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GB backplate to tank seal

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GB backplate to tank seal

Post  wha-tah-hey on Sat Oct 29, 2016 6:29 pm

I don't see a seal of any sort between the backplate and tank - do they have/need one?

Thanks
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Re: GB backplate to tank seal

Post  Marleysky on Sat Oct 29, 2016 7:05 pm

Not usually. The tank to back mate up thru a small groove. If you find you have leaks there, you can use dental floss, or a thin bead of silicon. TWO very important things when rebuilding a cox engine. clean and clean again, no dirt anywhere. Recommend reading Larry Rengers care and feeding of Cox engines located at the very bottom of the instructions sheets page.
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Re: GB backplate to tank seal

Post  crankbndr on Sat Oct 29, 2016 7:10 pm

There is a female grove in the tank back and a male lip on the backplate, I use toothpick and solvent to clean very well.
Then I lap the two together with polishing compound the seal is excellent. However the screw holes are another story, sometimes use some silicone to fill the screw holes
after the screws are set. It is easy to remove for disassembly. While the backplate is off I flush the passages with a syringe pushed on the fuel inlet, remove the needle.
I use a white cup with fuel and you won't believe what comes out and goes to bottom of cup.
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Re: GB backplate to tank seal

Post  wha-tah-hey on Sat Oct 29, 2016 7:12 pm

Thanks, reading now. Very Happy

I got my 2 eBay beauts today, so I'll go through 'em and clean 'em well before I try running.

Thanks, guys!
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Re: GB backplate to tank seal

Post  Ken Cook on Sat Oct 29, 2016 7:38 pm

I use dental floss in the groove on the tank. A real pain in the rear to install. It starts to lay in real nice and then it comes out. It never goes in the first time. I generally lay a bit of oil on my fingers and pull the dental floss through it. I overlap it about 1/8" and assemble. As for the screws, I tear the top of a Q-tip and off and shred it into a 2-3 pieces. Roll it in between your fingers into a string. Wrap it around the the top of the screw heads in the direction that tightens it up on the screw shaft when your tightening it. This makes a great seal and is service removable unlike silicone.
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Re: GB backplate to tank seal

Post  wha-tah-hey on Sat Oct 29, 2016 7:52 pm

Thanks for the tips, Ken.
I wet dental floss with saliva - worked it around with both thumbs to keep it from pulling out and wasn't too much hassle.
I'll try the Q-tip seal on the screws.
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Re: GB backplate to tank seal

Post  RknRusty on Sat Oct 29, 2016 8:17 pm



Off topic but important:
Sometimes the threads in the back of the crankcase are not tapped far enough into the holes. This gives you the false impression that the tank is seated tightly to the case. The holes are plenty deep, so you can use a flat tipped 2-56 bottoming tap and add a few more turns to the threads. I've seen the thread depth vary widely, and it took me a while to figure out what the problem was when the tank to crankcase joint was wet.
Rusty

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Re: GB backplate to tank seal

Post  wha-tah-hey on Sat Oct 29, 2016 8:25 pm

Good to know, Rusty, and not really so much off-topic for me.
Though I've owned a few Coxes for several years, I never fiddled with 'em,
Having finally grown into the wisdom to appreciate 'em, I'm now trying to find out all I can about them, so your tip is plenty timely for me.
Saves me having to post that question one day. Very Happy

Thanks.
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Re: GB backplate to tank seal

Post  Marleysky on Sat Oct 29, 2016 10:34 pm

Another good read for cox engine rebuilding is by Paul Gilbeault
Located at ex models web page: http://coxengines.ca/files/MRP.pdf
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Re: GB backplate to tank seal

Post  wha-tah-hey on Sat Oct 29, 2016 11:04 pm

Marleysky wrote:Another good read for cox engine rebuilding is by Paul Gilbeault
Located at ex models web page: http://coxengines.ca/files/MRP.pdf

