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Cox Engine of The Month
September-2021
crankbndr's

"Cherry Bomb" .051 engine

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Post  anm2 Sat Aug 03, 2013 7:59 am

I recently purchased an OK Cub .074 engine off ebay and it arrived this week. It is a very nice engine and looks almost new, however it has definitely been run, its just been well cared for. It is probably post 1950 based on the metal tank and no serial number. It started up very nicely and runs very smoothly, however, RPM seems very low. It came with a 7-5 (I think it is 7-5 might be 7-4) prop which it ran at about 8,000 RPM on 10%. I also ran a 6-3 prop and it made about 9,000 rpm. Even with 25% nitro it doesn't seem to pick up much RPM. The glow plug is strong and it really starts up very easily and the piston head looks obviously used, but overall in good condition. Is this what I should expect from the engine or does anyone have any ideas on how to pick up the power. What size prop should I use on this engine? Very nice display model, but I am hoping to fly it too. Thanks, Andy
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Post  SuperDave Sat Aug 03, 2013 8:57 am

Andy:

How strong is the compression on the Cub .74? You may be missing some gaskets: head, crankcase, etc.

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Post  anm2 Sat Aug 03, 2013 9:10 am

@SuperDave wrote:Andy:

How strong is the compression on the Cub .74?  You may be missing some gaskets: head, crankcase, etc.

SuperDave

The compression for this engine seems a little weak, however, it is just like every other OK Cub I have owned and/or held. I have frankly never had an OK Cub that had compression as good as a Cox. I have a brand new Cub .049 RB and I would say the .074 compression is almost as good. I did remove the Glow head and the gasket for the glow head was present and in good condition. Since this is a front rotary valve, and not a reed engine, I am bench testing it using an external tank, so have not been using the metal tank. I have not removed the metal tank, however, and I certainly should check that out. The ease with which this starts, however, tends to make me think that the gaskets are probably not the problem. If all my engines started as easily as this one, I would be a happy man! Thanks for the input.
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Post  anm2 Sat Aug 03, 2013 9:56 am

@SuperDave wrote:Andy:

How strong is the compression on the Cub .74?  You may be missing some gaskets: head, crankcase, etc.

SuperDave
Dave,

I removed the tank, and there was a backplate. I removed the backplate and found the gasket was in place and looked OK. I tightened it back up and then retested it. I am still only getting 8500 RPM on a 6-4 prop with 25% Nitro. May be just worn out. Thanks again. Andy
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Post  SuperDave Sat Aug 03, 2013 10:22 am

Andy:

When you "flip" the prop on the Cub does it seem to have good compression compared to othe similar sized engines?

EDIT:   I repeat:  Does it have head gasket?

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Post  anm2 Sat Aug 03, 2013 12:12 pm

@SuperDave wrote:Andy:

When you "flip" the prop on the Cub does it seem to have good compression compared to othe similar sized engines?

EDIT:   I repeat:  Does it have head gasket?

SD
Dave,

Yes, it does. I checked and there is a good head gasket. Andy
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Post  ticomareado Mon Oct 07, 2013 3:44 pm

Probably the biggest waste of c.i. dispacement in glow engine history. Loud, vibrate like a buckboard on the Oregon Trail and never more that 10k no matter what you do short of sending them to Ron Valentine.
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Post  wha-tah-hey Thu Dec 05, 2013 10:44 pm

Andy -

I think there should also be a bronze base gasket bettween cyl & crankcase.

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Post  Mark Boesen Thu Dec 05, 2013 10:57 pm

It's a little low, but about par for the course, Cubs had pretty mild timing to help with starting, they also had terrible quality control, worse then Testors/McCoy some ran decent, very few ran great, most ran like yours. Use lots of castor and have fun with it. You're ok on prop size, you might check and see if you have more then one copper head gasket.
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Post  anm2 Fri Dec 06, 2013 5:24 am

@Mark Boesen wrote:It's a little low, but about par for the course, Cubs had pretty mild timing to help with starting, they also had terrible quality control, worse then Testors/McCoy some ran decent, very few ran great, most ran like yours. Use lots of castor and have fun with it. You're ok on prop size, you might check and see if you have more then one copper head gasket.
With regard to the copper head gasket, are you suggesting I add another one, or that another one may be already on it? Andy
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Post  RknRusty Fri Dec 06, 2013 6:13 am

Andy, I think he means it might have two stuck together and looking like one at first glance.

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Post  gcb Sun Jan 05, 2014 5:23 pm

For goodness sake, you are running an engine that originally appeared in the early fifties. It ain't no NORVEL.
Run it with a 6x4 prop and ~25% nitro, 20% castor. It is designed to start easy and provide moderate (low by today's standards) power. Size the plane accordingly. We flew on 26' Dacron lines back then.
Good luck and enjoy it for what it is.

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