Well it was good while it lasted

For some reason, my post came out about three hours late. Stupid computers!

How close do the pistons come to the crank shaft on the bottom of the stroke? It seems like you would want them about as long as possible. All the little short skirt Honda pistons I have ever had needed to be that short to keep from hitting the crankshaft on the down stroke. I even had one where I trimmed and trimmed it until it worked which would have been a much easier job if the crank wasn't already in the engine. I know that longer is generally heavier but, long stroke, big torque and you don't want those babies turning around in there to come down frontwards.

motorson said:

How close do the pistons come to the crank shaft on the bottom of the stroke? It seems like you would want them about as long as possible. All the little short skirt Honda pistons I have ever had needed to be that short to keep from hitting the crankshaft on the down stroke. I even had one where I trimmed and trimmed it until it worked which would have been a much easier job if the crank wasn't already in the engine. I know that longer is generally heavier but, long stroke, big torque and you don't want those babies turning around in there to come down frontwards.

Click to expand...

There is about 1/2 mile clearance. I would prefer longer pistons skirts and lower wrist pins holes also. Next time. Jim

comnoz said:

motorson said:

How close do the pistons come to the crank shaft on the bottom of the stroke? It seems like you would want them about as long as possible. All the little short skirt Honda pistons I have ever had needed to be that short to keep from hitting the crankshaft on the down stroke. I even had one where I trimmed and trimmed it until it worked which would have been a much easier job if the crank wasn't already in the engine. I know that longer is generally heavier but, long stroke, big torque and you don't want those babies turning around in there to come down frontwards.

Click to expand...

There is about 1/2 mile clearance. I would prefer longer pistons skirts and lower wrist pins holes also. Next time. Jim

Click to expand...

Is it because of the big bore...the rocking? :?: that is an 850 bore size???....Standard +20/40/60 range???
Tom
CNN

CanukNortonNut said:

comnoz said:

motorson said:

How close do the pistons come to the crank shaft on the bottom of the stroke? It seems like you would want them about as long as possible. All the little short skirt Honda pistons I have ever had needed to be that short to keep from hitting the crankshaft on the down stroke. I even had one where I trimmed and trimmed it until it worked which would have been a much easier job if the crank wasn't already in the engine. I know that longer is generally heavier but, long stroke, big torque and you don't want those babies turning around in there to come down frontwards.

Click to expand...

There is about 1/2 mile clearance. I would prefer longer pistons skirts and lower wrist pins holes also. Next time. Jim

Click to expand...

Is it because of the big bore...the rocking? :?: that is an 850 bore size???....Standard +20/40/60 range???
Tom
CNN

Click to expand...

This is an 80mm bore. The pistons rock because the wrist pin position is high and the skirt is too short for the necessary amount of piston clearance. .0065 inch with forged JE pistons. Jim

Ring gap only matters at slow idle so the increased gap is not what causes blow by & smoking, its the rim wear loss of stealing tension on top of rounded sealing square edges, which is what Wes and I found on 3 kn type filtered engine that left the bores ok enough just a touch up resaled new rings fast. if comnoz ring edges still good and finds bores pretty round w/o scores then what's left but head sealing on cylinder tunnels, ring seats &or intake guides passing gas, but why?

Steve, you mention K&N "type" filters. I have found that there is a big difference between real K&N and K&N offshore clones, which are plentiful and cheap. I had one clone type disintegrate at the bottom side of the filter. This was one of the types that has two layers of quite heavy steel screen with the filter paper in between. The paper filter started to break up and go thru the engine, only I did not notice it for some time. This did not do the engine any favours, in fact it needs a redo now.

I have not seen this happen with proper K&N filters. Lesson learned, pay the extra for real K& Ns but still check the filter for deterioration by holding it up to a light as part of maintenance/oiling.

A machinist friend rode his fully laden Rapide from Vancouver to Tierra del Fuego , S.A, then home. I don't recall/the total mioeage, but it was a lot and there were many miles of gravel road covered.
The following year he pointed it North and rode to the Arctic circle then home. Again there were lots of gravel miles done.
Once home and with about 40,000 miles on the bike since rebuild, he decided to strip the engine to check wear.
The cross hatch marks from honing were still plainly visible. Ovality and taper were all well within his very fussy machinist standards.

Same K&N filters on the bike the whole way.

Glen

hobot said:

Ring gap only matters at slow idle so the increased gap is not what causes blow by & smoking, its the rim wear loss of stealing tension on top of rounded sealing square edges, which is what Wes and I found on 3 kn type filtered engine that left the bores ok enough just a touch up resaled new rings fast. if comnoz ring edges still good and finds bores pretty round w/o scores then what's left but head sealing on cylinder tunnels, ring seats &or intake guides passing gas, but why?

Click to expand...

The last motor had over 80,000 miles on the pistons and had been run all of that time with K&N filters. The leakdown rate was still in the low teens the last time I checked it before it's fatal trip. Rings just don't seal well when they are not square to the bore. Jim

Wes and I only experienced the famous k/n brand not knock offs but can accept its not the issue on comnoz or other examples getting very hi mileage, so i guess the leakage points to issue I asked about on light wt shorty pistons rocking on bore sealing life. I was a bit surprised watching how close Combat piston skirts come to the factory flywheel at BDC.

I got a set of Hastings pistons & rings for a Ford 260 V8 rebuilt motor. The top ring gap was too wide at well over .025" I complained to Hastings and they reassured me that it would not be a problem. They provided data sheets and specs as evidence that the wide gap was within tolerance. So I put it together and it puffed smoke out the ex pipes (tight new bore, re-sized guides and quality guide seals). I was pissed. Then I got a new custom set of rings with a tighter end gap. Rebuilt the motor again - presto - no more smoking and years later it still doesn't smoke or burn oil.

