new leakdown results

comnoz said:

jseng1 said:

Also - the Norton cylinder axis is already staggered about 1/4" to the front of the crank axis. I wouldn't move it further.

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Not on any Norton twin I have measured.
I have always found the barrel axis somewhere between 0 and .040 behind the crank.

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You are right Comnoz - the cylinders are behind the crank not in front (my mistake to depend on 30+ year old memory). I just checked 750 and 850 cases and I'm measuring about .090". But the bored cylinders could be out of line.

Back then I made a tube that fit snugly between the superblends, fitted the cylinders and ran a T square down the front and then the rear of the bore. The cylinders were staggered about .090" to the rear. This was back in the late 1980s and it was the first bike I started using the lightweight forged pistons in. I was forced to go to lightweight pistons because my cranks and cases were cracking. I used the stock rods so I had to lower the head about 3/8". That meant milling the head, an entire fin off the cylinders, the bottom of the cylinders and the cases as well. So I wanted to check the crank axis relative to the bore before I started.

the first 850 lightweight forged pistons (weighed less than 750 hepolites) note missing fin on milled cylinders.

just did a quick re-check on 750 & 850 cases - cylinders are staggered about .090" to rear (not front) according to my measurements, but the bored cylinders I'm using could be out of line.

The cylinders on my motor were originally bored about .030 to the rear of the crank. When I installed the sleeves I offset the bore and put them directly over the crank centerline. Jim

Have you had the head off yet ? Which way is the cylinder worn ? Which way do the pistons typically tip on compression vs expansion ?

When I had new pistons fit Jimmy said that there was a swirling wear pattern on the back of the cylinder. He had not seen such an odd pattern before. Jimmy did a little bit of everything two strokes, Harleys, 1000cc Yamahas for some sort or midget racing, etc.

Greg

gjr said:

Have you had the head off yet ? Which way is the cylinder worn ? Which way do the pistons typically tip on compression vs expansion ?

When I had new pistons fit Jimmy said that there was a swirling wear pattern on the back of the cylinder. He had not seen such an odd pattern before. Jimmy did a little bit of everything two strokes, Harleys, 1000cc Yamahas for some sort or midget racing, etc.

Greg

Click to expand...

The cylinder wear seems to be most on the front side of the bore about 1/3rd of the way down. .0007 out at the front but the bores had no scratches.

I don't know for sure what is going on.

Here is the front and back sides of the pistons when I tore the engine down at 9000 miles.

They had been fit a .0058 clearance. Looking at the wear on the pistons I would say they were almost too tight but they don't show real evidence of rocking.







Here are front and back shots of another pair of high pin pistons that were made from the same drawing. These didn't have gas ports but were fitted at .0065 clearance in gray cast liners and raced several times. They show clear evidence of rocking. They wore the cylinder bores on the front side also. [severely]

Front


Back


And then here is another high pin piston produced from the same drawing. It does have gas ports. It was fitted in an aluminum barrel with austentic liners at .004 clearance and raced for most of a season. It showed nearly perfect wear. It's mate in the other side of the motor became junk when a Ti valve broke.

Was easy to reproduce this pattern on my new pair $160 each mower pistons with chain saw file. Knowing what we do now how could anyone not want too?

AV304 profile
: A recess formed at the bottom edge is designed to introduce, retain and provide more oil faster to the main contact area. As shown in Figure 10, the stroke starts with plenty of oil on the entire skirt surface, partly due to oil retained in the oil reservoir at the top of the skirt. It is observed that oil continues to flow to the middle of the skirt through the bottom recess throughout the stroke. As the skirt is loaded by the side force, the main contact area is fully lubricated by the oil collected in the lower recess. More oil is introduced as the piston speed increases. This design shows the lowest friction during the expansion stroke. AV304 retains a lot of oil throughout the stroke. Oil retained in the reservoir formed in the upper skirt provides lubrication to the middle and bottom of the skirt as the skirt begins to tilt toward the non-thrust side. AV304 shows the lowest friction during the intake stroke. It is observed that when the intake stroke begins, oil is collected through the recessed area formed at the bottom of the skirt. It then spreads onto the main load bearing area in the middle of the skirt. The whole skirt surface is flooded with oil hroughout the stroke. The recessed area retains oil throughout the stroke. EFFECT OF ENGINE SPEED
From the end of compression stroke to 40° after TDCF, AV304 friction shows a different pattern from the other profiles. It is well known that, as engine speed increases, friction force during the expansion stroke increases. However, AV304 friction decreases with increased engine speed. As mentioned earlier, more oil is introduced as the piston speed increases due to the oil recess at the bottom of the skirt. AV304 is remarkably quieter than the other profile variants. It has been observed from a series of these and other engine tests that skirt slap noise is related to the hydrodynamic lubrication condition. Oil is also known to have a noise damping effect.
OMG! >>>
http://www.federalmogul.com/en-US/Media ... cript2.pdf


Oh yeah some chevy tech to apply too.
http://www.camarohomepage.com/ls3/

comnoz said:

gjr said:

Have you had the head off yet ? Which way is the cylinder worn ? Which way do the pistons typically tip on compression vs expansion ?

