Exploding rotor/stator

Group, I would like your input one more time... All this year I had been battling charging system problems, a month or so ago I pulled the primary cover and found that my rotor and stator had been coming in contact and were chewed up. It is original, 40 yrs old, I figured maybe they just started to fail and once something under that much centrifugal force starts to weaken, it is just going to go. I bought another rotor and stator on ebay, original used but in good condition, and reassembled. On my first ride, just about 20 min in, that combo completely exploded in my primary. Now I'm worrying that there is something other than bad luck that is causing these to fail. Can you think of something that I should check before I put my third rotor stator on the bike? I'll admit, the first failure I thought nothing of it and just swapped in a new one. After the second one, I did start looking for causes and I admit my primary chain was loose. I have tightened it now. (btw, my maintenance book states 3/8 in play of the chain on the tight side, is that 3/8 up and 3/8 down for a total of 3/4in total travel? Or 3/8 in total, ie 3/16 up and down?) Is there anything else I should look for before putting my third combo in and going for a ride? I inspected all the teeth on the sprockets, they look fine.
Do you believe I just had ready bad luck? Do you believe my loose primary chain could have bound causing failure that has since been remedied? Or is there a third more sinister problem I have yet considered that is going to put me in a really bad mood?

There has to be an air gap between the stator and the rotor, I don't have the specifications to hand, but this is important.

westernjesus said:

I bought another rotor and stator on ebay, original used but in good condition, and reassembled. On my first ride, just about 20 min in, that combo completely exploded in my primary. Now I'm worrying that there is something other than bad luck that is causing these to fail. Can you think of something that I should check before I put my third rotor stator on the bike?

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I suggest the next rotor you buy is the non-exploding (welded) type and not another dodgy ebay offering.

westernjesus said:

(btw, my maintenance book states 3/8 in play of the chain on the tight side, is that 3/8 up and 3/8 down for a total of 3/4in total travel? Or 3/8 in total, ie 3/16 up and down?)

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I don't know which "maintenance book" you are referring to, but the factory manual gives the following information in section C41, 6. (Primary chain adjustment): "....total up and down movement....3/8" in. (9.5mm)".
http://britmoto.com/manuals/Manuals/750_man.pdf

dave M said:

There has to be an air gap between the stator and the rotor, I don't have the specifications to hand, but this is important.

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I just happen to have my trusty Clymer here, and in no way am I an expert.
check rotor and stator with flat feeler guage, Ideally, it should be even all the way around but not less than 0.008-0.010 in.
( 0.203-0.254mm ) at any one point.
if the gap is less than this, it may be possible to realign the stator by loosening the nuts, pushing the stator in the direction of the smallest gap, and retighten nuts to proper tourque.

hope this helps

Check your crank nose carefully for nicks, and inspect the keyway for straight, clean edges. Use a new woodruff key.

I would buy a new Sparx 3-phase alternator & regulator/rectifier with a full warranty.

Thanks for the measurements. When I installed the second set I did confirm they were not in contact, but I didn't use a feeler gauge to check distance or that it was perfectly square (round?). I can't afford a new upgrade now, but I have another vintage set coming, as well as a set on both my triumph that I know is compatible, and maybe even my bsa, so if one of them are the welded type, I will use that. I'm not riding the triumph at the moment, so I don't mind scavenging a part temporarily to get the norton back on the road.
Thanks for clarifying the primary chain tension. But man, 3/8in total play is very tight. I wouldn't have thought that. I bet my primary chain pre-adjustment was about an inch or more. The tranny adjustment was pretty much all the way forward, note it is 2/3 back. Loosened up my drive chain dramatically when I moved the tranny.
I'll upgrade to a new Sparx type once confirmed this problem has stopped, and a couple paychecks down the road. Would be miserable if I ruined a new $200+ unit. Have been wanting to any how, since I have Sparx ignition.

Notice that you loosen the primary chain from dead tight, not from loose. Means you will have to loosen up the drive chain to get it right, then set the drive chain. I find 3/8" a bit tight myself.

Mike sent this one time.

