Topic: Is an F3 750 alternator better than a K2?

[Patrick]

Did Honda do anything to improve the alternator on the 750 between 1972 and 1978? I am installing a K2 engine in an F3 until I rebuild the F3 engine. I have the K2 alternator, but I also have the F3 alternator. The F3 motor won't be using it for a while, so if it's a better unti I would use it now. . I already have installed the F3 clutch in the K2. I would have thought that Honda, realizing the inherent weakness of the 750 alternator, would have done something to improve it for the bulked up final 750? Do I have anything to gain from the switch?

[754]

Weakness on a 750 alternator? never heard of that..

[MRieck]

The F3 has a much lighter rotor.

[MCRider]

Weakness on a 750 alternator? never heard of that..

Me neither. The stator part number changed from 31102-300-035 to 31102-300-045 from K-2 to F-3. Usually a major part upgrade will affect the center 3 digits. changes in the last three digits are usually (not always) small, may even simply be a different supplier with no change at all.

I think aftermarket stators are one size fits all, lowest common denominator.

AS said the rotor is definitely lighter, which is good for engine performance. But a K-2 can be made lighter and in any case rotor weight does not affect the alternator output.

Would you agree?

[754]

My K3 had a lighter rotor which can then be cut further down with no electric start. After you cut them down they will probably charge less,  but not in stock condition.

[Jerry Rxman Griffin aka MuthaF'er]

Looks like we now need a rotor weight chart. Who wants to start? Just be sure which engine you have. Perhaps include the engine serial number too. Please indicate either "w" for with starter clutch and parts or "w/o" for without.

[1080]

I was told once that the police bikes alternator's had more output.

Does any one know if there was a different part number for those?

[MCRider]

Looks like we now need a rotor weight chart. Who wants to start? Just be sure which engine you have. Perhaps include the engine serial number too. Please indicate either "w" for with starter clutch and parts or "w/o" for without.

I have 3 different ones, 2 stock, early and late, and one modified but still uses a starter. I'll weigh them, pictures, and get the serial numbers tonight.

[TwoTired]

Unless there are other changes to the alternator assembly.  Simply reducing the rotor mass will reduce output capacity.  At least, that's the physics of it.

The rotor is the magnetic core of the device.  The field coil and the rotor work together to form an electromagnet.  If you reduce the core mass, you reduce the electromagnetic strength, and thus the output capacity.

Since the alternator isn't always working to highest potential, the effects may be hard to notice, depending on the operating mode.  If you spend most of the operational time above, say, 3000 RPM.  The battery should stay charged.

However, if you add more load (brighter headlights, hot coils, etc.) or spend more time in traffic at low RPM, then your battery will be taxed more, which can lead to shorter battery life, and perhaps lack of ignition spark.

It's a trade off decision best made knowing the application of the modified device.

Cheers,

[MCRider]

Unless there are other changes to the alternator assembly.  Simply reducing the rotor mass will reduce output capacity.  At least, that's the physics of it.

The rotor is the magnetic core of the device.  The field coil and the rotor work together to form an electromagnet.  If you reduce the core mass, you reduce the electromagnetic strength, and thus the output capacity.

Since the alternator isn't always working to highest potential, the effects may be hard to notice, depending on the operating mode.  If you spend most of the operational time above, say, 3000 RPM.  The battery should stay charged.

However, if you add more load (brighter headlights, hot coils, etc.) or spend more time in traffic at low RPM, then your battery will be taxed more, which can lead to shorter battery life, and perhaps lack of ignition spark.

It's a trade off decision best made knowing the application of the modified device.

Cheers,

I don't want to assume anything here, so I have to ask: I can see this applies to the part of the rotor which is inserted into and spins inside the stator. Does it also apply to the part of the rotor which is not inserted into the stator and which is the part where the bulk of the lightening takes place?

The rotors are evidently lightened by Honda at the 76 model year or so. I've never heard of a stator upgrade or a drop in alternator performance because of this. Granted I don't hear or know everything and just want to learn.

[TwoTired]

It certainly matters where the metal for lightening is removed.  Heck, it also matters what alloy is chosen to become magnetized.

But, if the rotors aren't lightened near the outer circumference, and where the magnetic pole pieces move, their flywheel effect won't be altered very much.  Leading me to ask, "what's the point?"  Perhaps to use less metal and make the part cheaper?

Anybody got pics of the two different rotors showing the weight loss?  I only have the F2/3 and K8 motors.

Cheers,

[754]

There was a big flange between the starter clutch and the zigzag, the factory removed in 73 and later.

When I cut then down, I removed that, and made the back flush where the sprags sit (they were inset), then cut some off the outer endand bevel the edge (outside the zigzag). Also the strter spag mount bolts can be drilled out to 3/4 inch or more, can probably drill 3 more in there.

You end up with around 1/4 inch or so outside the zigzag on both sides.

-------------------------------------------------------------------------------------

Following is from RC catalog, sorry I cant put up the pic,

LIGHTWEIGHT ALTERNATORS

3 3/4 lb alternator to replace stock 8 1/2 lb unit.
Helps to prevent crankshaft breakage. Charging
system is not altered. Ideal for street racer who
needs a charging system but want performance.
Lets engine rev more quickly and safely.
Comes completely balanced.

