Author Topic: Late model A10 cranks in early style A10 cases???  (Read 838 times)

Online Sluggo

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I am looking at building up a hot rod motor, (NOT a restoration) for my rigid frame A7/A10 project. Basically want to make it look like a factory or BSA dealer race bike but for the street. 
(Basically Daytona race bike inspired but not a slave to it).

So I have some 52-53 A10 cases and while I have a 52 Plunger Golden Flash project sitting here I dont want to tear down the whole motor in that to research all the parts (Although I might end up doing it).  So, Last year I sold my OTHER Golden Flash project and spent a LOT of time researching what the cranks look like and many details about them.

Including custom builds with Norton cranks, A65 bits and other obscure BSA trivia.

I have some late type A10 Cranks,, RGS, Super rocket, Road Rocket type cranks and rods that seem to be very much lusted over. After many hours trying to cull info from forums and magazine articles I am as confused as ever.

#1) What makes the later cranks so much better? Besides slightly more robust construction?
#2) What are the challenges to using one of these cranks in earlier cases besides bearing dimensions?

I have searched the forum here, and some of the other ones and while tons of interesting topics and discussion, I dont see anything straight forward and simple as what I am looking for.
Basically a Build or tech thread of "How to upgrade and build an early A10 (1951-53) to late model hot rod A10 specs"

Of course I am open to suggestions on other possibilities.. (Norton crank? A65 bits? Other ideas?) But I dont want to make a total lifetime career out of this either. I have too many projects as it is and need to focus in on realistic builds and projects.  So,, wild
Un-ob-tain-ium parts and tech is very interesting (love that stuff!) I am trying to be pragmatic.

Basically use A-B-C- and machine D, check tolerances and here you go!  Maybe thats asking too much,, but not trying to reinvent the wheel here.  Just adopt well established ideas and tech and have reasonable expectations of being able to ride it on the street.  (IE: No 10,000 rpm+ race bikes with 2 speeds,, OFF and FULL Race)

If I missed such a discussion please point me to it, But I would welcome any advice.
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Online JulianS

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Re: Late model A10 cranks in early style A10 cases???
« Reply #1 on: 12.07. 2017 22:42 »
I fitted a 1962 A10 crank into a 1954 CA10 swinging arm case no issues.

The main bearings are same dimensions.

Online Black Sheep

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Re: Late model A10 cranks in early style A10 cases???
« Reply #2 on: 12.07. 2017 22:47 »
Just fit the big bearing crank to the earlier cases. A frequently done mod when these bikes were just transport. No weird science or necromancy required.
2 twins, 2 singles, lots of sheep

Online Klaus

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Re: Late model A10 cranks in early style A10 cases???
« Reply #3 on: 12.07. 2017 23:15 »
You can fit all cranks in this cases. The later cranks are big journal cranks with a bigger diameter at the conrod and a sludge trap. The bobwhight is fixed with three radial bolts. Any parts from the later unit engines is a pain and lot of work to fit.
If whant to have a realy smoth running engine make some improvements like lightening all parts upper the cams. It is possible to save a 80 gramms per valveunit.


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Online KiwiGF

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Re: Late model A10 cranks in early style A10 cases???
« Reply #4 on: 12.07. 2017 23:42 »
Most people view the later (large journal) crank as better beleiving it is a better design eg stronger, having less flex at high rpm.

The alternate view is that the earlier small journal crank is perfectly adequate on all but highly tuned engines (eg high compression RGS spec) - so there is little if anything to be gained by "upgrading" it to LJ.

Whilst the big end journals are bigger diameter on the LJ crank they are not as wide as on the SJ so the bearing surface is much the same.

If I was on a budget and had both SJ and LJ cranks I'd sell the LJ and use the $$$$ gore new billet rods for the SJ  *smile*

There are a few threads on the various cranks eg https://www.a7a10.net/forum/index.php?topic=11404.msg86441#msg86441

Once you've decided which crank to use there are plenty of threads on how to go about refurbing the bottom end eg whether to risk using old rods/bolts, timing side crank grinding and line boring, balancing etc. It's very important to get it right!



New Zealand

Last had an A10 in 1976, in 2011 it was time for my 2nd one.

1956 Flash Frame EA7-168x Eng. CA10 913x, left BSA together for Liverpool, 5th Dec 1955.

B31 “hot rod” (yeah right)

Armstrong Siddeley Whitley for rainy days (with wife).

GL1800 Goldwing not sure why yet

Online edboy

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Re: Late model A10 cranks in early style A10 cases???
« Reply #5 on: 13.07. 2017 02:14 »
my humble opinion is that the heavier l.g. will give a better top speed and best for your 23 tooth engine sprocket high gearing[? ]. good for motorway cruising. lighter crank for 21 teeth all round speeds and acceleration. both have their tunning pros and cons . big bore , carbs and valves are what matter.

Online Sluggo

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Re: Late model A10 cranks in early style A10 cases???
« Reply #6 on: 13.07. 2017 08:06 »
Thank you all (So far) for the replys.  I guess I need to get my crank notes out again. I spent some time going thru my cranks to supply a spare crank for the other Golden flash project and spent many hours trying to ID cranks.  I read many interesting posts about the various years and the transitional cranks, and some mystery cranks and a few setup for alternators. 
I got the idea that there was some issues and some years differences in the drive and timing sides on a few..
Going to the cabin for a few days but next week will dive back into crankshafts and engine parts,
I am still sorting old shop inventory and parts and long term project(s)  as I also have some other A10s.
Interesting point on the size of the dia of the journals vs widths of journals.  The issue is what rods.  I have some used late model rods, dont know until more sorting what I have for early rods if any.  I also did not know you could get H beam rods for the early small journal cranks.
On a few A65s I built, I was told by Capt Dirt and some other racers that the hot tip was to modify the A65 rods by using Norton steel caps, resizing the rods and new bolts.  This seems to be an accepted practice and seems to help rod life.  Dont know if that is viable on A10 rods.