"I've always found it difficult to measure the total up-and-down play of the chain, because you have to wiggle the chain up and down somehow and also measure how much it's moving, all through a small hole. An easier way is to measure a tight chain compared to a slack chain, since this is half the total up-and-down play. To do this, make two marks 3/16" apart [not 3/8"] on the end of a stick or piece of paper. Then put the bike on the centerstand and the gearbox in 4th, and lie on the ground next to the primary.

Now, if you move the rear tire in the forward direction with your foot, the bottom of the chain tightens and you get all the slack on top. If you move the rear tire in the backward direction, all the slack is on the bottom and the top is tight. So pick out some feature on the chain (top of link, bottom of link, rivet, etc.) , and compare it to the two marks as you move the wheel back and forth. You can easily see how much it's moving compared with the two marks and loosen the chain appropriately. (BUT DON'T OVERSHOOT! If you get it TOO loose, you have to tighten it and start all over again).

Once the tight chain/slack chain play is 3/16", you know the total up and down play is 3/8". I also try to check it later when the engine is really hot. You can do this at the side of the road, and there should be at least SOME play then, or it needs to be a little looser.
Mike Taglieri"

If you do it this way I find pushing it with my fingers gives more that 3/8", but then it depends how hard you push it. You can almost get 3/8" when it's dead tight if you really push. You'll know it's too tight if it's hard to shift. Don't leave it like that, you'll bend the GB main shaft. The only trouble with a too loose primary chain it it whips around and may hit something.

Dave
69S

Before you check for clearance between the rotor & stator, find a spacer to take the place of the outer cover on the central stud that holds the cover one. Then fit the nut that holds the cover on to simulate the conditons when assembled. I have had problems in the past with not enough shims on that stud behind the inner primary case so that the case distorts when you tighten the cover securing nut. That moves the stator & makes the rotor clearance bad. If that's the case you are going to have to strip primary down & use shims to get primary inner case lined up correctly.

Ian

A freind just had a simular problem with a p11- beeb running ok for years then let a magnet go repaced it with in a half our ride did another thought it was the air gap but ditroy a third, a very close inspection there was a nick in the stator wire causing the stator to short out.

westernjesus said:

All this year I had been battling charging system problems, a month or so ago I pulled the primary cover and found that my rotor and stator had been coming in contact and were chewed up.

Click to expand...

Was the stator wiped out all around or only on one side? Was the rotor loose on the shaft? Pictures?

westernjesus said:

On my first ride, just about 20 min in, that combo completely exploded in my primary.

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Did the rotor come apart? Pictures?

When you adjust the primary tension make sure you push the transmission all the way back against the adjuster before you take your measurements and tighten it up. It will creep backward if there's any slack, even after you've tightened it down. And there is a lot of slack between the bosses in the trans and the mounting studs; and between the studs and the adjuster collar.

I don't know why your rotor came apart, but here is a link with some information on preventing it:

primary-puzzle-t8150.html?hilit=primary puzzle#p85941

As for providing a gap between it the rotor and stator it is very easy to bend the posts for the stator. I just put a length of small pipe over mine and gave them a tweak in the needed direction.

rvich said:

I just put a length of small pipe over mine and gave them a tweak in the needed direction.

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That's what I did too, a piece of pipe and a large lever, you can control the bend much easier than whacking on it with a hammer. Leave the nuts on too. Or just use a socket that fits on the end of a ratchet.

Dave
69S

[/quote]Did the rotor come apart? Pictures?[/quote]

I'm doing this from my phone, so I'm sorry I can't embed an actual photo now, but here is a link to a photo I posted on my facebook page. It shows the two rotors. One is slightly chewed, that is the original. The second is the exploded one, catastrophic failure.
http://www.facebook.com/photo.php?fbid= ... e=1&ref=nf
Lots of good tips in this thread thus far. I will check into each and hope I don't need shims behind the inner primary!

You really gotta have clearance, and you need to check it at different crank positions. I ended up drilling the 3 stator holes slightly oversize to allow a minute bit of adjustment on mine.

I've had that catastrophic failure in the past and it's no fun. Primary chain, chaincase, bent mainshaft.