750 vs 850 balance

Hi,

Maybe a easy answer but I am wondering if the 750 and 850 bottom ends are the same or not. Is the counterbalance the same?

Thanks,
Dave

Yep same cranks rods but for 850 thicker bolts and heavier pistons so 750 list dry 54% BF and 56% in 850's to compensate some. Full of oil raises BF ~9 more points.

I would have thought full of oil decreases the balance factor. The 58% refers to the percentage of reciprocating weight which has been balanced by the counter-weight ? The oil increases the reciprocating weight ?

May have to wait till after weekend folks returning to workstations to find out.

The oil doesn't reciprocate, so how can it increase the reciprocating weight ?!!
It does decrease the balance factor though, since its on the opposing side of the crank to the balance weight.
C'mon you guys, this is really really basic stuff....

This question was asked just recently, and the piston weights for 850 v's 750 quoted.
850 were quite a deal heavier, (~75 gm each ?) so the actual balance weights applied to the crank is different for the 750 and 850.
Anyone find the link to that thread ?

P.S. Where did 54% and 56% come from,
I don't believe I have seen either of those numbers quoted before either ??

DaveK said:

Is the counterbalance the same?

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On my 1971 750 and 1974 850 the flywheels are different to look at, the 850 has more material on the heavy end and has 3 balance holes drilled.
Just to confuse things a little both 750 cranks I have, have the same part number on the flywheel but are drilled differently, one has 6 holes in succession in the heavy end, the other has 4 and 1 more drilled off that lower section.
About the only thing the same on all three crankshafts is the outer journal portion but they are secured differently, same diameter fasteners but all studs and nuts on the 850 and a combinations of studs and bolts on the 750's.

The pistons on the 850 are heavy, I picked up a NOS standard bore set of 'AE's for the 850 and the 750 has a set of +0.020" AE's, the good ones with the four slots out of the lower groove, they are light though compared to the 850 version.

Just to make sure it's clear, the 850 flywheel has a heavier counterbalance weight than the 750, mostly just to compensate for the heavier 850 pistons. Other than that, and with the exception of the MK III cranks, the 750 and 850 cranks are interchangeable.

Ken

Rohan, are you telling me that the big ends on a crankshaft don't reciprocate when the shaft rotates ? If the oil adds 50 gram inside the big ends, isn't that the same as adding 50 gram to the bearing caps ?

acotrel said:

Rohan, are you telling me that the big ends on a crankshaft don't reciprocate when the shaft rotates ? If the oil adds 50 gram inside the big ends, isn't that the same as adding 50 gram to the bearing caps ?

Click to expand...

The big ends don't reciprocate, they rotate with the crankshaft. The pistons and upper part of the rod reciprocate.

Ken

The 850 cranks, as mentioned, use different fasteners to join the halves and also are differently weighted. They are not the same castings. You'll probably run into an easier time getting a used 750 crank and grinding it until serviceable (if needed).

Note that if you're just talking about bottom ends, none of the bottom ends are a direct swap for each other without a lot of cascading changes. The main limiting factor on 750 to early 850 is the crankcase mouth size for the larger 850 barrels, and for the Mk3 vs early 850 it's the bolt pattern for the primary.

Manuals mixed up oil line hook up and 750 vs 850 mass figuring which was discussed-corrected by real Nortoneens world wide at least 15 yr ago

Postby jseng1 » Sat Mar 30, 2013 10:28 am
A dry BF on a stock Cmdo crank is about 7% higher than the same crank measured wet. So a 52% wet Cmdo crank would be about 59% dry. This changes with solid cranks that don't have an oil sludge trap.

Oil cannot sling and empty out of a cmdo crank because the oil holes are on the side - not top & bottom. The sludge trap is always 1/2 full.

Click to expand...

hobot said:

Manuals mixed up oil line hook up and 750 vs 850 mass figuring which was discussed-corrected by real Nortoneens world wide at least 15 yr ago

Postby jseng1 » Sat Mar 30, 2013 10:28 am
A dry BF on a stock Cmdo crank is about 7% higher than the same crank measured wet. So a 52% wet Cmdo crank would be about 59% dry. This changes with solid cranks that don't have an oil sludge trap.

Oil cannot sling and empty out of a cmdo crank because the oil holes are on the side - not top & bottom. The sludge trap is always 1/2 full.

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That doesn't sound right to me. I think you could count on the sludge trap being 100% full of oil.
If the crank rotated at a constant unfluctuating speed and the oil pressure was close enough to zero, yes it would only ever get to 1/2 full. For a start,remember that any air must be compressed to the same pressure as oil pressure (about 5 atmospheres).Crank acceleration/deceleration will still give that air a chance to escape.

