norton 850 crank assembly balancing

Drummer99 · Feb 8, 2014

I just had my 850 crank and evrything that connects balanced by a very reputable firm My crank had 3 half inch holes drilled into the flywheel. The shop told me that I was 12 grams light based on the balancing and basically filled the holes and touched it up here and there. I asked them why it would be out that much and given this is a basket job I have never driven the bike. They thought it might have been a 750 crank or that someone may have lightened it up. Are the 2 cranks different? and has anyone experienced a similar situation balancing? Would appreciate your thoughts before I start reassemble
Thx Drummer99

Rohan · Feb 8, 2014

12 grams is less than half an ounce !!
Oversize or different pistons could EASILY account for more than that...

Did the shop say what % factor they were working to.
Or if they allowed for the weight of the oil inside the crank.

Remember, reciprocating pistons can NEVER be prefectly counteracted by a (deliberately) out-of-balance flywheel,
its only a matter of getting it so that its more or less comfortable to the rider.
And in a Commando, the isolastics do most of that anyway..
So half an ounce is really neither here nor there in the scheme of things ?

P.S. Did they dynamically balance it. ?
Getting it perfectly balanced, side-to-side wise, may make it a shade smoother.

acotrel · Feb 8, 2014

'Remember, reciprocating pistons can NEVER be prefectly counteracted by a (deliberately) out-of-balance flywheel,
its only a matter of getting it so that its more or less comfortable to the rider.'

Stopping it from shagging the bearings and cases. You've use d the word 'never', I would question that. At what revs does the standard commando with the specification crank, run extremely smoothly - 3000 RPM ?

Rohan · Feb 9, 2014

Reciprocating pistons simply CANNOT EVER be perfectly balanced by deliberately out-of-balance cranks.
Period.
All you can do is get a best compromise.

Unless you rewrite the science to be complete nonsense...

Commandos are pretty smooth at all revs - thanks to the isolastics.
You need to be a little gentle with the throttle at lower revs - the old magazines used to call that the minimum non-snatch speed.
(Better designs of combustion chambers - and carburation/fuel injection - have a lot to do with this)

It has been mentioned here, before, that minimum bearing loads occur at 53%, was it ?
Co-incidentally, thats about the 52% BF quoted for Commando engines...

Fast Eddie · Feb 9, 2014

Drummer99 said:
I just had my 850 crank and evrything that connects balanced by a very reputable firm My crank had 3 half inch holes drilled into the flywheel. The shop told me that I was 12 grams light based on the balancing and basically filled the holes and touched it up here and there. I asked them why it would be out that much and given this is a basket job I have never driven the bike. They thought it might have been a 750 crank or that someone may have lightened it up. Are the 2 cranks different? and has anyone experienced a similar situation balancing? Would appreciate your thoughts before I start reassemble
Thx Drummer99

I'm sure that if you post some pictures, there'll be folk on here who will be able to tell what the crank is, whether or not its been lightened, etc.
I had a Norton crank lightened once, and the balancer had to add heavy metal slugs to compensate! An expensive waste of time that was!
I've just had a crank dynamically balanced, and he had to drill a lot of holes in it.

Dances with Shrapnel · Feb 9, 2014

Drummer99 said:
I just had my 850 crank and evrything that connects balanced by a very reputable firm My crank had 3 half inch holes drilled into the flywheel. The shop told me that I was 12 grams light based on the balancing and basically filled the holes and touched it up here and there. I asked them why it would be out that much and given this is a basket job I have never driven the bike. They thought it might have been a 750 crank or that someone may have lightened it up. Are the 2 cranks different? and has anyone experienced a similar situation balancing? Would appreciate your thoughts before I start reassemble
Thx Drummer99

Was the crank dismantled and cleaned before you sent it out for balance?

Maybe you had 12 grams of sludge in the sludge trap?

comnoz · Feb 9, 2014

Only 12 grams out.
I have not seen a factory crank that was that close yet. Jim

acotrel · Feb 9, 2014

'Reciprocating pistons simply CANNOT EVER be perfectly balanced by deliberately out-of-balance cranks.
Period.
All you can do is get a best compromise.'

Surely the 'compromise' relates to the revs at which the assembly smoothes out ? I love it when people state 'FACTS'. If the motor doesn't vibrate, it must be in 'perfect balance' ? Buy yourself a Rolls Royce, and balance a coin on the radiator.

Rohan · Feb 9, 2014

RR's don't have rubber engine mount pads, to help smooth things out ?
And a great heavy big chassis, to also help smooth things out.

RR's also have lots more cylinders, to help smooth things out.
Not 2 wacking great pistons, thumping up and down together...

But we diverge....

RennieK · Feb 9, 2014

IIRC The 850 crank is close to 5lbs heavier than a 750.

Rohan · Feb 9, 2014

acotrel said:
Surely the 'compromise' relates to the revs at which the assembly smoothes out ? .

Well yes - chassis specific tuning.
Engine balance doesn't change with rpm, Alan.
(As long as you are out of the minimum non-snatch engine speed at low rpm phase).

If everything smoothes out at some points in the engine rpm range,
then thats ALL to do with the sympathetic vibrations FROM THE CHASSIS that that engine is installed in.

acotrel said:
I love it when people state 'FACTS'. .

You should try it yourself sometime.....

ggryder · Feb 9, 2014

acotrel said:
'Reciprocating pistons simply CANNOT EVER be perfectly balanced by deliberately out-of-balance cranks.
Period.
All you can do is get a best compromise.'

Surely the 'compromise' relates to the revs at which the assembly smoothes out ? I love it when people state 'FACTS'. If the motor doesn't vibrate, it must be in 'perfect balance' ? Buy yourself a Rolls Royce, and balance a coin on the radiator.

It's disingenuous to suggest that a parallel twin, vibrating like hell, but in a conveniently compliant frame and/or engine mounts so that the rider is unaware, is "balanced" in the same way as, say, a straight six.

olChris · Feb 10, 2014

Dances with Shrapnel said:
Drummer99 said:

I just had my 850 crank and evrything that connects balanced by a very reputable firm My crank had 3 half inch holes drilled into the flywheel. The shop told me that I was 12 grams light based on the balancing and basically filled the holes and touched it up here and there. I asked them why it would be out that much and given this is a basket job I have never driven the bike. They thought it might have been a 750 crank or that someone may have lightened it up. Are the 2 cranks different? and has anyone experienced a similar situation balancing? Would appreciate your thoughts before I start reassemble
Thx Drummer99

Click to expand...

Was the crank dismantled and cleaned before you sent it out for balance?

Maybe you had 12 grams of sludge in the sludge trap?

The "sludge Trap"... I have read here and there that the trap gets cleaned out on rebuild and in particular, "Balancing"........... I havent yet to split my crankcase, therefore never investigated a norton crank, but......... if the "sludge trap" is cleaned out prior to balancing, (how many Gms worth ????) then once rebuild wouldn't the trap accumulate more sludge over time and then "unbalance" the crank ????.. IOr is the trap "plugged" so this doesnt happen again.......... or does it take 30k miles or years of wear and tear to "refill" the trap with "sludge" ????

Dances with Shrapnel · Feb 10, 2014

olChris said:
how many Gms worth ????

Probably on the order of 12 grams. I never weighed it but cleaned out plenty.

Furthermore, if the drill holes were plugged as mentioned by the OP, I suspect there was more than 12gms added.

olChris said:
once rebuild wouldn't the trap accumulate more sludge over time and then "unbalance" the crank ????..

Yes.

norton 850 crank assembly balancing

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