Comet Oil Use

[Martyn Goodwin]

Answering a few questions.

1. I had over 2,000 miles on the motor before I "gave it any stick" in fact I only really "got into it" after 3,000 miles. I suspect that the problem happened around 2,500 miles - that's when the breather started to noticeably smoke.
2. It was/is a forged CP (Carello) piston, running in a new muff/liner that was set up with 0.004" bottom clearance
3. The top ring had broken about 3/8" from its gap and it appears that the small bit of broken ring jammed and was then torn upwards, destroying the top ring grove as the piston descended. There is no sign of any seizure in the piston skirt
4. The liner/bore is badly damaged at a point near TDC, in line with the damage on the piston, couple of deep scratches plus a small part of the cast iron seemingly torn from its face.



Martyn

 

[Comet Rider]

Hi Martyn,

This sounds oh so familiar to the CP pistons that failed for my dad.

In both cases the lands closed up and nipped the rings, and on the second failure it also siezed the little end.

Full examination was done by an international piston manufacturer who diagnosed the following;

1/ poor specification of ring groove width (To tight)
2/ insufficient clearance for wrist/gudgeon pin
4/ wrong spec for piston skirt clearance

The replacement made by Wossner who make pistons for the likes of Porche AMG etc) has not given a moments concern since install

HTH
Neil

 

[nkt267]

When I got my Carrillo rod from ClevTrev he made a point that the small end(wrist pin) clearance Must be .875 + 1.5 thou,leaving a gap for the oil to fill.
I used a new Omega piston and found that if you ream to 7/8ths(0.875") then the gudgeon pin felt tight.
I had a small end cause problems before and I think that this caused the piston to sieze( well one side virtually vapourised).
Since then I bought a correctly sized 0.875+1.5 thou reamer and fingers crossed all is ok..John

 

[greg brillus]

Hi Martyn, A close look at the piston looks like the oil ring gaps are in line......this is not good.....make sure they are 180 degrees appart on assembly, this with any piston/ ring combination....cheers..Greg.

 

[Martyn Goodwin]

Just to complete the picture, these are the best shots I could get of the liner damage.

 

[davidd]

Martyn,

I am very sorry that you have had such terrible luck. I know that Trevor believed it was blow-by and I was pretty sure it was. Also, the two articles on breathers were reasonably sure that most breather problems are due to blow-by. I may have promised not to mention this, but a leak-down test would have shown this problem in a few minutes. When my bike ran poorly at Cresson Texas I did a leak-down test and it was 60%. I was expecting it to be no more than 5%. It came apart and the compression ring had shattered:

This is a JE piston and I hope to switch to CP because they have a much better reputation. Most of the Vincents at Bonneville use CP. So, I would not argue with Neil, but I would note that items 1,2 and 4 are mostly owner determined items. One should check the ring clearances. I don't think I have ever used the recommended skirt clearance, and when I order Carrillo rods I use a separate part number that has half a thou more clearance in the wrist pin, which has nothing to do with the piston. I am sure CP has made mistakes, but I do not think they are incompetent.

David

 

[Bill Thomas]

Hello David, Can you tell us what skirt clearance you use, Cheers Bill.

 

[Martyn Goodwin]

Just testing to see if I have the photo thing sorted.

This is/was my CP piston

 

[vibrac]

When I got my Carrillo rod from ClevTrev he made a point that the small end(wrist pin) clearance Must be .875 + 1.5 thou,leaving a gap for the oil to fill.
I used a new Omega piston and found that if you ream to 7/8ths(0.875") then the gudgeon pin felt tight.
I had a small end cause problems before and I think that this caused the piston to sieze( well one side virtually vapourised).
Since then I bought a correctly sized 0.875+1.5 thou reamer and fingers crossed all is ok..John

You are a better man with a reamer than me then, I always use a brake cylinder hone no chance of digging in and not only is it a good finish, it allows me to vary amount removed as its very controllable

 

[clevtrev]

You are a better man with a reamer than me then, I always use a brake cylinder hone no chance of digging in and not only is it a good finish, it allows me to vary amount removed as its very controllable

What does a brake cylinder hone, use as a cutter.