Yes, very much so. That one was recommended previously in another thread and I have it in my files also.
I'm not (yet Very Happy ) interested in high performance, but Paul has a lot of very useful info applicable to any Cox engine.
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Re: GB backplate to tank seal

Post  RknRusty on Sat Oct 29, 2016 11:28 pm

wha-tah-hey wrote:...
I'm not (yet Very Happy ) interested in high performance, but Paul has a lot of very useful info applicable to any Cox engine.
You're taking the right approach. Aside from exotic modifications to internal cylinder, NVA, and venturi architecture, most of the high performance is gained simply in the form of plain old fitment and elimination of air leaks.
Rusty

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Re: GB backplate to tank seal

Post  wha-tah-hey on Sun Oct 30, 2016 9:24 am

Tach and pure racing are as legitimate as any interest, and I was at first taken with the notion of a hot engine.
Then I realized that my real interest was simply sport flying and I had no idea whether a stock engine wouldn't do just fine for my needs.
"Don't fix it if it ain't broke" applies here for me unless I find I do need more power.
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Re: GB backplate to tank seal

Post  wha-tah-hey on Sun Oct 30, 2016 12:24 pm

And a timely lesson - I got 2 eBay product engines yesterday, 1 with a Tee Dee head.
I ran 'em on the bench and the standard head gives ~800 more rpm over the TD on Cox 15%.
Higher nitro might reverse those results but it seems achieving high rpm is a process, not necessarily simple.
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Re: GB backplate to tank seal

Post  fredvon4 on Sun Oct 30, 2016 1:54 pm

EXACTLY!

Cox standard
Cox High Compression
Adapter for normal glow plug
Merlin drop in with clamp (GloBee style)
Galbraith cut for Nelson plug

Plug heat range

Piston fit
Piston TDC height
SPI amount
Head gaskets

Crank to case interference
Thrust washer to case interference
By pass and boost port configuration

and a whole lot of other variables like venturi size, NVA and air leaks

All combined make for a very wide range of power/torque/rpm differences---- and other run issues

OR, all is so well that the engine practically starts itself and will spin a 6x3 at 18,000 RPM

Don't forget fuel Vitamin N and total oil are significant contributors also

Too bad this lil buggers are no longer found for $4+/- each.....

I still prefer the copper Beryllium reeds... and that is yet another area that can make a huge difference in rum and power


lots to fuss with
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Re: GB backplate to tank seal

Post  wha-tah-hey on Sun Oct 30, 2016 2:51 pm

And a BIG thanks to you, Fred - you just put the lid on my yearnings for high performance. Too much fiddling for me these days! Very Happy
And I'm glad you mentioned the reeds - both engines came with the CuBe reeds and I was gonna try the .005" mylar - wouldn't seal in either (think they're too thick - the spring warps 'em?) so back to the CuBe.
The mylar work fine in my GB and Pee Wees though (tying all this to the OP Very Happy ).
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Re: GB backplate to tank seal

Post  crankbndr on Sun Oct 30, 2016 3:16 pm

A good cleaning is probably the most important thing on an old run engine, like Ken said somewhere, even the crank will have old castor varnish.
Ive found that even on old engines never run the crank needs cleaning and the bore, I drop the cylinders in hoppes 9 for a few min. and they are like new.
Flush intake with fuel and they run like new.
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Re: GB backplate to tank seal

Post  wha-tah-hey on Sun Oct 30, 2016 3:26 pm

Point belatedly taken, Crank - yesterday's cleaning wasn't good enough.
I ran both and they're really flaky, so I'm gonna take 'em all the way down and be really fastidious about cleaning everything this time.
I'll be happy if they run consistently at the top rpm they showed today.
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Re: GB backplate to tank seal

Post  roddie on Sun Oct 30, 2016 6:52 pm

wha-tah-hey wrote:And a timely lesson - I got 2 eBay product engines yesterday, 1 with a Tee Dee head.
I ran 'em on the bench and the standard head gives ~800 more rpm over the TD on Cox 15%.
Higher nitro might reverse those results but it seems achieving high rpm is a process, not necessarily simple.

