Bsa Alternator, Crankshaft type.

[Harryc208]

Hi,

After reading posts about 12v electrics from the dynamo, 12v lighting sounds quite a nice asset and something I would like to have on my new build.

I understand some a10s/a7s where fitted with an alternator on the crankshaft, which I think, is what I have some parts for.

I think I have the, Alternator rotor and stator and the crankshaft, however primary chain cases are missing.

The crank I believe to be A7, it has a7 length rods at least

So I have some questions,

What numbers do I need to look for on the crankshaft, or what do I need to measure to know exactly what it is?

How much power do these things produce?

Will it rob any Hp from the engine compared to a dynamo?

And If fitted would the dynamo be needed anymore, or could it be left off?

Here’s a Picture of the parts I have:



Any help is much apreicated.

[Beezageezauk]

Hi Harry,

Check the splines on the drive end of your crankshaft.  If they are the normal ones, the ones that take the standard Cush Drive assembly, your crank can't be used with an alternator.

The Police A10's had the alternators fitted to make up the extra power needed for the radio and extra lights and the drive side of the cranks were totally different.

These Police models had a Triumph 4 spring clutch fitted with an integral shock absorber that did the same job as the cush drive.  The Triumph 4 spring clutch fitted to the later (non-police) A10's didn't have this integral shock absorber.

Beezageezauk.

[chaterlea25]

Hi Harry,
I believe that the alternator drive side crankcase is different to the standard one
also you need the A7/10 alternator primary cases
and as Beezageezauk says a shock absorber clutch
The parts that are different are listed but not illustrated at the back of the parts book
HTH
John O R

[iansoady]

I suspect you'll have great difficulty in getting the primary chaincases as there weren't that many bikes made in that configuration.

What do I know though.....

In answer to another part of your question, nominal output for an RM21 is 140 watts IIRC.

[muskrat]

G'day Harry,
                if their 6" rods it is an A65/50 crank.
 I think that ABSAF make outer alt covers for their GS's might fit A's.
Cheers

[trevinoz]

Musky,
                 There doesn't appear to be any radial bolts in the flywheel so I would say not A50/65.

   Trev.

[muskrat]

G'day Trev,
               I know I need new glasses but are we looking at the same pic. Ain't that one between the rods.
Easiest way measure the drive side bearing shaft, A7/10 is 30mm, A50/65 is 1.125" = 28.575mm.
Cheers

[Harryc208]

Il go measure up, and report back!

[Harryc208]

Right,

The Rods are more like 8" than 6"

The con rod ends measure 19mm ID

The Drive side Bearing shaft is 28.5mm.

And heres some more pictures:

[muskrat]

OK by the shaft size it's A50/65/70, all had con rods 6", A7 has 6" but sml journal, A10 has 6.468"
 A50/65 has 74mm stroke A70 has 85mm. All have 3/4" (19mm) sml end.
 You could sleeve the shaft to take the 30mm bearing and stroke an A7 
Cheers

[Harryc208]

Turning a sleeve wouldnt be a problem, however what about ballencing, if the a65 has 75mm pistons, is it not ballenced differntly?

So the A65 has the same stroke as an A7?

Thanks for everybodys patience

[muskrat]

 No A7 stroke is 72.6mm. So that's what I meant by stroking an A7. You wouldn't put it in an A10 but in an A7 the piston size is only 1/2mm bigger so balance won't be far out. Might need a 1mm thicker head gasket. Find a set of alt primary cases, etc, etc.
 Cheers

[trevinoz]

Sorry Musky,
                       I only looked at the small picture and took the holes to be balance holes.
Marvelous what blowing up the photo does!

  Trev.

[Harryc208]

I see,

Didnt know the stroke was so similar, Its An A7 it would be going in, just a shame it isnt an A7 crank in the first place.

Thanks for the Info everybody.

[muskrat]

Well it's really better Harry. The A7 never had the large journal big ends. I snapped a few std and lightened A7 cranks when racing. That's why I was looking into it, in fact I saw the A65 crank with sleeved drive side today. might have to take the sleeve off as I now have an A65 project.
Do a lot of measuring and checking. What year is it, alloy or iron head?
Cheers

[Harryc208]

Engine cases are CA7 SS 8223  which I think is 1961?

The rest of the engine parts will vary with age.

Its got the twin carb Alloy head.

Ok so to fit the a65 crank,

Turn a sleave to adopt the A7 bearing.
Get hold of some alternator chaincases.

Would I fit a7 pistons, turn a smaller ID bushing to replace the small end bushing of the a65 conrods?

[muskrat]

 Yep, I forgot A7 pins are 11/16", A10 & 65 are 3/4".
Finding the primary covers will be the hard part. If you know a good alloy welder and machinist you could convert the originals. I saw a pic of a home made one not long back.
A7's only have thin flange barrels so go easy on the compression, say 8.5:1, 9:1 max. I've broke a few of those as well.
Cheers

[Harryc208]

Already considered adapting the chain case,

Could cut a hole the size of the stator in the outer chain case, turn a ring to house the stator and tig weld it onto the case.

Among the parts I have, there is some bored out cylinders to take Bonneville pistons. The trouble is on the a7 crank and rods they don't reach the top of the cylinders, I’m wondering how far they would go up with the a65 crank and rods. Time permitting tomorrow I may have a mock up and see.

[muskrat]

Fitting, Fitting I know. I had 67mm trident pistons on 2.5mm shortened A10 rods, but they were sml journal.
The crank will only push them up by 0.6mm. I'd sleeve the cylinders back to std and use A7 pistons.
Cheers

[Harryc208]

Now I have thought about it, when I looked at the bonneville conversion before the thing I didnt like was that the barrels overlapped the edge of the head.
That engine in question was partly stripped when I got it, the head was off, so I think it had blown the head gasket.