Piston cracks - CMB 67 greenie.

[Terry Keeley]

Glad I caught it this time, a chunk broke off a couple years ago and she blew up big time. Thought I fixed it by chamfering the inside of the skirt, not quite.

What's happening is the bottom corner of the skirt is hitting the counterweight ABDC, there's faint marks on the crank. You can also see it on the piston looking with magnification.

I'm gonna round that corner off on the next one, doesn't look like sub porting will be an issue.

Anyone else have little scuff marks on the counterweight? Might not be an issue normally, I'm winding it up pretty good in my SAW boat.

 

[Grimracer]

none for me.. (heat racing only)

Grim

 

[Lennard Cuenco]

first i heard of 67 shrapnel, thx for tipping us off (heat racing ofc)

 

[steve wood]

rebuilt one with cracks and chuncks missing

 

[Terry Keeley]

Marks to look for on counterweight:

 

[Joe Wiebelhaus]

Terry,

I would look for another problem, the long end of the skirt (exhaust side), where the crack is, will "never" hit the counterweight, the only place the skirt can hit is ABDC, right under the wrist pin.

The radius of the piston at that point is beyond the face of the crank, unless you have a different style crank then I have ??

 

[Terry Keeley]

Terry,

I would look for another problem, the long end of the skirt (exhaust side), where the crack is, will "never" hit the counterweight, the only place the skirt can hit is ABDC, right under the wrist pin.

The radius of the piston at that point is beyond the face of the crank, unless you have a different style crank then I have ??

Ya, I thought that too at first, that the skirt could only contact directly under the wrist pin, so I chamfered the skirt in that area like we did on the P45's.

But that's not where it hit in my case, if you look at the marks when the crank is in the case they line up perfectly with the corners of the piston skirt. When I look at that spot of the piston with a loupe you can see it's been hitting. You are right that "The radius of the piston at that point is beyond the face of the crank", if I'd seen that before I'd realize chamfering the skirt there would do nothing.

Think flex in the crank at higher rpm plus dwell time ABDC allow it to happen.

 

[tnrcboatracer]

I also don't see how this is possible The R of the counterweight and the distance the corner is away from the wrist pin would meant that the skirt area in line with the wrist pin is 2-3mm below the arc of the CW. It is easier to draw. That is a bunch of crank flex....... Piston rocking at BDC?

 

[Terry Keeley]

I also don't see how this is possible The R of the counterweight and the distance the corner is away from the wrist pin would meant that the skirt area in line with the wrist pin is 2-3mm below the arc of the CW. It is easier to draw. That is a bunch of crank flex....... Piston rocking at BDC?

Rocking might be part of it, I'm just telling you what I see...

 

[Jack ODonnell]

Terry; Looks like you have a bigger problem with the way the rear bearing is sliding on the crankshaft. J.

 

[Terry Keeley]

Terry; Looks like you have a bigger problem with the way the rear bearing is sliding on the crankshaft. J.

Yes sir, that's been an ongoing issue.

Tried holding it with green locktite but still slipping. Might havta try hard chroming and finish grind to size next.

Or it might go away when I get the revs back in the mid 20K range with my new props.

 

[Mike Cathey]

Terry; Looks like you have a bigger problem with the way the rear bearing is sliding on the crankshaft. J.

Yes sir, that's been an ongoing issue.

Tried holding it with green locktite but still slipping. Might havta try hard chroming and finish grind to size next.

Or it might go away when I get the revs back in the mid 20K range with my new props.

The crank looks galled where the bearing sits. I wonder if the crank is wobbling around a bit. Rick Brake of RB Mods does a great job of indexing cranks to make sure they are true.

I have never had an issue with a crack where you have (I chamfer my pistons like you do as well) but I had a problem with the 90 degree angle of the skirt with that little tip piece breaking off and trashing a motor. Rounding that edge solved the problem. The way I understand it is that anywhere you have a 90 degree outside corner like that creates a hot spot and softens the metal.

 

[Terry Keeley]

Yup, I'm sure I'm getting some crank flex or "wobbling", I am winding them up close to 30K.

Here's what happens if you don't catch it in time:

Crack on the other side:

 

[Don Ferrette]

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

 

[Andy Brown]

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

I think Terry is using an Eagle Tree system on board. Seems like he posted data from his 67 here about a year or two ago. I remember he was turning 28K according to the data when he first started running the CMB 67.

 

[Ty Faust]

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

I think Terry is using an Eagle Tree system on board. Seems like he posted data from his 67 here about a year or two ago. I remember he was turning 28K according to the data when he first started running the CMB 67.

Andy

I think my 67 that you are doing should turn 30 k Plus !!

 

[dr gary turner]

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.

 

[Andy Brown]

Terry; Looks like you have a bigger problem with the way the rear bearing is sliding on the crankshaft. J.

Yes sir, that's been an ongoing issue.

Tried holding it with green locktite but still slipping. Might havta try hard chroming and finish grind to size next.

Or it might go away when I get the revs back in the mid 20K range with my new props.

The crank/bearing issue is standard for all 67/80 engines built by CMB. They always fit the bearing loose on the crank. Virtually every MAC 67/84 crank looks the same way after a short amount of run time.

It has never proven to be a problem. A tighter fit may be better, but the fit they use is working as evidenced by all of the CMB/MAC 67,80,84 engines that have set records over the last 15 years.

 

[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!

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.

 

[Don Ferrette]

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.