What exactly are you using for 15% fuel Terry? The engines will run on that % nitro.. but ideally you want 25%+. You can order it in pints.. direct from Sig Manufacturing Co. up in Iowa. I've had great luck with their "Champion 25 and 35" blends. Don't run just any glow-fuel in your Cox engine. I apologize if that's general knowledge on your part.. or if repeat-discussion.

Variables are plenty.. but make a checklist and keep data of your tweaks/combinations/configurations. You already have various data/trouble-shooting resources to work with. You'll make those engines sing! It's just a matter of experimentation.

Tankless/product engines (like your recent acquisitions) are easier to deal with IMO. They eliminate some variables. You have good compression in both engines. Try swapping plugs.. one might have a better coil/element than the other.. or an insulator/seal leak. Was there more than one head-gasket used on the engine with the Tee Dee head? Just curious. There's usually a reason why someone runs one on a reed-valve engine.. but not necessarily. It could have just been thrown on there to sell it. Maybe try your GB plug in place of it.. and see if there's a marked difference in performance.

The reeds/retainers.............. plenty of discussion to be found here on that. I tend to use a reed that was designed for the retaining-system. Some people thin-out a wire/circlip retainer designed for a .001" thickness (CuBe) reed.. to retain a thicker .005" Mylar reed, whether star-shaped or oblong. It just has to work.. and an easy way to tell.. is to apply the blow/suck method using your mouth over the reed-holder. When blowing (not forcefully) the reed should seal. If air passes or it "squeaks"... check/clean the seat/reed.. refit/re-orient the circlip and try again until a definite open/close action is established. I've found the later Cox "cap-style retainer/Mylar-reed" combination to be much less sensitive to this attention to function.. but whichever; it's basically the heart of the reed-valve engine. Don't ever assume that  a reed is functioning in a "stale" engine. The reed can become stuck to the seat if run.. then left/put-away un-cleaned for a time. A good soak in solvent will loosen it. The copper reeds will deform/crease easily.. so care must be taken if one is stuck-tight.

I'll need to "practice what I preach".. with Ron Cribbs coming up to RI early next month!
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Re: GB backplate to tank seal

Post  wha-tah-hey on Sun Oct 30, 2016 7:29 pm

Hi Roddie -
As a benchmark I'm using my GB, which again keeps us on-topic (sorta). Very Happy Huh...
It's a can of original Cox Super Power 15% - found it new last year in a 2nd-hand shop - 2$ Very Happy
When I buy more fuel, I'll buy 25%.
I have a feeling these both may have been assembled to sell.
The one came with TD head (1 gasket) and a spinner but less 1 horseshoe backplate screw; the other had no c/c gasket, a marred standard head, a heavily oxidized spring starter, a horseshoe backplate but a Sure-Start brass prop driver ; both have the CuBe star reed in good condition and sealing properly.
Both appear to be in pretty good shape though, no varnish or external scuffs, etc.
I've torn the 2nd mentioned down and cleaned it thoroughly - I'll do the other tomorrow and give 'em a bit of a break-in before I see how they run.
For $21, I'm not complaining. Very Happy
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Re: GB backplate to tank seal

Post  pkrankow on Mon Oct 31, 2016 2:04 pm

Flaky operation is usually an air leak. Aluminum safe sealant sparingly applied to the gaskets and rim of the tank and under the heads of the screws can be a game changer for the better.

I get stuff called "anaerobic gasket maker" by Permatex from the auto parts store. It is kinda pricey, like $10 for a 1 oz tube, but is enough for dozens of engines.

Phil
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Re: GB backplate to tank seal

Post  wha-tah-hey on Mon Oct 31, 2016 2:21 pm

And I'm finding out that leaks can be a chore to isolate - great solution, Phil - thanks!
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