New CMB 67 HR Advice

[Tim van Schyndel]

Purchasing a new CMB 67 HR for a rigger. Just after some basic starting points to get me going. Running 50% Nitro mix

1. What is the best size hole for the water cooled head?
2. Head clearance?
3. Pipe?
4. Best Plugs or temp range of plugs to try?
5. Anything else I need to know about this motor?

Thanks in advance for the advice!

Tim

 

[Tom Foley]

Nice engines , I love mine .

1 - a .040 " - .050 " hole will work fine in the cooling cap .

2 - head clearance around .013 " works well , mine is at .012 on 60 % Nitro - 18 % oil Syn castor blend (Technology fuel )

3 - I have the black AB muffled pipe that works on just about anything ! Al Hobbs has them .

4 - KB HP , OD 99 - OD 299 , Mcoy MC - 9 all work well .

5 - Intake drum rotor clearance must be increased . Take the drum housing to the sink with soapy water and some 400 wet or dry paper gradually and evenly sand the inside of the drum bushing in a circular manner working your way to 800 grit then wash thoroughly and test fit the drum . Also increase the chamfer on the drum bushing at the point nearest drive side of the rotor . some guys will add an oil hole as well . If you are not comfortable doing these things send them to one of the engine guys here . For the first couple of outings I would pull the housing to see if there is any bronze color on the rotor from the rotor bushing . If so re-sand again .

 

[Tim van Schyndel]

Tom

Thanks for the great advice!!

As usual, always good assistance on Int Waters!

Tim

 

[Tim Kish]

Definitely check the rotor clearance as Tom said,I set mine at .001 per side.

Awesome long running motors otherwise.

 

[Tim van Schyndel]

Thanks Tim

Just confirming whether the 0.001 is in inches???

Tim

 

[Tim Kish]

Yes .001 per side minimum, I have mine at .0025 dia. on my 67 and 91s with no issue at all and they were all way to tight from CMB with the 67 only having .001 total clearance.

 

[Stuart Barr]

To increase the drum clearance, it is best to use lapping compound to lap the drum to the housing. I use #15 diamond compound and light oil first. Finish with #3 compound. After a days running, pull the drum and look for bushing deposit on drum. Sometimes you have to lap twice to get the proper clearance.

Stu

 

[Tom Foley]

To increase the drum clearance, it is best to use lapping compound to lap the drum to the housing. I use #15 diamond compound and light oil first. Finish with #3 compound. After a days running, pull the drum and look for bushing deposit on drum. Sometimes you have to lap twice to get the proper clearance.

Stu

Stu , where is the most convenient place to get the compound from ?

 

[Tim Kish]

I actually hone mine and use a bore gauge for accurate measurements, it also leaves a finish that's not to smooth if you use the correct stone .

To smooth is almost as bad as no clearance.

Tim

 

[Don Ferrette]

To increase the drum clearance, it is best to use lapping compound to lap the drum to the housing. I use #15 diamond compound and light oil first. Finish with #3 compound. After a days running, pull the drum and look for bushing deposit on drum. Sometimes you have to lap twice to get the proper clearance.

Stu

Stu , where is the most convenient place to get the compound from ?

http://www.mcmaster.com/#diamond-lapping-compounds/=qel7e4

 

[Tom Foley]

Thanks !

 

[Terry Keeley]

Wonder if one of those "Flexhones" would do a good job?

http://www.brushresearch.com/product-line.php?line=1

 

[steve wood]

Wonder if one of those "Flexhones" would do a good job?

http://www.brushresearch.com/product-line.php?line=1

probably round the inlet edges too much

 

[Tim Kish]

Ya you have to be really careful with honing it's very easy to bellmouth the ends especially on a short piece.

 

[Terry Keeley]

I just wrapping enough 400 grit around something close to the right size for a tight fit, a few twists was all it took to take a half thou or so out.

Dunno how you'd keep the hole round and straight otherwise unless you has a machine hone or lapping setup.

Found the thread from last Spring if interested:

http://www.intlwaters.com/index.php?showtopic=56099&hl=drum

 

[Mark Stein]

possibly a brake cylinder hone

 

[Tim van Schyndel]

I just wrapping enough 400 grit around something close to the right size for a tight fit, a few twists was all it took to take a half thou or so out.

Dunno how you'd keep the hole round and straight otherwise unless you has a machine hone or lapping setup.

Found the thread from last Spring if interested:

http://www.intlwaters.com/index.php?showtopic=56099&hl=drum

Thanks Terry

I just want to confirm that it is just the front part of the drum housing that is being relieved. I noticed in the photos that the the scuffing ring on the steel drum by the brass bushing is only at the front.

Am I correct in this assumption or do I still relieve the entire bush?

Tim

 

[Tom Foley]

The whole bushing needs to be opened up , but the area near the drive end (rear) is where the loading is from the crank driving it , also hotter there .

 

[Tim van Schyndel]

Thanks Tom and everyone for the great advice!

One last question.

How does the 67/80 large volume CMB pipe compare for the CMB AB 67 pipe?

Again thanks in advance!

 

[Hydro Junkie]

Okay guys, two fast clarification questions:

1) Does the age of a NIB greenie matter or is this something that covers all engines?

2) Does using a square drive at the flywheel make a difference as there isn't the axial load on the bearings like there is using round collet