Is This Really an Issue?

[Dan Beaman]

In looking at rotors I came across this.

There are two Bosch alternator rotors that look the same but the one for the BMW is 1/2mm smaller and allows too much air gap and will not charge properly when used on the Moto Guzzi.

Correct measurement for the Moto Guzzi rotor is 73.5mm.

BMW measures 73mm. 1/2mm = .020 excessive air gap.

[Chuck in Indiana]

Yes.

[Murray]

Roto or slip ring, rotor yes/maybe slip ring no the spring on the brush will sort it. Some one some where suggest they alts were used commonly on some GM cars and continental aircraft motors if you have the guzzi part number try plugging that into a google machine and see what it spits out.

[wirespokes]

The diameter he's talking about is the outer diameter.

Some of the early BMWs (/5 - 1970-3) had 73.4mm rotors. Their resistance was around 7 ohms rather than about 3 like the newer ones.

Probably best getting a Guzzi rotor and not messing with the BMW version. They look the same and I wouldn't have guessed they'd be different.

I guess you could change to a BMW stator to match the BMW rotor.

[Loop Tonti EV]

This was discussed at my local dealer years ago. If I remember correctly the BMW stator was the same. Due to crank flex BMW required more air gap to keep the rotor from contacting the stator and that was the reason for the slightly smaller diameter rotor.

[Dan Beaman]

One of the reasons I’m asking is that after all my work talked about on another thread (https://wildguzzi.com/forum/index.php?topic=102980.0), I’ve finally come to the conclusion that the issue is my rotor, despite testing with correct resistance. MGCycle has one that they say is for both a Guzzi and BMW for $75 and Harper’s is Guzzi specific for $125. I trust both places and have had good service from both. That’s a substantial price difference.

[cliffrod]

Dan- I can't vouch for the rotor but do know I went to BMW in Daytona in 94, bought new BMW brushes for my V7 Sport, installed them and they've been there doing fine ever since.  Pretty certain I was told by the person who sent me to BMW & at the BMW counter that the rotor was the same..  I know I bout some dyna coils and Italian bike jumper cables (because my bike was always dead...) from Harper's at Daytona that year. They're likely the ones that suggested I get brushes at the local BMW dealership.

We went through a long trial of electrical woes with my bike.  During the rebuild, I replaced the trashed original harness with an entire NOS wiring harness, hoping to ward off such things.  Then brushes when rotor tested fine but the bike was still going dead. Then extra insulation in certain spots, trying to account for possible incidnetal but unproven contact.  Still no consistent charging. 

Following a friends advice,  I finally source the problem to corrosion-completely hidden from view- in the great looking riveted joints within the fuse box.   I had not replaced the old fuse box, which was only approx 20 yrs old at that time.  The cumulative effect of so much resistance was significant.  I drilled out the rivets to reveal the corrosion, polished the contacts and soldered all back together.  Not as pretty or PC as a replacement new fuse box, but it worked for dirt cheap.  After that corrosion was eliminated, I've done nothing else and had no such electrical problems for years.

I had a G5 briefly and thought it had a similar fuse box similar or identical to my V7 sport.  Before replacing the rotor without confirming it is the problem, i would check such rivets joints.