Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board A65 crankshaft

Interested in the weight of a late A65 1971-72 ??

I take it that you mean the lightened crank with the scalloped flywheel. My 1972 NOS A65 crank is bob on 10kg. I'm thinking of putting it into my 71 Firebird with the full width flywheel.

Does anybody know if there's any significant improvement with the lighter crank and what the weight of the earlier 1970s crank is?

The object was to improve balance but it makes little if any difference.
Some blokes ran the a50 flywheel and reckoned it was better, i tried it
and once again noticed very little difference.

I had one of the lighter cranks in a late OIF Thunderbolt. Vibration was horrible. It was so bad it cracked the frame in several places including the seat support frame loop, the engine mounting bracket that is welded to the oil tank part of the frame, causing the oil tank to leak and the seams of the fuel tank split as well. When one of the conrods let go and destroyed the engine, I put in a replacement engine with a heavier crank which was much better. That was 30 years ago and I'm still using the bike and the welded up bits are still intact.

Here is the crank out of my '72 A65T with engine date code 'NG'. Very sharp machined edges on the scalloped flywheel.
The bathroom scale says 10.2kg. No idea of how accurate that is, but don't have anything else to use.

JJC, your vibration was more likely due to bad balancing than lighter weight. If the balance were the same, the imbalance forces between the crank and rods/pistons would be the same. It would just allow the crank to spin up/down faster.
Dan Macias made a crank for the Triumph where the most of the flywheel weight was placed in the outside webs next to the bearings. It reduced the flex of the crank. Not too relevant here though since you are starting with a stock crank.

not saying u are wrong , just that i dont understand.

if u replace the (say) 70 crank with the full flywheel with one of the lighter ones in the pictures how come it doesnt bugger up the balancing ? My understanding is that balance is done with **% of the reciprocating weight of rods pistons etc

please explain ?

Appreciate all the response. I agree with Dmadigan in regard to JJC issues with excessive vibration. The light weight crankshafts we used in the Routt 750 long rod and 825 short rod Triumph motors was not an issue. Careful blanching is part of the solution. A big part.
In any event, thanks for the responses. And yes, I and a close friend are going to run light crankshafts in our A65s.I'll try to keep y'all informed on how it all works out

What I meant was replacing a heavy crank with a light one with the same balance factor will not make a difference in the vibration. Just changing cranks without rebalancing will likely always end up with more vibration.

Knowing the long-term history of the bike and having owned it since it had low mileage, I am confident the engine left the factory as I received it. That is, with the lightweight crank and very bad vibes.

got it

The factory only worked to a static balance % which directed the vibration in a direction in the vertical plane to suit the frame harmonics and the expected revs of a road bike. What they left untouched was the side to side balance which is eliminated by doing a dynamic balance, you can be lucky and get a crank close to a good dynamic balance with low vibes or bad bad one and get lots of vibes.

So get your crank dynamically balanced, you will need to send out your rods, pistons etc out so their weight can be included and they should also be weighed and made matching in weight left to right. The balancers first task will be check the static balance and correct any errors before moving on to the dynamic balance.

There was a rumour that the late type a65 cranks were dynamically balanced,
but i think that was cobblers myself. I statically balanced my one and
it's not bad, better in fact than some i've ridden with dynamically balanced
cranks, worse than some others, that's how it goes. It's a vertical twin with a
central flywheel, it's going to vibrate, they all do. If you get a bloke who is
prepared to do the balancing properly and gets it right, you should get sweet
spots you can ride in, that's the best you can hope for.