67 CMB GreenHead specs for 1/8 Scale

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longballlumber

Well-Known Member
Joined
Apr 17, 2003
Messages
1,548
I am helping "guide" a local friend thru the process of checking out a used .67 CMB GreenHead motor before he fires it up.  I am far from a motor guy, but I think I can help him thru the basics if I can get a little help from you guys.  The numbers I am requesting are intended to be conservative.  I want the boat to get off the beach an on pipe....

Head Clearance?

is there a diameter range on the lower rod needles to look for? (how many are there)

I can help check bearings and P/L fit.

Pipe length (don't know the pipe brand yet)

Any info will be appreciated.  I will follow and let you guys know how it goes..

Thanks,

Ball
 
I've run them a bunch just not in a Scale, did help a buddy with one in a scale boat tho.

I like the head at 0.012".  Needles check new at about 0.0590", after 75 -100 runs they'll wear a tenth or so that's when I replace them along with the bearings.  If they look blue at all I change them also.  Can't remember off the top of my head how many there are but the space will be almost completely filled.

I'd run a bigger volume pipe on the long side to start, maybe 12" C/L of cylinder to mid point of the dead band.  Don't cut the low end needle out!  When you get it right it'll idle down to a crawl and go!

You don't havta do anything to them!  They're stump pullers!  :)
 
Last edited by a moderator:
Back again,

Different engine, but still a 67 CMB green head. I was helping a friend by tearing down to check the bearings. In the process I found the crank pin had some damage. Can anyone tell me what would cause something like this. I kind of expected to find some damage on the needles, but they all looked fine and measured at .0586
 

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I've had that happen to a 45 crank. Needles looked fine as well. Only thing I could think of was something passed thru the motor or lean run. Possible bad material on the crank. Mine looked like the metal pealed off of it which led me to think a material issue, but I had burned some plugs up in it and the rotor has that standard CMB groove in it that they all get which had me thinking something passed thru. I've not seen it any of our other CMB motors, it's been a 1 time thing on a motor I beat up pretty good.

PS. If you need bearings, I got plenty in stock for that motor.
 
Most scale guys I know are usually at or really close to ,015 on head clearance. Best way to tell if the needle bearings or crank pin is worn is if when you roll the engine over by hand (with a glow plug in) and if you feel the thump-thump at the top of the stroke when you roll at back and forth somethings not right.
 
Back again,

Different engine, but still a 67 CMB green head. I was helping a friend by tearing down to check the bearings. In the process I found the crank pin had some damage. Can anyone tell me what would cause something like this. I kind of expected to find some damage on the needles, but they all looked fine and measured at .0586

Brinelling. Looks like the needles wore too much before being replaced and started skidding.

I'd replace the crank and needles and have a good look at the rod. Or if you know someone have it hard chromed and re-ground.

Tell Todd not to be so dang hard on stuff! :D
 
Most scale guys I know are usually at or really close to ,015 on head clearance. Best way to tell if the needle bearings or crank pin is worn is if when you roll the engine over by hand (with a glow plug in) and if you feel the thump-thump at the top of the stroke when you roll at back and forth somethings not right.
That's close to what I was told as well, that being .016.
Terry, what kind of boat do you run your engines in that you can run them that tight? I had serious detonation issues running tighter than .16
 
That's close to what I was told as well, that being .016.
Terry, what kind of boat do you run your engines in that you can run them that tight? I had serious detonation issues running tighter than .16

That's in my SAW boat with 70% nitro.

From my experience detonation is usually caused by over-leaning the needle which is needed to burn the fuel when an engine is over-cooled...
 
I see that happen a lot with our cranks I’m our CMB 67 andn45. We took some lapping compound and cleaned it up a little and replaced the needle bearings. No issues as of yet. Our head clearance is .008. We make a plug last a season.
 
That's in my SAW boat with 70% nitro.

From my experience detonation is usually caused by over-leaning the needle which is needed to burn the fuel when an engine is over-cooled...
My plugs tend to last for several races and I know I'm not running lean. I had one of my engines at roughly .010 and, after one season, the piston and button both looked like they had been shot with a sand blaster. After shimming it up to .016, it hasn't been an issue. Could be a coincidence though, who knows
 
My plugs tend to last for several races and I know I'm not running lean. I had one of my engines at roughly .010 and, after one season, the piston and button both looked like they had been shot with a sand blaster. After shimming it up to .016, it hasn't been an issue. Could be a coincidence though, who knows

What does your complete water system flow compared to your main needle flow?

What fuel/plug/pipe are you running?
 
Terry, to answer your questions in order:
1) Don't know right off hand, just using the set up that was in it when I bought it
2) 50% of various brands as I've been given or bought fuel from various sources
3) ODonnell Purple
4) Looks to be a CMB pipe with a muffler added to it. I'll have to take some pictures and let you look at them
 
Terry, to answer your questions in order:
1) Don't know right off hand, just using the set up that was in it when I bought it
2) 50% of various brands as I've been given or bought fuel from various sources
3) ODonnell Purple
4) Looks to be a CMB pipe with a muffler added to it. I'll have to take some pictures and let you look at them

Like I said:

From my experience detonation is usually caused by over-leaning the needle which is needed to burn the fuel when an engine is over-cooled.
 
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