You need 0.005" at the bottom of the skirt

Any less and you seriously risk seizure and you'll get no sympathy from me.

 

Well, I do have about .005 at the bottom of the skirt (at the top of the barrel). But less at the bottom of the barrel so it look like I need to work on the bottom of the barrel some more.

It would appear that there is not much difference between the diameter of the top of the skirt (just below the bottom ring land) and the bottom of the skirt. I expected there to be more flair of the skirt than that. Is that pretty common?

Art.

 

I've used .0045 for 30 years with no issues.

 

As a side note, I measured my EMGO pistons and found that there is very little taper to the pistons. The top of the skirt (area just below the rings) should be smaller than the bottom of the skirt. I found a max of 0.002 difference. Healy says they cut a tiny radius and you cannot measure the taper as it is not straight. I have a lot of respect for John, but my pistons have no such radius and are pretty much dead straight.

I had .008 clearance and signs of some "issue" at the top of the skirt. Healy thought it was seizure, but I think it was from overly rich jetting and the gas washing down the walls. I'm about to put it back together with 0.0083 clearance and new Hastings rings.

The gray burnish marks on my pistons polished out.

I had wanted 4.5 mils. But my shop used auto racing clearances and delivered 8 mils. Should make more power, but will be tad noisy.

regards,
Rob

 

I'd much prefer to file the pistons down, than make good cylinder bigger.
Hone the bottom of the cylinder, but don't take any more off the top of the cylinder. If you've got more than 0.0045" clearance at the bottom of the skirt at the bottom of the barrel, that's a good start.
If there's not enough taper on the piston skirts, file some taper on them.

 

Tapered skirts may be a thing of the past.

From http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=553492&page=1

The reason for the piston taper from top to bottom had more to do with the available production machinery "in the day" than than where the needed extra clearance it provided could be produced economically. If you put the piston on a Shadow Graph you would see that the area at the top of the skirt (just under the oil ring) is a slight radius.

Because the width of this slight radius is narrower than the width of faces of a micrometer, and so slight, it is hard to measure. With modern CNC machinery, with only a few lines of code, one can place the taper exactly where it is needed. Where in the past the only way to economically get that clearance under the oil ring was to cut a taper across the face of the piston. While the taper was never needed for the length of the face of the skirt the technology did not exist to economically mass produce the pistons any other way.

The real question is why is this feature needed in the first place and it has to do with heat and how it moves through the piston. Heat flows where there is material for it to flow through. The piston pin bosses, with their greater mass, get most of the heat that flows out of the top of the piston. This is why four corner seizures are common. But there is a thick section of the piston just behind, and below, the oil ring. There is just enough of it to cause some mischief - seizure.

Where there is heat, and aluminum to expand from being exposed to it, you need more clearance. This is why you need a bit more clearance just below the oil ring. The area behind and below the oil ring is quite thick as compared to the skirt. There is enough to require some extra clearance in this area. The taper in the old days, and now the short radius serves this purpose.

 

"...the area at the top of the skirt has a slight radius..." I don't see what he's getting at here. What area? Does he mean the skirt, viewed/measured vertically immediately below the oil ring, has a slight radius? And when he says the "width" of the radius, does he simply mean the radius? Or does he mean the arc of the circle that would be made if the radius was fully drawn? And if measuring with a micrometer is difficult, can't it be measured using wire or feeler gauges on a plate? Or is he saying that the radius instead of the taper is a recent method for providing extra clearance, but you can't see it (as on Rob's JCC piston) because it is so slight? (I think I may have answered my own question - still not sure though)

 

You may as well ask John to clarify.

 

I think you are in for an early re-work, if you install pistons with .008+ clearance.

Time will tell.

 

I'm hijacking this thread and do not mean to.

Bike is back together. Bike pulls like a tractor. NO MORE SMOKE. See my Grasping at Straws thread so I don't continue to hijack this thread.

regards,
Rob

 

.045 then cross hatch hone

 

I have the greatest respect for GPZ, as a man who has used .0045 for 30 years. I asked for and got .0045 when I last rebuilt my '59 a while back. I have done over 10,000 miles in hot weather at speeds up to 80mph with no issues in that department.

 

I think we are into hair splitting here again, something we seem drawn to do on these old tarts. If someone at Meriden specifies .045" to .061", then who cares just what it comes at, between those numbers.

To be sure that less than .045" will likely seize the piston, and over .061" will introduce some piston slap, but can we all agree not to become so anal about .005 of an inch!! RR

 

As the OP I have, naturally, followed this thread with great interest.

First let me note, based on the first few replies, I quickly moved on, happily going with the .005/.0055 clearance that I had.

However, may I ask, why are the accepted clearances here on the forum so different than the manual? The numbers that I see are .0085/.0106 (top of the skirt) and .0046/.0061 at the bottom.

Is it a matter of old materials/technology? Or are today's practices just different? The numbers at the bottom of the skirt are right about where I am now but more than the numbers commonly suggested. However, the numbers for the top of the skirt are really a lot more than that.

I noted the fact that the new pistons seem to be pretty straight with out much, if any, taper. But, if the clearance I am going to use is about half what the manual suggests for the top, is it just a matter of these new materials not expanding as much?

So, what am I doing, misreading the manual? Or, is the manual wrong? Or, am I just misinterpreting what I am reading?

Remember, I am not challenging any thing any one has said. If it's good enough for Mr. Pete and GrandPaul, it's plenty good enough for me.

Art.

 

Is it a matter of old materials/technology?
Art.

JCC (Hepolite, Emgo) pistons specify 0.004" at bottom of the skirt at right angles to the gudgeon (wrist) pin according to the sheet that comes with them.
Apparently it's to do with modern materials/technology...

I am running my T120V at that clearance with no issues.

 

Well I showed the sheet that came with the pistons to the machinist at Banda Engineering (Portsmouth Docks, very good engineers) and he was happy to go with that.

He didn't think it was "too tight" and I certainly didn't feel I was trying to be "clever".