1967 Norton Atlas pistons and rods

Hi All
Just received my .020 over forged lightweight pistons from Jim Schmidt. Waited just about 2 months to get them just as he had told me. Great guy to deal with. Here’s the question…the pistons I bought were for use with the stock rods. They were identified as lightweight “medium” compression but when I measure them they appear to be very close to a stock Commando flat top which I think where something like 8.9 :1 but lighter by substantial grams. In some ways I wished I had sprung for the long rod / short piston approach that he sells but was trying to be sensible relative to cost for street use. Would like to have realized a drastic reduction in vibration. That said anybody here use the lightweight pistons with stock rods and if so what was the reduction in vibration like? Also any problems with stock rods breaking with standard ( not racing) road work? Thanks

jms said:

Hi All
Just received my .020 over forged lightweight pistons from Jim Schmidt. Waited just about 2 months to get them just as he had told me. Great guy to deal with. Here’s the question…the pistons I bought were for use with the stock rods. They were identified as lightweight “medium” compression but when I measure them they appear to be very close to a stock Commando flat top which I think where something like 8.9 :1 but lighter by substantial grams. In some ways I wished I had sprung for the long rod / short piston approach that he sells but was trying to be sensible relative to cost for street use. Would like to have realized a drastic reduction in vibration. That said anybody here use the lightweight pistons with stock rods and if so what was the reduction in vibration like? Also any problems with stock rods breaking with standard ( not racing) road work? Thanks

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You won't or shouldn't break anything on the street using the stock rods.

There will be some vibration relief with the lighter weight pistons but not a lot unless you had your crank balanced for the reciprocating weights of the rods, pistons, wrist (gudgeon) pins, and clips. I have JS long rods pistons cam lifters springs and so on in my engine and then some. It is all balanced, but not vibration free in a short solid mounted frame. The featherbed might be a little different, but Jim has a featherbed and has said you can't get rid of the vibration completely. I only ride on the street. Was getting all the JS parts a waste of money? I think it might depend on one's definition of waste, and I'm probably not the right guy to listen to. I can't take the money with me when I go. Might as well spend it while I'm still breathing and have a little fun.

By the way, I think your analysis of the medium compression pistons being nearly the same height above the deck as a Commando flat top piston to be reasonable. If you want a tad more compression, you can assemble the engine without a barrel base gasket using a sealer like Yamabond, Max Torque Permatex Optimum 27036, or another sealer than can take high heat. Barrels and cases mating surfaces have to be in very good condition to get a good seal. You can also use a thinner head gasket. The composite gasket compresses down more than .040, and you can get a .032 or .021 copper gasket from Jim if you have enough clearance.

All interesting viewpoints. Thanks. I’ve read that the Factory used those dished cast. 7.6:1 compression ratio pistons ( which I have two good standard bore pair if anybody is interested ) in an effort to reduce vibration once they punched out the 650 Dommie to 750. Why they didn’t play with the longer rod shorter piston approach seems like an engineering miss back then. Regardless if it is true that higher compression means more vibration, then I’m trying to keep low. With a .020 base gasket and a .040 standard head gasket I’m guessing that it puts me somewhere around 8.5:1 CR

IMHO, with regular conservative street riding (some spirited) the vibration factor comparing the ‘72 Combat I had and my Atlas with Commando pistons, the Commando buzzed more. I did have the isolastics set on the tight side, but no where as tight as a solid mounted featherbed. The only thing I have had that vibrated worse than the Combat was an ‘07 Triumph Thruxton. At one point I had all three at the same time and rode them all regularly, so it was a side-by-side comparison. With all that you’re doing, you’re mitigating the vibration as much as you can and probably will have a smooth machine. Isn’t the vibration, smell of the gas and hot oil part of it though? If it wasn’t for me, I’d ride a Honda.

kernel65 said:

IMHO, with regular conservative street riding (some spirited) the vibration factor comparing the ‘72 Combat I had and my Atlas with Commando pistons, the Commando buzzed more. I did have the isolastics set on the tight side, but no where as tight as a solid mounted featherbed. The only thing I have had that vibrated worse than the Combat was an ‘07 Triumph Thruxton. At one point I had all three at the same time and rode them all regularly, so it was a side-by-side comparison. With all that you’re doing, you’re mitigating the vibration as much as you can and probably will have a smooth machine. Isn’t the vibration, smell of the gas and hot oil part of it though? If it wasn’t for me, I’d ride a Honda.

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Haha. In 1980 I bought a 1952 Vincent Rapide from a guy here in Connecticut and believe me it lived up to its name “ Plumbers Nightmare”. To your point it was all part of the experience and fun. It was my only transportation at the time and I finally let it go for a Moto Guzzi Lemans 1. Wish I had it today! Good point Kernel