Piston Balancing and dilling

Hobot · Apr 18, 2015

Great tale supraflea and likely one your most lasting vivid memories so essentially priceless compared to current stable Commando, so ya need 2 of em eh so at least one of em worth writing home or here about. Now back to the still illogical race engine extremists please try everything and get back to us saner folks.

jseng1 · Apr 19, 2015

Supaflee said:
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

Supraflee

Sounds like you drilled holes between the pin and the top of the piston. If so then this is why your piston broke. The Dominator pistons are long with the pin located further downward. Drilling in the high stress area between the crown and the pin can cause a piston to break because the heavy crown wants to pull loose (there isn't much reinforcment between the pin and the crown on a Dominator).

Look at the photo below of the lightweight pistons. The big advantage is the high pin location. The area below the pin centerline is low stress. There is a thick web connecting the pin boss to the crown that will never break. Drilling below the pin centerline as in the photo is the safe way to do it. You don't have to worry about drilled JS pistons and balancing them may actually reduce stress. . They have been tested above 8000 RPM. The piston below now has 20,000 miles on it and shows no evidence of rocking and has a leak down of less than 1% (earlier photo).

Hobot · Apr 19, 2015

I've been sucked into JSM holey camp but learning a lot about hole sizes and material thickness for surface area increase vs decrease and staggered holes worse strength wise this is how I see my next factory parts 750 pistons though a wee bit bigger holes and spaced more apart. Some could just be dimples for oil holders in high contact areas.

jug · May 2, 2015

Jim, I'm going to balance the pistons when they arrive (should be in a few days according to the USPS tracking site), but I'm wondering how this will affect the balance factor of the engine, if at all. :?
jug

jseng1 · May 3, 2015

jug said:
Jim, I'm going to balance the pistons when they arrive (should be in a few days according to the USPS tracking site), but I'm wondering how this will affect the balance factor of the engine, if at all. :?
jug

changes balance factor by about 1 to 1-1\2%
not to worry

Anonymous · May 5, 2015

Hi jseng1.
Perhaps, instead of holes to reduce friction you may trial "...numerous small, angled grooves, semi-circular at their apex..." and maybe do away with rings entirely which would make my recent ring purchase wasted.
Reference: http://m.motoring.com.au/news/2013/auss ... ings-40773

Perhaps, a crankless engine and Norton life would be so simple.
http://museumvictoria.com.au/collection ... r-inventor
Ta.

Dances with Shrapnel · May 5, 2015

needing said:
Hi jseng1.
Perhaps, instead of holes to reduce friction you may trial "...numerous small, angled grooves, semi-circular at their apex..." and maybe do away with rings entirely which would make my recent ring purchase wasted.
Reference: http://m.motoring.com.au/news/2013/auss ... ings-40773

Perhaps, a crankless engine and Norton life would be so simple.
http://museumvictoria.com.au/collection ... r-inventor
Ta.

OT relevance?
Ta.

Crankless (swash plate) technology is now a common application in hydraulic motors and pumps with some experiments in IC engines.

Anonymous · May 5, 2015

Hi Dances with Shrapnel.

Point 1. is picking up on the original jseng1 thread post e.g.
"... at this point you may just be drilling the pistons for lightness or extra skirt lubrication to reduce wear..." and "...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...".
So, less rings = less weight, and - Scallops on wear face = less weight and better lube and perhaps air cushioning.

Point 2. Musing for edification.
Ta.

Dances with Shrapnel · May 5, 2015

Fair enoungh, piston balancing (the major jist of this thread) and drilling (presumably to achieve balance, not lightening).

jseng1 · May 20, 2015

I just got this email from Robert C about stock Hepolite pistons and bore wear.

"I thought my own original [barrels] were in quite good condition. Gave them a Hone and they looked fine except for one extra bit of wear not removed by the hone lower down the bore, after reading your item about drilling the pistons put 2 and 2 together and was thinking this was wear from the piston pushing against the side due to the weight imbalance just as the piston changes direction checked the piston clearance with feeler gauges between piston and bore and was about 0.007"

This is interesting because you don't always see wear further down in the bore but usually up top near the ring ridge. .007" is a lot of clearance for cast pistons and suggests that cast pistons cause just as much wear as forged pistons. Whether its the off balance of the pistons or some other reason still needs to be established.

p400 · Jun 19, 2015

Has this subject been discussed at any other technical website ?
Maybe a publication in the past 100 years?

Dances with Shrapnel · Jun 19, 2015

p400 said:
Has this subject been discussed at any other technical website ?
Maybe a publication in the past 100 years?

Which topic; balance of pistons fore and aft of the wrist pin or drilling piston skirts to achieve balance?

jseng1 · Jun 19, 2015

Well some people are trying it.

Just got this photo today from a Chap down under who did his own piston balancing according to the instructions on my webpage. Not a bad job from the looks of it.

Piston Balancing and dilling

Hobot

jseng1

Hobot

jug

jseng1

Anonymous

Guest

Dances with Shrapnel

Anonymous

Guest

Dances with Shrapnel

jseng1

p400

Attachments

Dances with Shrapnel

jseng1

Similar threads