When I had new pistons fit Jimmy said that there was a swirling wear pattern on the back of the cylinder. He had not seen such an odd pattern before. Jimmy did a little bit of everything two strokes, Harleys, 1000cc Yamahas for some sort or midget racing, etc.

Greg

Click to expand...

The cylinder wear seems to be most on the front side of the bore about 1/3rd of the way down. .0007 out at the front but the bores had no scratches.

I don't know for sure what is going on.

Here is the front and back sides of the pistons when I tore the engine down at 9000 miles.

They had been fit a .0058 clearance. Looking at the wear on the pistons I would say they were almost too tight but they don't show real evidence of rocking.

Here are front and back shots of another pair of high pin pistons that were made from the same drawing. These didn't have gas ports but were fitted at .0065 clearance in gray cast liners and raced several times. They show clear evidence of rocking. They wore the cylinder bores on the front side also. [severely]

Front

Back

And then here is another high pin piston produced from the same drawing. It does have gas ports. It was fitted in an aluminum barrel with austentic liners at .004 clearance and raced for most of a season. It showed nearly perfect wear. It's mate in the other side of the motor became junk when a Ti valve broke.

Click to expand...

Jim,

Please help me understand what I'm missing here (I can be a bit thick on occasions)...

I understand the issue of forged pistons in air cooled motors requiring bigger tolerances than in liquid cooled motors, as being because: air cooled barrels will 'tighten up' when they reach operating temperature. Therefore extra cold clearance is required to allow for this.
But... this means that when the engine is up to operating temperature, the extra tolerance is gone.
Therefore, there is no extra tolerance (to cause rock or slap or etc) in an engine at operating temperature.

That's how my mind views it anyhow... What am I missing?

Fast Eddie said:

Jim,

Please help me understand what I'm missing here (I can be a bit thick on occasions)...

I understand the issue of forged pistons in air cooled motors requiring bigger tolerances than in liquid cooled motors, as being because: air cooled barrels will 'tighten up' when they reach operating temperature. Therefore extra cold clearance is required to allow for this.
But... this means that when the engine is up to operating temperature, the extra tolerance is gone.
Therefore, there is no extra tolerance (to cause rock or slap or etc) in an engine at operating temperature.

That's how my mind views it anyhow... What am I missing?

Click to expand...

You are not taking into account the fact that the piston clearance is only at it's minimum at the end of the longest hill, straightaway or pull, on the hottest day.
At any other time the piston temp on an air cooled motor is going to be much lower so the piston clearance is going to be much greater. Piston temp in an air cooled motor can vary by 300 degrees F between full throttle and light cruise. Jim

comnoz said:

Fast Eddie said:

Jim,

Please help me understand what I'm missing here (I can be a bit thick on occasions)...

I understand the issue of forged pistons in air cooled motors requiring bigger tolerances than in liquid cooled motors, as being because: air cooled barrels will 'tighten up' when they reach operating temperature. Therefore extra cold clearance is required to allow for this.
But... this means that when the engine is up to operating temperature, the extra tolerance is gone.
Therefore, there is no extra tolerance (to cause rock or slap or etc) in an engine at operating temperature.

That's how my mind views it anyhow... What am I missing?

Click to expand...

You are not taking into account the fact that the piston clearance is only at it's minimum at the end of the longest hill, straightaway or pull, on the hottest day.
At any other time the piston temp on an air cooled motor is going to be much lower so the piston clearance is going to be much greater. Piston temp in an air cooled motor can vary by 300 degrees F between full throttle and light cruise. Jim

Click to expand...

Indeed I wasn't !

Do you still have the 9000 mile rings laying around ? If so, are they worn asymmetrically ( I'll bet it's hard to say which side was the front and which was the back) ? Did you have the oil analyzed at 9000 miles ? Did it show a lot of chrome besides the iron ?

Greg

Its the alloy piston expanding at max fuel-heat load more than the bore and the bore expanding more at top than bottom to compensate for by trial-error per engine combo conditions. Maybe Jim's experinece means we've hit=surpassed limits of fitting ashtry pistons in air cooled clunkers like race compound tires on street bikes, not that great till heated then wonderful till cords show too soon. The solution everyone else uses but Nortons, piston coolling oil jets that equals or exceed the total engine lubrication volumes. No straight forward or even possible placement for jets in the crank cases like most modern engines [tight clearances with rods/crank in the way] so only option I see is rod big end rim notches like my big lawnmower that's also Bore Tech'd. Top of bore expands more that bottom so made sense to me to tapper bore bigger at bottom so when heated evens up clearances top to bottom with swollen piston working like crazy. Before spring grass erupts Kohlter engine will get jugs and piston/rings chilled with some saw chains before use too.

http://www.diversifiedcryogenics.com/motorsports.html
http://www.300below.com/motorsports/

Confusing thoughts on piston crown vents into ring lands causing blow by.

