Engine build prep (2014)

motorson said:

There are three things that can cause that kind of wear on a piston. I have had 3 "soft siezes" on my engine, two of the times it was because the piston was fitted just a bit too tight. The last time though it was because the timing was too far advanced. The third thing would be oil related which I doubt in your case. And, trailing in 4th place would be a coating that doesn't hold up to the pressure and heat of a Norton motor. I also doubt that in this case. If I'm right then that leaves only clearance and timing.

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How can a piston only seize on one side....?

Fast Eddie said:

How can a piston only seize on one side....?

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Because it rotates in only one direction.

swooshdave said:

Because it rotates in only one direction.

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?? Pistons don’t rotate. They go up and down! I guess what you mean is that crank rotates in one direction? But I don’t see how that’s relevant here.

Due to the rotation of the crank, it’s the front face of a piston that usually see’s more force when it descends during the firing stroke. That may show up as excess wear on the thrust face. But not as a ‘one sided seizure’. Anyway, it’s irrelevant here as my damage is on the rear face.

For a piston to seize, in the context discussed, ie lack of clearance, or over expansion due to heat, it means the piston is essentially to big for the hole. It can’t only be too big on one side. When this happens, although one side may suffer more than the other, the piston will show clear signs of distress on both front and rear faces due to the piston expanding and ‘seizing’ in the bore.

You can see from the pics that the front face of my piston look perfectly fine. The rear face is terrible with deep grooves cut into it.

As far as I can deduce, there is no way a seizure could have applied the force required to do this damage to the rear of the piston whilst leaving the front face unscathed.

But, I do only have a layman’s view, if someone knows different please do correct me...

Nigel

Maybe next time try a Teflon coating on the piston skirts, you may lose some during running in certainly won't make a mess like that. Where possible I like to use Teflon buttons in stead of gudgeon/wrist pin circlips.

Best regards

Esme

Esmerela said:

Nigel

Maybe next time try a Teflon coating on the piston skirts, you may lose some during running in certainly won't make a mess like that. Where possible I like to use Teflon buttons in stead of gudgeon/wrist pin circlips.

Best regards

Esme

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I used Teflon coated skirts before Esme, including on my prior pistons in this engine. Those were also ceramic coated on the crowns. As were the combustion chambers. Some of that came off, quite large parts of it actually, and it’s very hard and sharp when it does and I believe it contributed to the issues I had.

So, in general, after two failures on two sets of pistons I am now rather averse to any aftermarket coating on pistons (coatings applied by the piston manufacturer would be different).

I used to do both Teflon and ceramic coating myself with no issues, but don’t have easy access to a blasting cabinet anymore (they need blasting with a specific grit before coating).

I only know of one company, Camcoat, who do this service and they have applied all of the coatings that have failed on me. So I am definitely not using them again.

If you know of a reputable U.K. based firm, please let me know.

How long before it nips up? I have an engine that nips up or certainly gets close in less than a minute. I pulled the jugs off and put on an old set
with old pistons all past high limit. Same thing happened. So not the clearance you would say. OP gauge looked good.
Time to do what Nigel did and pull it apart and examine.
Picture worth a thousand words. A teardown worth a lot more...

Fast Eddie said:

I used Teflon coated skirts before Esme, including on my prior pistons in this engine. Those were also ceramic coated on the crowns. As were the combustion chambers. Some of that came off, quite large parts of it actually, and it’s very hard and sharp when it does and I believe it contributed to the issues I had .

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Nigel,

a scenario whereby part of the flaked compustion chamber coating is trapped between piston and cylinder wall causing rubbing is ruled out because the ring land seems unaffected.

The rhs piston is in perfect condition?

Did you have an issue with the left hand combustion chamber running a bit hot due to probably lean gas mixture? Ceramic coating has a bad side effect in that heat dissipation from piston to cylinder is greatly reduced. Nearly all heat dissipation occurs through oil splash. The cooling effect of fresh gas charges is limited due to coating and carbon build-up at the piston crown.

Now, we know that the cylinder is hotter at the rear side than in the front, causing cylinder bore to distort in an oval fashion "east-west" which means piston/bore tolerance will shrink at the rear ("south"). Another sign of a thermal problem is that friction marks extend along part of the circumference and appear to be rather equal while increasing down the skirt. Increased expansion at the skirt may be explained by lack of transverse stiffening which is present near the gudgeon pin, thus the skirt will expand unrestrained.

If friction marks were caused by pressure due to acceleration forces and crankshaft displacements, I would expect to see concentrated wear marks up and down the "south" line (with ref. to the piston itself).

Did you check diameter of the coated piston along the circumference prior to fitting? Piston diameter variations at various vertical cross-section planes have to fit the requirements of an air cooled engine. Some makers apply their experience with watercooled engines. Variations in clearance with circumferential angle will obviously be different for an air-cooled engine. The amount of oil cooling is another factor. Pistons for engines which have directed oiling jets see vastly improved cooling over pistons which operate essentially in an oil mist.

http://blog.wiseco.com/piston-to-wall-clearance-myths-mysteries-and-misconceptions-explained
https://www.dieselnet.com/tech/combustion_piston-cool.php#heat

-Knut

Hi Knut,

My previous piston failure was the one that suffered flaking ceramic. Not this one. This one I suspect the andosing has caused issue.

