Aftermarket Crank Pulley

[930LDR]

Thinking about either trying to have my crank pulley balanced or possibly just replace it during my rebuild. Remember it having an odd little wobble to it prior to tear down. OEM part is obviously expensive and have seen a few aftermarket lightened replacement units but haven't heard much in the way of pros and cons to them.

Alternatively should I just see if I can get the original balanced?

[chris_seven]

If the pulley 'wobbles' it may be bent or not fitted squarely (difficult to imagine) on the end of the crank.

If it is bent you probably need to replace it as I wouldn't trust trying to bend it back to shape.

If it just fitted badly it is likely to be reusable.

I can't see balance being much of an issue. The mass moment of a small diameter pulley doesn't contribute much to the overall engine out of balance and any slight eccentricity would most likely be insignificant.

[911pcars]

Since it rotates with the crank and it has mass, it's one of the parts to include when balancing the engine.

However, at this point, I suggest you make sure it runs true, then replace as necessary. I would also suggest that the exact TDC mark may not (or not continue to) be accurate (this affects ign. and valve timing). That's another detail best ironed out during a rebuild unless you're very anal.

Sherwood

[Weissach911]

Originally Posted by 911pcars

Since it rotates with the crank and it has mass, it's one of the parts to include when balancing the engine. Sherwood

Does this mean that the 911 crank is assembly balanced with the pulley fitted as there are no signs of any component balancing on the standard pulley.

[911pcars]

Originally Posted by Weissach911

Does this mean that the 911 crank is assembly balanced with the pulley fitted as there are no signs of any component balancing on the standard pulley.

That would depend on the balancer's routine. For a 911, the pulley is probably balanced separately, same with the other rotating parts (flywheel, pressure plate, etc.). Typically, the crank is balanced with an equivalent amount of weight from the reciprocating pieces (pistons, rods, piston pins, rings, fasteners, etc.) Confirm with the shop.

Sherwood

[Henry Schmidt]

We make a power pulley (two sizes) that offers light weight and additional markings to insure proper timing both when setting the cams and ignition.
The kit comes with a factory 12.9 flanged crank bolt and new locator pin.

[930LDR]

Henry - What are the pros and cons with something like that versus a factory pulley? Certainly looks nice and a nice savings over an OEM unit.

[Walt Fricke]

I'm pretty sure 911 cranks are balanced without use of the "bobweights" which are used on V8 cranks to simulate the rods and pistons and such. I've had a couple balanced, and that's what I recall. Has to do with the opposed nature of the reciprocating mass, or something. Convenient.

The balancer I used (alas, RIP) really wanted to balance everything from the pressure plate to the flywheel pulley as a unit. He wanted the IS gear and dizzy gear on, with the keys installed. I had to talk him into "zero balancing" the external parts, so they could be, as I knew at least the flywheel/clutch parts would be, separately replaced.

[mytoy]

The stock pulley is a balanced unit. But it is balanced separately from the crankshaft. It is balanced for rotational force only. Otherwise how would you be able to replace a damaged pulley? The same for a clutch assembly.

[chris_seven]

I agree that Boxer engines generally have excellent primary balance and have no rocking couple so they are very smooth. There is a small effect due to the axial position of opposing pistons but I believe this is generally neglected.

When we discuss balance we always conveniently ignore the most important aspect of any balancing procedure which is the standard to which the assembly is balanced and this would make a significant difference to how the individual parts are considered.

If we balanced a crank to ISO G1.0 at 6000rpm as an example we would need to do some very careful work and would need a very high spec balancer. It then may be necessary to balance with the pulley.

If, however, we balanced to G6.3 at 1500rpm then we may find that the low weight of the engine pulley and its small diameter means we don't have to worry.

To talk about zero balance is almost meaningless without stating the speed of rotation and the precision of the electronics used for measurement.

I would never hand over the bottom end assembly of a race motor to a balancing shop with out discussing balance level and asking for some test certificate to show that the work has been done correctly.

We have an engine balancing shop nearby who uses an 'soft pedestal' machine manufactured in the Sixties and I doubt the electonics is capable of mesuring better the G6.3, the owner/operator is unaware that soft pedestal machines need adjusting for every different mass being installed and yet happily balances a modern Zetec Crank.

The engine manufacturer uses a $6 Million Computerised Schenk Hard Pedestal machine that is truely state of the art.

If you look at standard pulleys there are no witness marks to show that they have ever been balanced and there are no balancing features present.

The point I tried to make was that I don't believe there is any real problem with changing a pulley as it's mass and position are much to small to cause any lateral vibration as long as it isn't bent.

The Aluminium Pulley that Henry makes must be so light that balancing is just a waste of time and money.

[304065]

Chris, fascinating and I never cease to be amazed by your knowledge.

What do you mean "hard pedestal," Something like this? For vibration isolation from surface vehicle movements, wind, etc.? (This from Mahr metrology lab in Rhode Island, USA)



I know who Schenk is, they were the manufacturer of the original factory dynos from the 1960s, with the huge 12" gauge faces!

Edit: the Wikipedia article does an OK job of describing the differences between hard bearing and soft bearing, including citing the same limitations on calibration, but doesn't actually describe what the differences are.
http://en.wikipedia.org/wiki/Balancing_machine

[mytoy]

Originally Posted by chris_seven

I agree that Boxer engines generally have excellent primary balance and have no rocking couple so they are very smooth. There is a small effect due to the axial position of opposing pistons but I believe this is generally neglected.

When we discuss balance we always conveniently ignore the most important aspect of any balancing procedure which is the standard to which the assembly is balanced and this would make a significant difference to how the individual parts are considered.

If we balanced a crank to ISO G1.0 at 6000rpm as an example we would need to do some very careful work and would need a very high spec balancer. It then may be necessary to balance with the pulley.

