Piston cracks - CMB 67 greenie.

[Terry Keeley]

30K?? And how are you arriving at that number??

Actually most of my faster runs have been in the 27 - 28K range, need less R-P-M and mo' M-P-H!

Thanks T, was wondering if you were data logging or still messin' with audio tachs.

You can see that one hopped a couple times before my driver got out of it!

 

[Terry Keeley]

Had a look at my other motor that's just starting to break in, no cracks so far.

Here's what I've done to try and stop it:

 

[Rick Doane]

Terry:

I can fix that problem for you.

Regards...........Rick

 

[Terry Keeley]

Terry:

I can fix that problem for you.

Regards...........Rick

I'm all ears???

 

[Andy Brown]

30K?? And how are you arriving at that number??

Actually most of my faster runs have been in the 27 - 28K range, need less R-P-M and mo' M-P-H!

My CMB green 67 is about 6 yrs old. Still has a great P/ S fit thanks to my fellow Iowan who 6 yrs ago changed the location of the seal of the piston. Changed the bearings for the first time in Jan raced it twice and the bearings failed at Hobert. Tonight I looked closely at the motor. NO cracks in the piston which has the sharp edges. I have the SAME mark on the bottom of the crank!!! IT IS NOT FROM HITTING THE PISTON. THE BOTTOM OF THE CASE HAS A FAINT LINE THE WIDTH OF THE CRANK WHERE IT HAS RUBBED. The 67 are shakers and I think the crank flexes and touches the bottom of the case. The crank shows also the same fit issues of the bearings which could related to balance issues as well. Terry, look closely at the bottom of the case.

Hey Doc: I've had the rod rub the trench in the case but it's clean as a whistle where the counterweight runs.

Looks like you are making SAW passes Terry. Why would you want less RPM?

 

[Terry Keeley]

Looks like you are making SAW passes Terry. Why would you want less RPM?

Well, if I can turn my new props that fast I should really have something!

Spinning my wheels with what's available now tho...

 

[Rick Doane]

PM sent.

 

[dave roach]

Terry

I wouldn't run a stock piston in a 67 26-28k I would get a Brad Chirsty piston and problem gone.

Dave

 

[DaveThynne]

Ive got the exact piece missing shown in the more severly damaged piston in my 67 greenhead. Ive ran this engine quite conservative with average rpm and load. I dont think its rpm related. My crank and case are not marked from the crank flex

 

[Keith Halverson]

A fair amount of flow thru the engine at BDC. Is it rocking/snapping the piston against the cylinder wall. A good tuned pipe could cause that kind of vacuum. I could see where that could possibly crack the skirt or turn it into a failing tuning fork. The piston and cylinder should show that kind of outside wear.

Oil content correct?

Custom piston time it seems. Either way, CMB should be made aware of this. Both with the crank marks and the piston.

 

[shoboat]

Rick,

Has the solution.

Terry,

Get it ready for Flint, MI.

Hopefully we will see the graph unfold in a few weeks.

Thanks For Sharing,

Mark Sholund

 

[Tom Foley]

A fair amount of flow thru the engine at BDC. Is it rocking/snapping the piston against the cylinder wall. A good tuned pipe could cause that kind of vacuum. I could see where that could possibly crack the skirt or turn it into a failing tuning fork. The piston and cylinder should show that kind of outside wear.

Oil content correct?

Custom piston time it seems. Either way, CMB should be made aware of this. Both with the crank marks and the piston.

Parts sales is where it's at !

 

[shoboat]

Tom,

Not if you have Rick Doane set it up for you.

Keep Testing,

Mark Sholund

 

[Keith Halverson]

Modded parts sales Tom,,ha.

Ed Bridges at one time used a Siemens turbine balancer for his cranks,, to get things right. They sure hauled the mail and ate gallons before needing work. Still have a couple 18TZ's unused taped shut with the full mods including the rod spacer.

 

[LarryBeals]

Round the lower points on the piston skirt this tip is rubbing at high rpms

 

[Terry Keeley]

Just an update.

Made a dozen or so hard runs with the rounded corners - no sign of marks on the piston or crank, and no cracks!

Did have some marks on the case this time as Doc mentioned, cleaned it up with 400 grit:

Stuff moves around in there more than we think...

 

[Joe Wiebelhaus]

Terry,

Movement like bearing slop, front to back and side to side ?

Piston flapping side to side, yea, happens in car engines with cut skirts.

crank or rod stretching, dont think so.... it fairly cold inside those engines when running.

 

[Terry Keeley]

Terry,

Movement like bearing slop, front to back and side to side ?

Piston flapping side to side, yea, happens in car engines with cut skirts.

crank or rod stretching, dont think so.... it fairly cold inside those engines when running.

I think the crank flexes and moves around radially a fair bit, didn't measure the clnc between the crank web and case but bet it's 0.010" or more where it rubbed.

It might not move axially (in and out) much if you take the prop load on the front bearing, maybe when you get off the throttle, who knows.

Also common to get rub marks where the crank seals in the case aft of the front bearing, there's typically a thou or two clnc per side there.

Think the skirt gets caught right at that bottom corner by the groove in the crank web from a combination of crank flex and piston rock ABDC.