This time, neither piston or head had any ceramic coating. No Teflon coating either. Just anodising.

Yes, the anodised piston was used to size and bore to, so all good in that regard.

Ign timing was 28 degrees. Carburation was really good when last on the dyno. So I can’t think of any reason for excess heat. It’s been used hard in some hot weather on track days last summer, but nothing it shouldn’t be able to handle. Faster guys (& girls) than me race these things in hotter climates!

Your comment about distortion and expansion are interesting however, especially given the lightness of the piston.

The timing side piston was good, if not perfect. That showed signs of rubbing (no scoring) on the FRONT face and a perfect rear face. That’s more what I’d expect to see. But it’s a tad strange that it’s the front of one and the rear of the other piston that’s unscathed.

I conversed with a certain well know husband and wife team of Norton experts today, who raised the prospect of a possible ‘cockeyed bore’ which is something I’d never thought of. I guess I need to find someone other than my usual ‘boring man’ to get an honest appraisal of this possibility.

Onder said:

How long before it nips up? I have an engine that nips up or certainly gets close in less than a minute. I pulled the jugs off and put on an old set
with old pistons all past high limit. Same thing happened. So not the clearance you would say. OP gauge looked good.
Time to do what Nigel did and pull it apart and examine.
Picture worth a thousand words. A teardown worth a lot more...

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It didn’t nip up.

I only discovered this when I took it apart for a ‘look see’...

swooshdave said:

Because it rotates in only one direction.

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Piston will "seize" in any place where there is a high spot and not enough piston/bore clearance, and since pistons are as a rule below the piston rings, oval - it is usually on the front or back piston skirt.

Bernhard said:

Piston will "seize" in any place where there is a high spot and not enough piston/bore clearance, and since they are as a rule below the piston rings oval - it is usually on the front or back piston skirt.

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The piston ‘floats’... it can’t have lack of clearence on only one side of the piston and plenty of clearance on the other side of the same piston and seize.

A seizure due to a lack of clearance, by definition, means lack of clearance on both side of the piston. That’s when it will seize. Leaving witness marks, to a greater or lesser extent, on both sides.

If it was a seizure due to lack of clearence that had caused the very excessive damage on the rear of my piston, there would be some witness marks on the front.

Surely we can agree on that !?!?

What clearance were the pistons originally fit at? Apologies if it was stated previously and I missed it.

They were at .0055”

Fast Eddie said:

I conversed with a certain well know husband and wife team of Norton experts today, who raised the prospect of a possible ‘cockeyed bore’ which is something I’d never thought of. I guess I need to find someone other than my usual ‘boring man’ to get an honest appraisal of this possibility.

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That may very well be the cause. Jim Comstock has experienced bore distortion numerous times apparently. I remember a thread in which he specified always to use a torque plate. You may want to check with your "boring man" if a torque plate was used. Also, do a search here for "torque plate" and recap what Jim wrote.

-Knut

Nigel the cock eyed bore i thought had applied to 750 barrels. Certainly when the head and barrel were off mine sometime in the late 70s , I took the rings off the pistons and smeared them with prussian blue and turned engine over to see if there were even witness marks. Not exactly an accurate procedure but the best i could thin of at the time. There would have been a reason for me to do that- something i had read but long since forgotten

mdt-son said:

That may very well be the cause. Jim Comstock has experienced bore distortion numerous times apparently. I remember a thread in which he specified always to use a torque plate. You may want to check with your "boring man" if a torque plate was used. Also, do a search here for "torque plate" and recap what Jim wrote.

-Knut

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I failed to find a borer in England who uses torque plates (I’m happy to be corrected on this BTW if someone knows otherwise).

I considered making some, but then realised that the reality is that (almost) none of the thousands of Norton’s on U.K. roads and tracks have had torque plates used on them.

Not sure if that makes it right or wrong, but it is what it is !

Fast Eddie said:

I failed to find a borer in England who uses torque plates (I’m happy to be corrected on this BTW if someone knows otherwise).

I considered making some, but then realised that the reality is that (almost) none of the thousands of Norton’s on U.K. roads and tracks have had torque plates used on them.

Not sure if that makes it right or wrong, but it is what it is !

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When considering the manufacture of my aluminium barrels, I was going to have a number of t5orque plates manufactured to allow people access to them when boring the cylinders for their particular piston choice. I think this was an important step for the integrity of the barrels.

Fast Eddie said:

I considered making some, but then realised that the reality is that (almost) none of the thousands of Norton’s on U.K. roads and tracks have had torque plates used on them.

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Herd mentality should never take precedence over sound engineering. I take it you were informed that boring without a torque plate is a pitfall. To me, the assurance knowing cylinder distortion isn't causing problems would be worth the cost of having a torque plate made!

-Knut

And for those folks who don't know what a torque plate is (which included me until today) there are a couple of nice video's here..



Cheers,

cliffa.

thank you cliffa
Jim