Another option is to modify the piston so that the cylinder pressure is not ‘denied access’ to the inside diameter of the piston ring. There are machined design features that can be added to pistons to allow the cylinder pressure to act on the inside diameter of the top ring, even when the ring is forced against the top of the ring groove. https://www.highpowermedia.com/blog/376 ... ng-flutter

hobot said:

Confusing thoughts on piston crown vents into ring lands causing blow by.

Another option is to modify the piston so that the cylinder pressure is not ‘denied access’ to the inside diameter of the piston ring. There are machined design features that can be added to pistons to allow the cylinder pressure to act on the inside diameter of the top ring, even when the ring is forced against the top of the ring groove. https://www.highpowermedia.com/blog/376 ... ng-flutter

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That is what I have, known as radial gas ports. Jim

Yes so confusing to me those were mentioned as possible factor in your current poor sealing.

hobot said:

Yes so confusing to me those were mentioned as possible factor in your current poor sealing.

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Me too. Jim

Hm, its said it takes 10,000 hours of practice for pieople to master sport or music skills so implies Peel's got to learn to max out within 10, 000 mil3s before power drops too much.. Current practice to delay/avoid ring flutter is to make 2nd ring gap about 20% larger than top so nil pressure build up below 1st ring. IIRC that's what I asked Ken to do in Peel.








5

comnoz said:

hobot said:

Yes so confusing to me those were mentioned as possible factor in your current poor sealing.

Click to expand...

Me too. Jim

Click to expand...

Not poor sealing when new - but accelerated ring wear because of the extra pressure created by the radial ports - which then led to worn rings and a big ring end gap (and I'm guessing it possibly caused some bore wear???) - which usually means leakage and blow by. I think comnoz mentioned something about the rings being worn because of the radial jets in your previous leakdown thread a week or two ago.

Comnoz - What I want to know is how the pistons with .004" clearance managed to survive and if that worked well - why did you give the other set (that didn't work well) .0065" clearance?

jseng1 said:

Jim C - What I want to know is how the pistons with .004" clearance managed to survive and if that worked well - why did you give the other set (that didn't work well) .0065" clearance?

Click to expand...

One of the significant differences that Jim C mentioned was that the .004" clearance was for alloy cylinders with austenitic liners. Austenitic cast irons can have much higher thermal expansion rates than conventional grey cast iron. Some of the austenitic ductile irons have a 60% higher thermal expansion (CTE) than the common gray cast irons. That means that .004" clearance in the austenitic cylinders could easily give you the same (or even more) hot clearance as .0065" in a common cast iron cylinder. What I'm curious about is more details on what alloy the liners were, and why Jim tried them, and why he isn't still using them.

Ken

lcrken said:

jseng1 said:

Jim C - What I want to know is how the pistons with .004" clearance managed to survive and if that worked well - why did you give the other set (that didn't work well) .0065" clearance?

Click to expand...

One of the significant differences that Jim C mentioned was that the .004" clearance was for alloy cylinders with austenitic liners. Austenitic cast irons can have much higher thermal expansion rates than conventional grey cast iron. Some of the austenitic ductile irons have a 60% higher thermal expansion (CTE) than the common gray cast irons. That means that .004" clearance in the austenitic cylinders could easily give you the same (or even more) hot clearance as .0065" in a common cast iron cylinder. What I'm curious about is more details on what alloy the liners were, and why Jim tried them, and why he isn't still using them.

Ken

Click to expand...

I have not been using them because no one in the US will make them anymore. There is not enough demand since aluminum and nikasil became the norm. I found one place in the UK that will do them on special order but they are pricey.

I do have austentic liners for my new billet aluminum cylinders. I had to turn them from bar stock. Jim

Ah, the leakdown. So many variables.

First I'll assume you are dead certain the cylinders are round, at least at the top where the readings are taken.

I spent a number of years improving leakdown on H-D Sportster 883 racers, which have some significant problems. After the usual cylinder shape issues, simplify your life with a couple simple checks; Spray a bit of WD40 in the cylinders and spin the engine sans spark a couple times. If LD improves significantly, it's the rings.
To the point, there are two critical ring sealing points, neither of which has anything to do with end gap.
First, and most importantly, is the piston ring lands. Any defect here will add +5-10% leak. It's also very difficult to confirm this, and impossible to correct without removing the pistons. Look on the lower land (pressure surface) of both compression rings and note any pimpling, unequal discoloration, scratches or defect. If your side clearance is not on the wide side, use some fine jeweler's rouge to polish the land and ring together. Be very careful about the ring ends while assembling/dessembling the rings.
Second is the cylinder wall surface itself. I've always had best luck in iron bores by using a plateau hone, or as I did being a cheapskate by wrapping a rightly-sized ball hone with 600 grit wet-and-dry sandpaper and honing the bore until you have an equal finish everywhere in the ring travel area. Then, assemble the (very clean) rings pistons and bores with ZERO lubrication except for a light coat of WD40 on the rings and pistons.

Start normally, and by the time the engine oil has splashed the walls, the rings will have made love to everything and you should end up with as good as .75% leak, which was very achievable in the H-Ds.

http://www.enginebuildermag.com/2000/09 ... -finishes/