WTB: one or two metal backplates witout vents/fuel nipples for .049

I myself am a fan of the metal backplates. I've had the same problems yours describing and I contributed it to the plastic. Larry Renger, (former engineer of Cox) stated that many of the plastic backplates would eventually fail due to a mistake made in the molds. The problem was addressed by placing a radius around each screw penetration on the inside. All of my 1980's plastic backplates failed. I purchased 10 and all of them were exceptional runner aside from one which I could never figure out why and turned it into a donor engine. I discovered though after returning into the hobby and getting all of my engines back into use that the metal backplates are equally as bad. After several attempts to figure out why some would run better and some longer, I came to the conclusion that airleaks may be suspect. I figured out that if you close the needle all the way down, using a syringe to pressurize the tank with your finger over the overflow this would indicate airleaks. It does this very well and enables you to pinpoint many problems. If air is leaking due to a poor o-ring gasket you can certainly hear it directly at the screen. If the reed is leaking, you can hear it internally. This will also tell if the needle is leaking around the threads. This is where I discovered that many of the tank back that I owned which were metal were actually cracked around the holes. The screws themselves are very problematic in which I found brand new screws usually replace the leak. I did find several hairline cracks though in the casting. I've found tank to backplate joints leaking. Wet a piece of dental floss (unwaxed) and carefully line the v-groove of the tank and this will provide a good gasket and seal. I've found the plastic tank backs to do a better job at sealing the screw area. I've also crashed and had the backplate stay on the plane while the case and screws flew off. Ken

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shawn cook wrote: I myself am a fan of the metal backplates. I've had the same problems yours describing and I contributed it to the plastic. Larry Renger, (former engineer of Cox) stated that many of the plastic backplates would eventually fail due to a mistake made in the molds. The problem was addressed by placing a radius around each screw penetration on the inside. All of my 1980's plastic backplates failed. I purchased 10 and all of them were exceptional runner aside from one which I could never figure out why and turned it into a donor engine. I discovered though after returning into the hobby and getting all of my engines back into use that the metal backplates are equally as bad. After several attempts to figure out why some would run better and some longer, I came to the conclusion that airleaks may be suspect. I figured out that if you close the needle all the way down, using a syringe to pressurize the tank with your finger over the overflow this would indicate airleaks. It does this very well and enables you to pinpoint many problems. If air is leaking due to a poor o-ring gasket you can certainly hear it directly at the screen. If the reed is leaking, you can hear it internally. This will also tell if the needle is leaking around the threads. This is where I discovered that many of the tank back that I owned which were metal were actually cracked around the holes. The screws themselves are very problematic in which I found brand new screws usually replace the leak. I did find several hairline cracks though in the casting. I've found tank to backplate joints leaking. Wet a piece of dental floss (unwaxed) and carefully line the v-groove of the tank and this will provide a good gasket and seal. I've found the plastic tank backs to do a better job at sealing the screw area. I've also crashed and had the backplate stay on the plane while the case and screws flew off. Ken

Thanks for the writeup, Ken. I have a metal Bee backplate that trouble has followed wherever I install it. I bet it has hairline cracks. I lapped it to the tank and skipped the dental floss, though I am familiar with that trick. I will investigate further. As for the needle leaking, I've subscribed to the washer and fuel tube seal and always replace the spring with that whether it seems to need it or not. It would be nice if the 5-40 screws had a tad wider head.
Looks like a couple of members have some backplates that should fix me up. Thanks for the reply,
Rusty

Silk thread works well like the dental floss. It is what is used in full sized reciprocating aircraft engines between the case halves.

I'm a big fan of the thread in the tank bowl groove as well when building a max performance engine. Most recently I tried a teflon-like thread, and it sealed up really well. Stuff is pricey though. I would stock it, but not sure how many people would actually buy.

I tend to prefer to wrap the needle valve with a little teflon tape until its pretty tight within the spraybar assembly. That said, the fuel tubing and washer setup works well too.

Matt