Piston Balancing and dilling

John Magyar has an idea that I think is worth pursuing. Norton pistons are a bit offset with deeper intake valve pockets than exhaust pockets. This makes them heavier on one side and that weight tends to force the piston to rock when it reverses direction at the top and bottom of the stroke. At high RPM that side pressure may add up to enough force to wear on the cylinder wall whereas a balanced piston may not have as much rocking force and cause less wear.

So I had an idea for balancing the piston.

First I mounted a piston in a lathe and used a center drill to place a tiny dent in the center of the under crown.



The dent location is marked with a black spot.



Using a pointed wire in a vice I placed the pistons on the point. The pistons tilted toward the heavy exhaust side (smaller valve pocket). You can see that I placed a penny (about 3 grams) on the intake side of the piston top to level it out.



Someone can calculate how much this mass would multiply out to at high RPM.

Now comes a lesson in piston drilling. First layout & mark the pistons to locate the holes in an equidistant manner.



Mount the piston in a vice and step drill the piston. At this point you may just be drilling the pistons for lightness or extra skirt lubrication to reduce wear. If you're balancing the piston then you want the check it as you increase the size of the holes.



Drilled pistons below. These pistons are drilled in both skirts just for lightness but you get the idea.



You should chamfer the holes with a chamfer bit – tilting the drill so the complete circumference of the hole is chamfered. Piston below is a lightened Hepolite piston.



This may only be for perfectionists and whether or not piston balancing is worth it and makes any difference has yet to be proved. Larger valve pockets would increase the imbalance. Combustion forces may be greater than the unbalanced forces. No matter what - I think there is value in getting extra lubrication to the cylinder wall and I have no doubt that any weight you can take off pistons goes a long way in taking stress off our vibration prone Nortons.

Re: Piston Balancing

you have drilled both sides of the pistons, are the holes bigger on one side?

Re: Piston Balancing

jseng1 said:

This makes them heavier on one side and that weight tends to force the piston to rock when it reverses direction at the top and bottom of the stroke. At high RPM that side pressure may add up to enough force to wear on the cylinder wall whereas a balanced piston may not have as much rocking force and cause less wear.

Click to expand...

Very interesting! Are the wrist pins centered fore-and-aft in your pistons? Usually, they're biased towards the back (I think); or is it the front? Either way, if that's the case, the combustion forces would overcome any imbalance. If not, then rock on!

Nathan

Re: Piston Balancing

madass140 said:

you have drilled both sides of the pistons, are the holes bigger on one side?

Click to expand...

In that pic the pistons are just drilled in both skirts for lightness but you get the idea.

Re: Piston Balancing

Nater_Potater said:

jseng1 said:

This makes them heavier on one side and that weight tends to force the piston to rock when it reverses direction at the top and bottom of the stroke. At high RPM that side pressure may add up to enough force to wear on the cylinder wall whereas a balanced piston may not have as much rocking force and cause less wear.

Click to expand...

Very interesting! Are the wrist pins centered fore-and-aft in your pistons? Usually, they're biased towards the back (I think); or is it the front? Either way, if that's the case, the combustion forces would overcome any imbalance. If not, then rock on!

Nathan

Click to expand...

Nort cranks are off set slightly. I don't think the pins are offset.

well its certainly something I've never heard of doing, now how to determine if it helps.

madass140 said:

well its certainly something I've never heard of doing, now how to determine if it helps.

Click to expand...

As mentioned in a previous thread here on this.
In older bikes, pistons were quite commonly holy in the skirt region, for better lube.
But back then they only added a teaspoon'worth of oil every few miles.
Teens and 1920s, of course.


As to whether it helps in any significant way in modern bikes.... ??

BTW, oddly enough, adding holes to skirts like that probably makes them stronger.
Spreading stresses, and all that...

The question was posed as "what is the imbalance fore and aft of the wrist pin". What I had in mind were Norton race pistons with significant domes and resulting valve pocket cut outs.