Thanks, I have a 850 bottom end with 750 jugs and wondered if my setup would be smoother then a stock 750. I also have higher compression pistons an combat head with Norris d cam. My father in law has a 850 but it buzzes his hands too much and finds mine much smoother.

Why would the con-rods be different between two 1971 crankshafts.
The flywheels are the same, the balance holes differ and notice the ones out of the fastback have a different model number R3R1R and no oil jet holes.
Pre Commando perhaps ?

The dies for forging/stamping things out do wear out, and hence need to be replaced.
It would be very surprising if they were all the same from first to last.... ?

DaveK said:

My father in law has a 850 but it buzzes his hands too much and finds mine much smoother.

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His isolastics are probably worn, and need to be adjusted or replaced.
Been done lately ??

Carbs too been synched lately ?

But we diverge, sort of...

Good point to ponder x-file. No oil pressure could be obtained if oil flowed out trap fast enough if didn't compress the air to a just a bubble so i agree trap should be essentially full all the time. I just tell builders what to shoot for. Division of labor is a great thing to those just wanting to be a pilot. Great avatar timewrap i wonder if a cycle clock version could be constructed.

lcrken said:

acotrel said:

Rohan, are you telling me that the big ends on a crankshaft don't reciprocate when the shaft rotates ? If the oil adds 50 gram inside the big ends, isn't that the same as adding 50 gram to the bearing caps ?

Click to expand...

The big ends don't reciprocate, they rotate with the crankshaft. The pistons and upper part of the rod reciprocate.

Ken

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Ken, if you plot the vertical distance of the big ends vs time, as the shaft rotates, don't you get a sine wave ? If you add mass to the big end, it creates forces when it experiences the accelerations that sine wave indicates. The fact that the mass is not moved away by using a conrod doesn't change it's effect on the balance by much. When you balance the assembly using the method In tuning for speed, you weigh the rods, pistons, gudgeons etc, and balance a percentage of it. How do you allow for heavier bearing caps ? Or oil in the big end journal ?

acotrel said:

lcrken said:

acotrel said:

Rohan, are you telling me that the big ends on a crankshaft don't reciprocate when the shaft rotates ? If the oil adds 50 gram inside the big ends, isn't that the same as adding 50 gram to the bearing caps ?

Click to expand...

The big ends don't reciprocate, they rotate with the crankshaft. The pistons and upper part of the rod reciprocate.

Ken

Click to expand...

Ken, if you plot the vertical distance of the big ends vs time, as the shaft rotates, don't you get a sine wave ? If you add mass to the big end, it creates forces when it experiences the accelerations that sine wave indicates. The fact that the mass is not moved away by using a conrod doesn't change it's effect on the balance by much. When you balance the assembly using the method In tuning for speed, you weigh the rods, pistons, gudgeons etc, and balance a percentage of it. How do you allow for heavier bearing caps ? Or oil in the big end journal ?

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Alan, I'm not disputing that adding weight to the big end changes the balance of the crankshaft. Clearly it does. My point was that it does not change the reciprocating weight, only the rotating weight. Adding weight to the big end will increase the magnitude of the centripetal force at the big end as the crankshaft rotates. The centripetal force is a constant magnitude at a given rpm, with the force vector rotating with the crank. If you resolve the force vector into horizontal and vertical components, you will get the sine wave, which is handy if you are wanting to calculate the vertical and horizontal components of the force on the main bearings for something like designing main bearing caps, but not much use otherwise.

That's why you count the oil in the crank as rotating weight instead of reciprocating weight in determining the balance factor. When you balance the assembly, as in "Tuning for Speed", you don't just weigh the rods, you determine the small end weight and the big end weight separately. The small end weight is counted as reciprocating weight, and the big end as rotating weight. If you want to calculate how much the oil in the journal changes the balance factor, all you have to do is plug the additional weight into the balance equation as part of the rotating weight and see what it does to the balance factor.

I've always used 50 grams as the weight of oil in the stock crankshaft, a number I got from Bob Milliken, the balancing guru in Southern California for flat track racers back in the '70s and '80s. The difference in calculated balance factor between including the oil and not including it is typically 5 or 6%. For example, a 750 crank that I used back in the day was balanced at 62% including the 50 g for oil. If you calculated the BF without the oil, you would get 67.9%.

Ken

Rohan said:

His isolastics are probably worn, and need to be adjusted or replaced.
Been done lately ??

Carbs too been synched lately ?

But we diverge, sort of...

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Yes, it isn't anything to do with worn isolastics or carbs just he is sensitive to vibration and most bikes are prone. (I don't have a problem on his bike)
Just wondering what others thought about the conclusion we came up with.