I run my 750 Nort rings at about .010" or .011" new end gap and I've experimented as tight as .008" without problems even though I was warned not to. Now I use gapless 2nd rings and after about 20,000 miles I can still stand on the kickstart about 30 seconds before it leaks down. My personal experience is that the ring end gap is important.

I agree, ring gap is important.
Not so much for high RPM power but it is important for controlling blowby and oil consumption which was my only complaints.
That is why I am installing new extreme duty rings and hoping they will hold a good end gap beyond 10,000 miles. Jim

I'm not saying too wide gaps is something desirable, I'm saying Ring gap importance is relative to the rpm, so yes wide gaps not good for idle or oil wiping but don't give much lost on sealing power making pressure. This is the mystery of comnoz's issue ie: lost of sealing, not yet oil smoking. I'm in the close gap camp too so pensive on Peel gap quessimates to minimally fudge for max expected heat loads plus rocking short skirts in loser than normal bores. But if Peels power holds for 10K miles like Jim's then should be worth it. Btw past Peel had TS gapless ring set and they assured me Bon Ami like power was something they sold.

I am not sure tight rings gaps will actually fix my problem although it should help. If the majority of the leakage that is causing my problem is due to piston rocking then the tight ring gap will not help much. Time will tell. Jim

We sure do appreciate your extensive expensive experimenting for us Jim. We lucked TF out you opened your shop to us while we can still make use of it. What logic would the rest of us use if we found rocking pistons ring wear in a couple rear tires use? What could be done different? Would appreciate a video demo of proper Commando leak down test.

comnoz said:

I agree, ring gap is important.
Not so much for high RPM power but it is important for controlling blowby and oil consumption which was my only complaints.
That is why I am installing new extreme duty rings and hoping they will hold a good end gap beyond 10,000 miles. Jim

Click to expand...

I would try plugging those gas jets to reduce ring wear. What about Nylatron buttons? I've heard of Harley guys using them on loose pistons to good effect but don't know the longevity. Venolia used to use nylatron buttons on drag pistons to help prevent scuffing. PIA to install.

jseng1 said:

comnoz said:

I agree, ring gap is important.
Not so much for high RPM power but it is important for controlling blowby and oil consumption which was my only complaints.
That is why I am installing new extreme duty rings and hoping they will hold a good end gap beyond 10,000 miles. Jim

Click to expand...

I would try plugging those gas jets to reduce ring wear. What about Nylatron buttons? I've heard of Harley guys using them on loose pistons to good effect but don't know the longevity. Venolia used to use nylatron buttons on drag pistons to help prevent scuffing. PIA to install.

Click to expand...

If they were gas jets in the top of the piston I would plug them. Since they are radial ports I don't know How I would successfully plug them. Buttons are neat -for a short time. Jim

.0065" is certainly going to result in pistons banging around in the holes. No ring are going to survive that for long. I assume the big clearance is needed due to the cast iron bores/forged aluminum pistons combination?
The skirts "look" pretty worn from what I can see in the pics with debris scratching (dirt past the filter).
But what I "think" is see doesn't mean much, only a feeler gage between the skirts and bores, or bore mikes expertly used, and a fingernail expertly used to feel the surface can tell the real story.

LUCKY DAVE said:

.0065" is certainly going to result in pistons banging around in the holes. No ring are going to survive that for long. I assume the big clearance is needed due to the cast iron bores/forged aluminum pistons combination?
The skirts "look" pretty worn from what I can see in the pics with debris scratching (dirt past the filter).
But what I "think" is see doesn't mean much, only a feeler gage between the skirts and bores, or bore mikes expertly used, and a fingernail expertly used to feel the surface can tell the real story.

Click to expand...

The pictures look worse than they look in person. There is virtually no measurable wear on the pistons. There is about .0007 wear in the bores front to rear. That is more than I would want at this mileage but not unusual in my experience with these pistons. Jim

Your static (cold) tests show a lot of leak down, but that may not reflect the dynamic (hot) conditions.
At .0065" clearance the running temperature of the pistons should be very high by the time they fit the bores as a press fit. I would think that a "conventional" fit of .0015" or so would result in substantially lower piston running temps. This would of course increase the effective hardness/strength of the pistons in running conditions.
Or maybe they would promptly seize.....what do I know? I'm just theorizing here, I have zero experience with this combination of parts.

LUCKY DAVE said:

Your static (cold) tests show a lot of leak down, but that may not reflect the dynamic (hot) conditions.
At .0065" clearance the running temperature of the pistons should be very high by the time they fit the bores as a press fit. I would think that a "conventional" fit of .0015" or so would result in substantially lower piston running temps. This would of course increase the effective hardness/strength of the pistons in running conditions.
Or maybe they would promptly seize.....what do I know? I'm just theorizing here, I have zero experience with this combination of parts.

Click to expand...

The leakdown tests were done on a hot motor.

Piston clearance is only going to be "tight" at full rpm, full load on a hot day. That is what they must be clearanced for to avoid a seizure under the "worst" conditions. That is piston temps in the 550 to 600F. range and requires .0065 clearance with a forged piston in an iron cylinder.

Pistons temps easily vary by 300 degrees between max load and an easy 50 mph cruise with an air cooled motor.

That means that much of the time the piston clearance is going to be "loose" .

It would be great if they could be set up at .0015 clearance but they would seize before they got out of the driveway. Jim