Magneto Weights

[Pushrod Twin]

Here is something which tickled my risibility, whatever that means. But I found it amusing anyway.
Just for no technical reason whatsoever, but because I had one in each hand and could feel a difference, I weighed my KVFTT against my KVF.
The difference of 24 grams didn't surprise me, what did surprised me was that the magnesium TT was heavier! How could that be?
I swapped ends of the scales just to prove the scales weren't dodgy.

 

[Peter Holmes]

The obvious conclusion is that you would be better off with a standard magneto as compared to a KVFTT magneto, you are in luck, I have a standard magneto in full working order, and even better news, I would be more than happy to swap it for your heavy KVFTT magneto. Christmas only comes once a year after all.

 

[Martyn Goodwin]

Is it possible that if you went to the summit of Everest then the weight (as opposed to mass ) would be lower? But then there are not that many bike roads.............

Xmas cheer is getting to me

 

[John Reynolds]

I presume that it is the weight of the manual advance/retard mechanism - and that's not including the additional weight of the control cable nor of the handlebar lever!!

 

[Pushrod Twin]

The obvious conclusion is that you would be better off with a standard magneto as compared to a KVFTT magneto, you are in luck, I have a standard magneto in full working order, and even better news, I would be more than happy to swap it for your heavy KVFTT magneto. Christmas only comes once a year after all.

Thanks Peter, your generosity is only exceeded by good humor.

 

[greg brillus]

The end cam ring housing including the manual advance parts, plus the alloy screw on end cap will add weight, but i'm pretty sure the racing magnetos including the more common K2-FTT mag's use a larger alnico magnet cast into the main housing. The input shaft on the racing mag's is one piece made of stainless or similar as opposed to the standard steel shaft with the brass end housing cast onto it (these come loose even on road engines) although there would be little change in weight for that item, the rest of the armature is pretty much the same as the standard ones. On the equivalent BTH mag's the housing that covers the magnet is taller thus having a larger magnet compared to the standard ones. Of the TT racing magnetos I have worked on and /or rebuilt, on the test bench, the split timing of the mag was always spot on, showing the extra effort that went into the building and set up of these mags from Lucas or BTH.

 

[Pushrod Twin]

I looked at the zinc die cast tower for the manual advance cable, plus the steel mushroom bit inside and pondered whether there was 24 grams in that, but was not in the mood to risk disturbing the timing. I have it locked in one spot and use the ATD.
If you have seen evidence of bigger magnets, I think that would more realistically account for it.
The important bit to me right now is that with a quick freshen up, it is back working as advertised. The freshen up consisted of removing the armature and replacing the seal, which also required replacing the bearing insulator, of course.

 

[bmetcalf]

24 g is 0.84 oz, so not a whole lot of weight.

 

[Pushrod Twin]

Quite right, but it would have been more if the body was aluminium, (or even aluminum) which I am led to believe later ones were.
The surprise was that the Mg bodied model was heavier, but Greg suggests that the TT magnets are larger, so I guess the Mg body can be seen as an attempt to compensate for the added deficiency.

 

[greg brillus]

Around 1950 Lucas changed the input shaft bearing from 15 mm to 18 mm, perhaps this was when they went from the magnesium housing to alloy on the racing mags. Obviously there was insufficient strength in and around that bearing, and maybe a corrosion issue with the magnesium housing. The racing mags are a much better mag than the standard ones, I have ridden bikes with them and they work fine, although the addition of the advance/retard lever on the bars is a bit of a pain, the engine tends to hold a fast idle unless you retard the timing every time you pull up and stop.