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#831216 11/27/20 5:23 pm
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Happy apres Thanksgiving to all the BSA folks on this side of the pond.. Happy Friday to everyone else ! smile

Has anyone the basic data about an A65 flywheel - the width (in several places) and the diameter ?

I ask because I am eyeballing my A10 crank and my A65 crank side to side. The A10 flywheel is considerably bigger. Photos below -

[Linked Image from i.ibb.co]
[Linked Image from i.ibb.co]
[Linked Image from i.ibb.co]

I anticipated shaving about 3/16th to 5/16th of an inch off the overall diameter of the A10 crank to make sure the A65 piston skirts didn't touch, but it looks like more than that to me right now. I am likely to thin down the width of the A10 flywheel as well.

However, I am asking about the A65 crank, because the one that emerged from the cases looks like it's been machined, with very sharp edges. Not the dull cast radius of the A10, and, I remember my 1969 A65 being similar to the A10 when I rebuilt that back in 1974. Maybe that's how the later OIF flywheels were. I don't know, hence the question.

The A10 crank is to the left or front in the above photos. A65 to the right.

Thanks for the attention.

Steve.

Last edited by S-NJ-W; 11/27/20 5:34 pm. Reason: position of each crank
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i think your A65 crank/ flywheel has been modified, my A65 crank is full width all the way round. An 1/8th" off each side of the flywheel was reccommended for A10s in Eddie Dows twin tips guide.


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Originally Posted by S-NJ-W
However, I am asking about the A65 crank, because the one that emerged from the cases looks like it's been machined, with very sharp edges. Not the dull cast radius of the A10, and, I remember my 1969 A65 being similar to the A10 when I rebuilt that back in 1974. Maybe that's how the later OIF flywheels were. I don't know, hence the question.
The very late OIF A65 flywheels were reduced to 40mm width around the big ends.

You may find this thread on BritBike useful for your conversion.

http://www.britbike.com/forums/ubbthreads.php/topics/758082/1


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The OD difference between A10 and A65 is about 5mm on the diameter. So I think yours has been machined down somewhat.

The fillets appear on the A70 and some late A65 cranks, though yours may have been added at a later time.


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Originally Posted by S-NJ-W
Has anyone the basic data about an A65 flywheel - the width (in several places) and the diameter ?

I ask because I am eyeballing my A10 crank and my A65 crank side to side. The A10 flywheel is considerably bigger. Photos below -

I anticipated shaving about 3/16th to 5/16th of an inch off the overall diameter of the A10 crank to make sure the A65 piston skirts didn't touch, but it looks like more than that to me right now. I am likely to thin down the width of the A10 flywheel as well.

However, I am asking about the A65 crank, because the one that emerged from the cases looks like it's been machined, with very sharp edges. Not the dull cast radius of the A10, and, I remember my 1969 A65 being similar to the A10 when I rebuilt that back in 1974. Maybe that's how the later OIF flywheels were. I don't know, hence the question.

The A10 crank is to the left or front in the above photos. A65 to the right.

Thanks for the attention.

Steve.

The A65/A50 flywheel is 2-3/8" wide.


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My engine number starts with the code NG/20xxx which I think means October of 1972. I have no idea when BSA stopped assembling engines, but I think this is a very late model. So, if the flywheel changed towards then end, then this is likely it. It doesn't have the steel capped con rods.

The basic dimensions of the two cranks/flywheels are:

A65 : 175mm diameter, 40mm width at the big ends and 60mm opposed to the big ends.
A10: 180mm diameter, 60mm width all around.

So, nobody knows the factory specs?

Servodyne, thanks for the link to that thread of yours, I had already found it and it was helping me out with the drive side modifications I need to make. I will also use a sleeve nut to secure the rotor.

That diameter difference corresponds to the numbers that you were quoting Allan, so thanks for that pointer.

Gavin's reference to Eddie Dow's tips can be found here : https://cybermotorcycle.com/archives/bsa-a10/tt11.htm
Thanks Gavin, I hadn't seen that before.

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Originally Posted by Gary E
The A65/A50 flywheel is 2-3/8" wide.

Thanks Gary.

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Originally Posted by S-NJ-W
The basic dimensions of the two cranks/flywheels are:

A65 : 175mm diameter, 40mm width at the big ends and 60mm opposed to the big ends.
A10: 180mm diameter, 60mm width all around.

So, nobody knows the factory specs?

It appears your very late A65 engine has the factory spec flywheel with the reduced width around the big ends. The A70 Flywheel is noticeably reduced further, judging from photo's that I have seen. I don't know the exact dimension as I haven't got access to one, so I decided to go for 35mm in order to get the balance factor in the right ball park before getting the crank dynamically balanced.

One other thing to consider when you reduce the diameter of the A10 flywheel, is the three retention bolt heads will now protrude from their counter bores and foul on the bottom of the barrel skirts. You can either make the counter bores deeper to clear the skirts or as I did, relieve around 2mm off the the skirts between the bores to clear the bolt heads.

Jim


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Or reduce the head depth on the bolts which is what mine have.


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If you must reduce diameter to get flywheel / piston clearance its probably a good idea to leave the width as is, taking a mm or two off the piston skirt below the gudgeon / wrist pin would be my prrefferred option. This is a relatively low stress area, unlike the fly wheel which is a stored bomb. I would compromise , a wee bit off the flywheel, and scallops on the skirts.


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The A10 flywheel fits in the A65 cases without any problems, but I anticipated reducing the diameter to clear the A65 piston skirts. However, if I don't need make it any smaller, I would prefer to keep it standard.