#1093 Deletes electric starter
#1094 With starter (recommended for small motors only)

--------------------------------------------------------------------------------------

I would put the weight of the 73 & later, at around 6 lbs or so, Honda had the right idea..

[TwoTired]

Could be semantics, as marketing people love to play with that.  But, I note the RC catalog statement:
"Charging system is not altered." 
...Does not necessarily mean the output capacity/capability isn't changed.  Only that you are not expected to make changes to the rest of the charging system to use the part being sold.

I submit that performance engines aren't well suited for inner city idle marathons, and who is going to build a high performance bike ideal for that low RPM application?

A street racer needs a charging system, sure.  But, does it need the same output capacity as a stocker?  I'm guessing the RC catalog writers don't think so.  Do street racers typically languish below 3000 RPM for long periods?  That could lead to an image problem, no. 

Anyway, removing metal on the engine side of the rotor likely has low impact on the output capacity, as the magnetic field is weakest there anyway.  Removing metal on the cover side near the outer circumference will have some impact, as magnetic core mass will be changed reducing the lines of flux crossing the stator windings.  Would have to do some load tests to see just how much, though.  If you keep the RPM up and the load down, the Vreg will just compensate to keep the alternator working harder/more often than before the core loss, in order to keep the battery topped up.

Cheers,

[scunny]

I thought the idea behind the lightened alternater(flywheel) was so the engine spun more freely, alternater output would be a secondary consideration, this is on the proviso that it's a track bike, no lugging around town with your lights on.

[MCRider]

I thought the idea behind the lightened alternater(flywheel) was so the engine spun more freely, alternater output would be a secondary consideration, this is on the proviso that it's a track bike, no lugging around town with your lights on.

True lightened is for more, quicker revs, but was also to failsafe the motor. It was discovered that the original rotor was too heavy and aggressive shifting could shear off the crank. I saw one first hand at a shop i worked at. Bike ran fine but wouldn't charge the battery. Alternator checked at "0". I as shop flunky was told to remove the alternator cover. Rotor fell out with piece of the crank still bolted inside. Requiring a complete rebuild, owner opted for an 836cc.

Anyhoo, I have all three. A heavy stocker, a lightened rotor which still had just a touch of the flange left so i could use the starter, and a light stocker. I think my lightened rotor is lighter than the light stock rotor, but not as light as that RC advertisement. Will weigh and photo each tonight. I've always run a lightened rotor on my street bikes, along with 100w headlight and an electric vest. Never had a problem charging the battery nor did I steer away from low rpm situations. Didn't know any better. However that is not an objective way of measuring output, I don't have hard numbers.

YMMV.

[MCRider]

I couldn't wait. My lightened rotor is 3.75lbs. About what RC advertised. As mentioned this was in my street bike and I never noticed diminished charging capacity.

Visually the Honda lightened rotor I have is SMALLER than my lightened rotor. It is in a black motor I bought off eBay which I guess is an F3?. It not only has maximum material removed (I'm not sure how the starter survives, must be beefier) it has holes (a lot of)  drilled into the circumference. It is still attached to the crank so it will be a while.

Bottom line, I'd run the F3 rotor and assume that there is no loss of charging power. With more and more demands on charging from 1969 to 1978 from accessories, I just don't see Honda downgrading the output.

The K2 rotors were simply overbuilt. The massive flange on the K2 surrounding the starter clutch contributed to a flywheel effect which would smooth out the drive quality and shifting. But that's for sissies. Give me quicker revs!

That's my assumption from the evidence, but no hard facts. That's what i would do, YMMV

[Patrick]

So it seems like it would only make sense to use the F3 rotor, if it wouldn't adversely affect charging. This bike will be a commuter, every day bike. I've already got a high performance 750 and with my son taking over my K5 750 I am lacking a daily driver for myself. Does anyone know if the field coil was changed when the rotor was lightened? If I use one part, do I need to use the entire later assembly? Every one has addressed the rotor thing, but no one has talked at all about the stator or field coil. Are those the same? The parts numbers on the fiche are the same, but I think the fiche just reflects the latest part number no matter how many times it may have been superceded. It shows the part numbers for the rotor on the K2 and F3 to be the same although they clearly must have started out with different numbers.

[754]

I will try to check my 71 750 partsbook for the early rotor number.

I have a 78F motor with dyno cover on it, but have not yet seen the rotor. At this moment I am not convinced it is any different than my 73 CB had.

I have read on here about500,s having charging issues but none on 750,s
 

I ran the lightened rotor for a long time, run a regular sealbeam and a 5.5 A battery, never had any problems.

[Artfrombama]

The 750F had a lighter alternator but a heavier chain?
I've read on this forum the earlier drive chain set is a little easier to pull.

[MCRider]

The 750F had a lighter alternator but a heavier chain?
I've read on this forum the earlier drive chain set is a little easier to pull.

That is true. The lighter alternator was not totally a performance issue. The crankshaft could break with the heavier rotor.

The later (78+?) bikes have a 630 chain, earlier are 530. Weight is weight, and rotating weight a little moreso. I'd prefer the 530 for performance, the 630 for touring street use, longevity, where the consumer wants to ignore maintenance.

A modern 530 O Ring chain is pretty darn good for touring also, a little more vulnerable to wear if neglected.

Lots of competing interests in design.