With the pistons Jim used, 3 grams imbalance is indicated. I would have simply put the piston with wrist pin on two precision knife edges on a machinist block but the pin and divot beneath the crown method seems to work - assuming the divot is in the vertical plane with the wrist pin.

With a stock rod to stroke ratio, an 89mm stroke and spinning at 7,500rpm this works out to about 3,600 g's. At 3 grams that results in 24 lbf (if I have the math and units correct). Now Jim's flat top example piston is rather symmetrical but I can see where one can get a significant imbalance with a high dome and significant differences between the intake and exhaust valve cut out and relief geometries.

Most any modern engine I have seen has very symmetrical piston crowns as most are four valve and rather flat.

Interesting stuff and maybe a few of the engine experts might want to chime in on the subject.

Re: Piston Balancing

madass140 said:

.....are the holes bigger on one side?

Click to expand...

This was my first thought too. Very interesting thread thanks for posting it Jim. At first glance it seems like removing more on one side would do it, or at least help to minimize the rocking. Cj

For some anti-logic of worth of balanced pistons worth - most the fastest turning engines have a combo of crankshaft and piston pins offset from bore centers as well as asymmetric shaped piston tops - so don't expect anything significant but removing a bit more rod jerk down inertia.

hobot said:

For some anti-logic of worth of balanced pistons worth - most the fastest turning engines have a combo of crankshaft and piston pins offset from bore centers as well as asymmetric shaped piston tops - so don't expect anything significant but removing a bit more rod jerk down inertia.

Click to expand...

Location of crank or wrist pin with respect to bore centerline has no bearing on this discussion but thanks for highlighting it.

It has everything to do with distribution of mass fore and aft of the wrist pin and the resulting moments around the wrist pin due to the variance (imbalance) of forces of acceleration.

Dances with Shrapnel said:

The question was posed as "what is the imbalance fore and aft of the wrist pin". What I had in mind were Norton race pistons with significant domes and resulting valve pocket cut outs.
........

With a stock rod to stroke ratio, an 89mm stroke and spinning at 7,500rpm this works out to about 3,600 g's. At 3 grams that results in 24 lbf (if I have the math and units correct). Now Jim's flat top example piston is rather symmetrical but I can see where one can get a significant imbalance with a high dome and significant differences between the intake and exhaust valve cut out and relief geometries.

Click to expand...

The bigger the valve pocket the bigger the imbalance. This happening a gazillion times per hour would seem to add up to some significant wear and stress. Its a new concept that hasn't been addressed. Someone would have to test with a balanced piston on one side and compare the results.

jseng1 said:

Dances with Shrapnel said:

The question was posed as "what is the imbalance fore and aft of the wrist pin". What I had in mind were Norton race pistons with significant domes and resulting valve pocket cut outs.
........

With a stock rod to stroke ratio, an 89mm stroke and spinning at 7,500rpm this works out to about 3,600 g's. At 3 grams that results in 24 lbf (if I have the math and units correct). Now Jim's flat top example piston is rather symmetrical but I can see where one can get a significant imbalance with a high dome and significant differences between the intake and exhaust valve cut out and relief geometries.

Click to expand...

The bigger the valve pocket the bigger the imbalance. This happening a gazillion times per hour would seem to add up to some significant wear and stress. Its a new concept that hasn't been addressed. Someone would have to test with a balanced piston on one side and compare the results.

Click to expand...

I am sure the concept has been out there somewhere but not necessarily in Norton circles. I began to wonder about this while experiencing phenomenal cylinder liner wear on our 500 and 750 USS engines. This is where I introduced the Bore-Tech solution to the accelerated cylinder wear problem I was experiencing. I found this to work back in 2005. I am all for testing but to me, this is one for just balancing and be done with it. In my opinion I don't think a small imbalance of 3 grams is anything near imperative for a street machine but for racing, maybe some attention should be given.

Trying to keep this all in context, the example above uses maximum acceleration. I seem to recall the two maximum accelerations are at TDC and BDC and they are roughly 4,000 g's and 2,000 g's (don't ask me which occurs where). I used a convenient online calculator for maximum acceleration with link given below:

http://www.bigboyzheadporting.com/PistonSpeed.htm

Regardless of cylinder pressure, these forces due to imbalance still remain and in my opinion are unnecessary.