It is interesting that the tips from Eddie Dow that Gavin pointed at talk about the balance factor for A10s, because my A10 flywheel has NO drilling on the flywheel periphery to adjust the balance. When I first looked at it I assumed that A10's weren't balanced. Mine looks like its exactly as cast. Maybe I am lucky that way.

I am going to hold off doing anything to the crank until I have a chance to chat with the folks at E&V Engineering who I was going to use for the big bore kit.

Gavin, it looks like E&V have the outrigger bearing kit for the gearbox main shaft you mentioned a long time ago.

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The central flywheel is a big gyro, and for that reason alone I would leave it alone if at all possible, its magic corner glue, A10s were by nature a bit smoother than A65s, lower revving, plus the heavy chassis helped suck up vibes, if you can get the final assembly matched for piston and rod weights , then get the crank dynamically balanced. it does make a difference. my longstroke A65 with an A10 crank is noticeably more vibratory than my big bore , an A10 crank and T140 pistons is not the best blend. Grunty, never rode it long enough to hate it, the big bore is way more civilized and still grunty and revvy without long term vibe damage ( apart from the number plate).


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My A65 is June 71 , full width fly wheel, I have seen elsewhere that later models had this trimmed , similar to your pic.A recent youtube vid of a stock A70 had cut down flywheels. I should have measured it last time it was out ,it weighs a lot, enough to stun a man falling one foot from a shoggily shelf.. They were meant to have been machined all over for better balance since 1970. It would be interesting to know the weight difference from full width to cut down, please record the weights.

Last edited by gavin eisler; 11/27/20 11:51 pm.

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here's what I know; and it ain't much...I recently took 3 cranks to EDv...2 large journal a10 cranks and a 1972 crank.the 72 crank is identical to the late crank you've pictured...my original idea was to fit the late flywheel to an a10 crank + a few other modifications. ..yes they will fit with alittle finesse ...in any event, the late crank of yours looks to these eyes , to be a late, the late a65 crankshaft just like it left the factory...Ed can confirm this...thats all I know


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Stick the a65 flywheel on it and have it balanced at around 60%, seems to work well on
the one i've just done.
Get a pair of a70 pistons if you can, they are around and are better than using t140 ones.


You will need to skim the barrel 20-40 thou or so as the crank is 1mm down on the a70.

Last edited by NickL; 11/28/20 1:31 am.
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One of the reasons edie dow said to take a bit off of each side of the flywheel
was that so many weren't Concentric on the crank. I'ts always a good idea if you have
a lathe to spin the assembled unit up and check that. Some are worse than others.
As a sidecar bloke, i never liked light flywheels much, you loose more than you gain.
The assembly i've just done required a lot of 'heavy metal' put in the crank cheeks,
rather than lightening but i was using steel rods and pistons that are a bit heavier.
By the time you fill it with oil and plug it put it on knife edges and get the correct
weights on it, you may as well get it balanced on a spinner, it's not that expensive.

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Thanks for all the input.

To be honest, I want to keep all the original A65 parts untouched. Once the bike is up and running, I will re-visit all those parts and refurbish as needed, in case I or my daughter (cough) ever want to put the bike back to original in the future. I was planning to build up the engine with the A10 crank and big bore kit on a different set of (1972) crank cases, but when I changed the title document to be Texan, the DMV used the engine number as the VIN. Still looking for a way around that.. Considering making my own VIN plate and riveting it to the frame somewhere.

I will indeed put the A10 crank on a lathe and take a look at how Concentric the flywheel is, but if at all possible, I don't want to modify it. But if needs must, then I will. I prefer to modify the A10 crank, not any of the A65 components.

It's a very interesting discussion about when the flywheel design switched over. Gavin says his bike is a '71, but all the A70 cranks used the modified flywheel and the steel capped rods. My understanding was that the A70s were only made in '71. My A65 came from the factory as a single carb model (A65T) *but* with a rev counter. The executor of the deceased owner's estate showed me a photo of the bike as new, in flamboyant red with a rev counter. The original head was in a cardboard box with a bunch of other stuff that came with the bike.

Just for the joy of a Saturday, here is a scan of the original, one owner title (V5 for the Brits) that changed hands with the bike. I blanked out the owners name and last few digits of the the engine number. The NG prior to the engine number I think means October of 1972. Sold on the 6th of February 1973, first registered for use on public roads on the 9th of March, 1973.

[Linked Image from i.ibb.co]

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My own t'bolt is a 72, built late 71.
Scalloped flywheel iron pump alley rods.
There were no definite dates for the changes, i've seen
several 71 bikes with alley pumps and steel capped rods.
They used what was on the shelf, not what brochures said.

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Originally Posted by NickL
They used what was on the shelf, not what brochures said.

Yes, agreed. I am guessing that means there was no 'build sheet'.
My 72 A65T has the scalloped flywheel, iron oil pump, alloy rods and caps.
It came from the factory in red with a rev counter.

It's those last two things that are interesting.
I don't think that A65T's were ever available in red or had a rev counter from the factory.
When I say 'red', I mean the tank and air boxes.
I can see a red tank sitting on a shelf at the factory, but not the single carb air boxes.

Maybe dealer options.

Last edited by S-NJ-W; 11/29/20 4:46 pm. Reason: spellin
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By the time 1971/72 came around the whole BSA group was in a shambolic state.
They were short of cash to buy in outside supplies.
In that situation a production manager , knowing that only if bikes were despatched would he get paid that week would probably instruct a line worker to put anything together to get a bike out of the door.
The official build sheets become somewhat irrelevant in those circumstances!
Just my two cents worth--from someone who was around there at the time.


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