Re: How long should it take to get some compression on a G.51 rebuild?

Bill Little

Re: How long should it take to get some compression on a G.51 rebuild?

Steve Helmick

Re: How long should it take to get some compression on a G.51 rebuild?

Martin Quartim

Re: How long should it take to get some compression on a G.51 rebuild?

Steve Helmick

Re: How long should it take to get some compression on a G.51 rebuild?

Steve Helmick

Re: How long should it take to get some compression on a G.51 rebuild?

Brett Buck

Re: How long should it take to get some compression on a G.51 rebuild?

Steve Helmick

Re: How long should it take to get some compression on a G.51 rebuild?

Bill Little

Re: How long should it take to get some compression on a G.51 rebuild?

RandySmith

Re: How long should it take to get some compression on a G.51 rebuild?

Steve Helmick

Re: How long should it take to get some compression on a G.51 rebuild?

RandySmith

Re: How long should it take to get some compression on a G.51 rebuild?

Steve Helmick

Re: How long should it take to get some compression on a G.51 rebuild?

Brett Buck

Re: How long should it take to get some compression on a G.51 rebuild?

Bill Little

Re: How long should it take to get some compression on a G.51 rebuild?

Steve Helmick

Re: How long should it take to get some compression on a G.51 rebuild?