Good discussion, thanks Jim

jseng1 said:

The bigger the valve pocket the bigger the imbalance.

Click to expand...

The bigger the piston dome the greater the potential for imbalance since the hemisphere is offset toward the exhaust side of the cylinder bores. This is additive to the mass bias between the valve pockets.

Maybe this is why (in part) the factory short strokes and Norman White went with a full hemi concentric with the cylinder bore.

Thanks Jim for initiating this and collecting the data.

here is an extreme example of a piston that has a VERY heavy pin imbalance.these things were scary heavy and cast to boot. this was also an engine you could push in the pistons without a ring compressor as the deck was not 90 degrees to the cylinder bore.

http://www.348-409.com/forum/threads/be ... 016/page-2

Dances with Shrapnel said:

jseng1 said:

The bigger the valve pocket the bigger the imbalance.

Click to expand...

The bigger the piston dome the greater the potential for imbalance since the hemisphere is offset toward the exhaust side of the cylinder bores. This is additive to the mass bias between the valve pockets.

Maybe this is why (in part) the factory short strokes and Norman White went with a full hemi concentric with the cylinder bore.

Thanks Jim for initiating this and collecting the data.

Click to expand...

Yes it may be a little worse with the domed pistons but not much because the JS Domed pistons are under milled for a consistent thickness of the dome. The domed pistons are still only 3 or 4 grams out of balance. Below is a photo of a Domed piston.



You can see where I placed a 3/8 diam piece of alum on the ex side to balance it. This piece is about .270" thick which is 3xs the thickness of the piston skirt. That means that three 3/8" diameter holes drilled into the piston skirts will just about balance the piston (see photo below).



Another big advantage here is that the drilled skirt will provide more lubrication to the cylinder wall on the front side (which shows the most wear).

The question now is - can end users drill the skirts or should I provide this service?

Jim, in answer to your question, you should offer the service, along with the extra lightening work, and a couple of other services.
You should add clear photos and info to your otherwise excellent web site for the optional work. The work would be a 'standardised operation' for you, once set up for it, so it would be very easy for you to set a standard price.
It would be as simple as:
Option package 1: correctly drilled piston skirts =$xx.
Option package 2: extra lightening of pistons (xx grams lighter) =$xx.
And then you should also add:
Option package 3: Teflon coated skirts =$xx.
Option package 4: ceramic thermal barrier coated crowns =$xx.

Fast Eddie said:

Jim, in answer to your question, ..... ...you should offer the service,. ..........
You could add clear photos and info to your otherwise excellent web site.....
Option package 1: correctly drilled piston skirts =$xx.
Option package 2: extra lightening of pistons (xx grams lighter) =$xx.
And then you should also add:
Option package 3: Teflon coated skirts =$xx.
Option package 4: ceramic thermal barrier coated crowns =$xx.

Click to expand...

+1..... :wink:

Way back in the late 60's I employed exactly this same theory and method of balancing the pistons in a 1952 Norton 500 Dominator which consistantly thrashed a "C" type Tiger 100 of the same era , (and he hated that)

I'd even gas welded crowns onto the pistons and shaped the skirts by hand , so as to provide just enough remaining metal to give a good thrust distribution surface, buttoned the gudgeons also .

Then one fatelul day at WOT ,,,,,KAAAAABOOOOOM !!the top came off the left piston and the engine simply destroyed itself . The top end of the piston was stuck in the top of the bore with a buckled exhaust valve punched well into the top of it.

Everything was wreaked, busted camshaft , both cases , bent both rods. The inside of the cases looked like they had been grit blasted with roadway metal .
I recall saying to myself at the time , that the holes in the skirt which I'd drilled created a stress raiser and the piston broke at that point.

Now I just leave my present bike's engine stock , apart from it being +040".
It's "75 Mk3 850 Interstate , and I like it how it is . The older I get , the faster I was.Lol
Supaflee