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Mind the Gap!

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Patbretagne
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Mind the Gap!

Postby Patbretagne » Tue Feb 28, 2006 1:33 pm

Mind the Gap!

On the London underground or Tube as it's known, in stations that are on a curve there is an announcement to warn people either at the end of the carriage or in the middle of the carriage, depending whether it is a concave or convex curve, that they should "Mind the Gap" for the 40 or so years that I have frequented the service, the voice hasn't changed at all, I believe that it is still the same recording all these years.
That however is by the way as one doesn't find many Cubs on the Tube!
However the word to extrapolate from this little saying is GAP, such a little word but to us who are into a C60 engine up to our elbows rather like a gynaecologist, GAP means a lot.
One has to check the big end clearance using Plastigage, the side slop in the crankshaft against the centre bearing, piston slap when there is too much gap between the piston and the cylinder, are there any others?
Oh yes! the piston ring end gap, stated in the Cubible as follows (I quote in inches as most of us are 'over there'):-
First two rings, compression end gap 0.001 to 0.002
Oil control scraper ring 0.0004 to 0.002
(These may be a little out from the US manual as I took them from a French C60 Engine manual I found over here and converted them with "Conversion Pro" a super prog for the computer)
NOW look at the photo of the rings off No 3 piston upside down in number three bore. Dear reader, does anything hit you whilst looking at this picture (Apart from "Her Indoors")?
Image
You are right the gaps are not correct. in fact they are as follows:-
Top Ring 0.088
Second Ring 0.064
Oil Control 0.105
The oil control is about 1 tenth of an inch instead of almost nothing; the other two are not so good either!

Question, is it time to change the rings or should we soldier on?

My initial diagnosis was correct when I heard gasses coming back down into the crankcase as the engine stopped and that the oil was starting to get very smelly.
Image
Whilst taking it to bits I was very interested in the contents of the oil pan. A photo doesn't do it justice but there was about 1/4 inch of hard packed goo over the majority of the pan under the oil, when disturbed with a brush and some Kerosene the liquid came up quite grey having dissolved the crud. So onto the next part of the saga.
BIG ENDS.
Have a decko at this next photo
Image
What do we see peeping through the soft wearing surface of the bearing shell? Copper! So it looks like all Viki's bearing surfaces from the engine have deposited themselves in the bottom of the pan. And so to the next point!
The upside down funnel/filter that is submerged under the oil surface. At sometime part of the gauze has become detached from it's crimped over side (rather like a preserving can top). This doesn't really matter because it is still well and truly in contact with the vertical side of the funnel ensuring that the oil has to pass through the gauze.
BUT for Monsieur le mechanique, this wasn't good enough, he had tied a piece of old French jean fabric over the upturned funnel so aid and improve on the original design. I am not sure what the filtering capabilities of jeans are, but their incapacity to pass thick oil when cold is I should think almost NIL! Fortunately one side of the cloth was undone so a certain amount of gauze was available to the oil to pass through.
Could this be the reason why the ends are run out? I think that the jeans had dropped off one side of the gauze before Viki came to us as we had quite a good oil pressure whilst working.
Isn't it fun finding out the whims of a do-it-yourself mechanic?
Pat

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George Willer
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Postby George Willer » Tue Feb 28, 2006 2:34 pm

Pat,

Those gaps look HUGE! Have you checked the cylinders for taper?

I've been successful repairing worn rings by replaceing them with the next oversize, after first filing the ends to give the right end gap at the less worn bottom of the stroke. I try carefully to end up with the minimum allowable... .007". I firmly believe a ring with only .001 or .002" end gap would expand enough when running to cause seizure.

The allowance in my manual is .007" to .017". I expect the rings to wear enough to reach .017" soon enough, so I try for the minimum allowable. Barney and Scruffy both have engines so treated.

An important but frequently overlooked detail is how the rings fit the SIDE of the groove. It's very important for the grooves to be clean and bright... so the rings can work as designed, alternately sealing against the sides of the grooves. The rings MUST be free to move in the grooves.

In extreme cases I've machined damaged sealing surfaces and used spacers beside the rings to bring the side play to specs.
George Willer
http://gwill.net

The most affectionate creature in the world is a wet dog. Ambrose Bierce

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Buzzard Wing
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Postby Buzzard Wing » Tue Feb 28, 2006 2:45 pm

mmmm the GAP sells jeans so I get the connection! :roll:

Not sure of the implication of the oil ring, but imagine that it would 'burn' oil because of it. Wouldn't those gaps allow oil into the combustion chamber, and alternately fuel/exhaust into the the bottom of the cyl? Also, without knowing the correct gaps I would guess that you would have lower compression?

I am thinking of dropping the oil pan soon and would be curious to see what 55 years has left there. May be wiser to leave it alone???? (leave a sleeping dog lay, as they say.)

Sorry to be of no help Pat, I am a 'shade tree' mechanic and when I have serious questions I ask my good friend who trains Chrylser mechanics.
1971 Cub (Rufus) 1950 Cub (Cathy) 1965 Lo Boy Fast Hitch (Nameless III) 1970 Cub 1000 Loader & Fast Hitch (Lee)

Eugene
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Postby Eugene » Tue Feb 28, 2006 7:25 pm

Time for a major overhaul. This time the mechanic can install the correct parts.

Wrapping the oil pickup and screen with cloth (jean material) would restrict the oil pumps ability to suck oil. I'm not sure why cloth. Metal window screen would have been a better choice.

Eugene


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