CMB 67 rotor end play

[WikiWiki1]

Can anybody tell me the correct drum rotor end play fro a CMB67? I have found some old info about this but nothing definitive.
I have 4 motors I'm going through and one has .015" and the others are at .023". I haven't measured one without the gasket so I don't
know what the minimum would be before stacking gaskets. Mine all have two gaskets. Being run several seasons all the motors have
markings from the rod bearings in the rotor face. I guess this must be normal as I don't see any way to adjust the rotor to avoid this.

Thanks, Brent

 

[Chris Wood]

.008 - .010 lap the back plate on a stone or 3000 paper.. also lap the disc ( they are not flat ) same 2-3000 grit. if you have a pc of glass you can tape down the sand paper to lap in. also take a diamond file and file the the slot so the disc just falls on the square pin.

ran many a motors that won many us1 championships with the same directions as above.

 

[FloridaScaleBoater]

Unless I'm missing something,
The question was about drum rotor clearance.
Not disc

 

[FloridaScaleBoater]

Is there a preferred way & number for drum clearance?

 

[SayMikey]

I think Chris is talking Evo Disc motors. WE are talking 67 hr motors. All Of mine have a bunch of end play. The seal is controlled by the bushing.

 

[WikiWiki1]

I find it interesting that there is no way to control this part of the engine. The drum sort of just floats free to move fwd and aft in the engine. With two gaskets it sets the end play around .023" so I guess that must be CMB's intent on the setup. I was hoping to find a way to address the wear on the drum face from the connecting rod.

Brent

 

[SayMikey]

I find it interesting that there is no way to control this part of the engine. The drum sort of just floats free to move fwd and aft in the engine. With two gaskets it sets the end play around .023" so I guess that must be CMB's intent on the setup. I was hoping to find a way to address the wear on the drum face from the connecting rod.

Brent

Brent are you running a conventional shaft not square drive? How are your engine bearings? Thrust from the crankshaft is pushing the crank forward. This is why some use square drive. Changing bearings at least twice a year will help with your issue

 

[WikiWiki1]

Yes I run square drive in 1/8 scale hydroplane. I do change the bearings a couple times a season even when they seem ok but it always seems to run better after. After years of not really getting into the technical stuff on these engines I thought I would spend some more time learning about them. The CMB 67 has allowed me to be lazy as it always runs pretty well. Usually my driving is the main problem. : )

Brent

 

[Chris Wood]

ya sorry i was thinking about the disc motor

 

[Terry Keeley]

Measure the width of the lower rod from time to time, you might be surprised what you find...

 

[Mike Cathey]

A little loose is better than a little tight because you never want the drum to pinch the rod against the crank if the motor gets hot (too lean or no water) because it will wipe out the bearings, break the crank pin or the rod. You definitely want the rod to have 3-4 thousands of clearance on the crank pin between the rod and the drum. Measure by assembling the lower end of the motor without the piston/sleeve and putting a feeler gauge down from the top of the case between the drum and the rod while pushing on the flywheel to find the max point of clearance. If there's not enough clearance add front case gaskets.

The Picco square case 67's were notorious for being too tight from the factory, along with weak rods (everybody around here at the time was replacing the stock rods with the RPM aftermarket rods) and the work around was using several gaskets on the front case.

And like Terry says that this stuff does wear out eventually. Piston bosses will eventually go out of round as well. If you roll your motor over by hand with the plug out and you feel clunk-clunk at TDC then you need to get some new rod needle bearings ASAP.

 

[Terry Keeley]

I've been toying with the idea of "fixing" the drum to the backplate somehow to prevent it from slamming forward into the rod every intake cycle but haven't figured out a simple way.

I seem to remember the K90 solved this somehow but can't remember how?

 

[Jeff Lutz]

I've been toying with the idea of "fixing" the drum to the backplate somehow to prevent it from slamming forward into the rod every intake cycle but haven't figured out a simple way.

I seem to remember the K90 solved this somehow but can't remember how?

The K90 drum valve had a bearing journal off the back and was bolted to a ball bearing.

 

[Terry Keeley]

I was thinking of trying to find a ball bearing with an ID the size of the drum and counter boring the housing to fit.

 

[Buckshot]

I was thinking of trying to find a ball bearing with an ID the size of the drum and counter boring the housing to fit.

Original K45 drum assembly:

 

[Terry Keeley]

Thanks, nice set up. Is there a hole to feed lube to the bearing?

 

[Buckshot]

Thanks, nice set up. Is there a hole to feed lube to the bearing?

No. Not sure how well it works as haven’t tried it yet, just parts I gathered to try on the latest K engine. (I never got to run the OG K motors.)

Sorry to hijack the thread.

 

[Al Hobbs]

The bolt holding the K90 rotor in place was left hand thread and an oil passage.

 

[Jeff Lutz]

Thanks, nice set up. Is there a hole to feed lube to the bearing?

I still have 4-K 90’s. The bolts that hold the rotors are center drilled on the end but do not go through to lube the bearing. Oil apparently makes it up there though,I never had a bearing issue at all on K90’s. As for the Cmb 67 HR and the new K45’s, I don’t see a issue with the floating rotors either. The lines on the rotor face are not as bad as they look and many engines like that have set big records. Vac .91 included. The bearing setups are a nice feature though. Jeff Lutz

 

[WikiWiki1]

I still have 4-K 90’s. The bolts that hold the rotors are center drilled on the end but do not go through to lube the bearing. Oil apparently makes it up there though,I never had a bearing issue at all on K90’s. As for the Cmb 67 HR and the new K45’s, I don’t see a issue with the floating rotors either. The lines on the rotor face are not as bad as they look and many engines like that have set big records. Vac .91 included. The bearing setups are a nice feature though. Jeff Lutz

I measured the depth of the marks in my rotors and most are around .001". One Idea I was thinking of was to machine a pocket into the rotor around the pin hole and install a thrust washer for the bearings to wear on then I could easily change it out. Who knows what other problems could arise from that though. But in reality they have been running that way for years so I guess I'll leave them as cmb designed it.

Brent