If, however, we balanced to G6.3 at 1500rpm then we may find that the low weight of the engine pulley and its small diameter means we don't have to worry.

To talk about zero balance is almost meaningless without stating the speed of rotation and the precision of the electronics used for measurement.

I would never hand over the bottom end assembly of a race motor to a balancing shop with out discussing balance level and asking for some test certificate to show that the work has been done correctly.

We have an engine balancing shop nearby who uses an 'soft pedestal' machine manufactured in the Sixties and I doubt the electonics is capable of mesuring better the G6.3, the owner/operator is unaware that soft pedestal machines need adjusting for every different mass being installed and yet happily balances a modern Zetec Crank.

The engine manufacturer uses a $6 Million Computerised Schenk Hard Pedestal machine that is truely state of the art.

If you look at standard pulleys there are no witness marks to show that they have ever been balanced and there are no balancing features present.
The point I tried to make was that I don't believe there is any real problem with changing a pulley as it's mass and position are much to small to cause any lateral vibration as long as it isn't bent.

The Aluminium Pulley that Henry makes must be so light that balancing is just a waste of time and money.

Chris

Not trying to argue with you but just wondering why my pulley from my 83 has balancing holes drilled opposite the locating pin. Here is a picture of what I am talking about.

[BURN-BROS]

That is a cast iron pully. Since it is not fully machined, a balance must be performed in order to get it back to "zero". This balance is in relation to itself, not with a crank.

[930LDR]

My stock 930 pulley looks as though it has some weights of sorts tacked onto the face of it. Positioning is far from symetrical so I'm guessing it was an effort to balance.

[BURN-BROS]

Some of the pullies are stamped steel and welded together to save money and need a bit of help to balance. The later engines used this method. This may be why your pulley bent so easily.

The early single groove pulleys were good quality fully machined billets that required no balancing.

Henry's pulley is pretty cool as it's much easier to see the timing marks and it looks good as well.

[chris_seven]

Originally Posted by 304065

What do you mean "hard pedestal," Something like this? For vibration isolation from surface vehicle movements, wind, etc.? (This from Mahr metrology lab in Rhode Island, USA)

Edit: the Wikipedia article does an OK job of describing the differences between hard bearing and soft bearing, including citing the same limitations on calibration, but doesn't actually describe what the differences are.
Balancing machine - Wikipedia, the free encyclopedia

Not quite

Soft pedestal machines mount the 'rotor' needing balancing on soft supports and measure displacement, usually with a very accurate LVIT (Shaevitz or HBM).
The motion is caused by the centrifugal force of the residual unbalance left in the rotor.

There used to be a tendancy to balance with these machines to a level of displacment say 2 mils and assume that this was satisfactory.

The problem is that even though CF increases with the square of speed the the rest of the rotor body, which acts as a restraining/restoring force also increases so changing speed doesn't change the displacement

A linear soft suspension will exhibit the same displacement at all speeds as long as the rotor mass does not change. Forces increase exponentially with speed, but a soft suspension does not constrain the part, therefore it is allowed to displace at will due to a given heavy spot

Simply because the rotor body behaves as a parasatic mass and the restraining force also increases as does the unbalance so no significant displacement difference in terms of mils can be observed.

You must convert from mils to unblance units such as oz-in or gr-in in order to know the balance quality and this was often not carried out.

Used correctly there isn't much wrong with these machines but I believe they need much more expertise and understanding form the operator than Hrad Pedestal systems.

Considering the same rotor installed in a hard bearing machine, the approach is different. The hard bearing machine uses strain gauges to measure the force exerted on them by the unbalance in the rotor and there should be minimal float.

Therefore at the same speed as a soft bearing machine, the force will produce a certain reading on the meters and will this force will increase if the speed of the balancing machine is increased resulting in a larger reading being shown on the meters.

Working on the basis of 'If you can't see it, you can't correct for it', I tend to prefer hard pedestal machines

Originally Posted by 304065

I know who Schenk is, they were the manufacturer of the original factory dynos from the 1960s, with the huge 12" gauge faces!

Schenk used to be the bane of my life, they were Germany's leading manufacturer of test systems and offered the following equipment:

Engine Dynos of all shapes and sizes
Tensile Test machines
Fatigue and Creep Testing machines including 1000Hz elastomer testers
Balancing Machines and Systems
Spin Test Chambers for busting disc and triaxial testing.

They were a really tough act and I spent 25 years competing with them at many levels and all over the world.

I would also say that I hadn't considered the later and longer pulleys fitted to later cars I was only thinking about the early nicley made parts (Doh!)

[Henry Schmidt]

I subscribe to the theory that you balance each piece individually and then check the entire assembly. That way, if you need to replace one component (I.E. pressure plate, flywheel or whatever) the engine will still be close. If you only balance the assembly it might be disastrous to replace one component.
Flat six engines don't need to be that close. We always balance to 1/2 a gram which is much closer than the factory specs.

[pirin911]

Hi Henry:
Does your power pulley provide for the A/C also?

Thanks,
Francisco Soto

[ONQRACIng]

I agree with Henry. We balance all spinning parts individually to each other, example, all rods are made to weight the same, all pistons (pins and clips and rings sets) are made to weight the same, flywheel is trued pulley is trued as well, crank is also trued. When all is said and done, we are also within 1/2 Gram.

Andrew

[johnman001]

Originally Posted by Henry Schmidt

We make a power pulley (two sizes) that offers light weight and additional markings to insure proper timing both when setting the cams and ignition.
The kit comes with a factory 12.9 flanged crank bolt and new locator pin.

Henry,

How much is that "kit"? I want one for the 3.2 that I am building right now.