But I have some opinions on service life of an alloy rod. If affordable, I would prefer a NEW H beam rod. 

I also, in my experience have found a lot of alloy engine cases out of tolerance either from shifting-age-distortion, or poor manufacturing.  But on most motors we do a full engine blue printing.  If I can find the pictures I have a series of photos from my old friend Sir Edward. A tool & die maker originally from the UK.  Eddy was a perfectionist and genius and I learned much from him.  (A few other engine and tuning gurus as well)  So, any build like this will be fully blueprinted and tolerances fully optimized.

Secondly, I have a really awesome dynamic balancing guy.  Ernie does Airplane props and certain types of industrial equipment as his day job but loves to do MC engines on the side.
been using him for 20 years and very happy with him.  Its rare I will rebuild an engine with out fully balancing the parts.  We experimented a few years back before my health issues with some A65 motors and different balancing figures and crank weights.  Capt Dirt told me the key with the A65s was to take the lightest Triumph unit crank and match that weight with the BSA. 
We did, and that motor revved like a dirt bike 2 stroke.  I would have to get out my build notes,, but the BSA unit cranks are a LOT heavier than the Triumph unit cranks.  I have a pile of flywheels that are NOS from BSA from a dealer.  Still in the wrappers.  I can post part numbers and weights but BSA offered a number of flywheels for the Unit twins.  I will look at if they can fit (Probably not) a preunit crank, but there is some options.

But on this bike,, I dont think I want a motor that revs like a 2 stroke dirt bike.  With a long rod motor like these motors have, Torque is where its at.  I doubt Ill do much interstate/freeway riding.  Mostly country roads and comfortable RPMS at 70 mph is my goal but strong pull from off idle to 70-80 mph is where I see this going.

Thanks as well for the suggestion on the engine parts weights.  I will have to look into that.
On the Nortons, I am a big fan of the lightweight cam followers on the twins. Ironically many use BSA lifters.
See: http://www.jsmotorsport.com/products.asp

Sir Eddy made his own versions for the Norton LSR race bike,,  And I have some development parts here that Kenny Dreer was playing with. He was using modified VW lifters. I can post a pix of that as well.  Simple but effective designs.

I have some alloy heads as well, and some performance cams, but I think I have some useable stock 356/357 cams.  Need to see what I have for pistons,. But I know I have some NOS pistons, as well as some others.

I will update and post some parts pictures next week. 
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Online edboy

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Re: Late model A10 cranks in early style A10 cases???
« Reply #7 on: 13.07. 2017 11:33 »
sounds like you want a high reving, big bore  a65 if 80mph is your limit.

Online Sluggo

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Re: Late model A10 cranks in early style A10 cases???
« Reply #8 on: 13.07. 2017 21:43 »
No not really,, I dont know all the gearing options on a A10, but on a typical Triumph unit bike using the stock rear sprocket, I use a 20 tooth countershaft sprocket and that allows plenty of grunt off the line (With the rest of the gearing stock) but allows a comfortable RPM at 70 mph.
(Not too high)  With most of these old engines, and alloy rods, I try to keep RPMS south of the 5000 rpm limit and cruising speeds around 3500 rpm approx.  Torque is where its at.

Stan Shentons Triumph Speed tuning book as well as Factory Triumph literature state that they considered the useful service life of a stock alloy rod as ONE 500 mile long race. Makes you think doesnt it?

I have a few friends who are the belief that they can wring the snot out of their old British bikes and rev well up to 7500 rpm and sustained speeds, and It amazes me that they dont blow up more motors.  I have an old engineering paper for industrial equipment that shows duty cycles vs service life and I feel its a compelling argument that a 70 year old motorcycle engine needs to be treated gently if using original or used parts.

That being said,,, I was involved with Sir Eddy from the beginning and donated many of the parts to build his land speed record racing Norton with 750 cases but running in the 500cc class, but nothing stock in that motor, 270 deg crank, and all kinds of trick stuff.  The design was to safely rev to 10,000 RPM but FAG bearings are only rated to 8000 rpm on a good day, so every step was a challenge. Originally we planned to develop the platform on road race tracks before going to the salt.  But he had the temerity to die before it was completed.  His son took it over and most of the original people are RIP or no longer involved.

My basic idea for this bike is the 51 rigid frame I posted about (with the dog leg), Semi unit A10 cases. Late type crank, still up in the air on rods (MAP cycle has new H beams for $450 and I might still get my dealer discount). Alloy head and pistons still up in the air. Probably use one of the 356/357 cams I already have, or order new one.  Have not decided on dual or single carb and which type.... and looking at Maggie vs EI, Would like a Bob Newby or MAP belt drive if I can swing it, and probably going to do a Suzuki Clutch convesion if i dont go with the newby belt drive.
Blue print the heck out of it. (optimize ALL the dimensions) and dynamically balance it.  I am still in mock up and collection mode and before I start building, have to finish my ongoing Triumph and Norton projects.

My point of joining this forum is info gathering and assistance in ID'ing parts for some of my many projects.  I appreciate any and all feedback and advice.
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Offline bikerboy

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Re: Late model A10 cranks in early style A10 cases???
« Reply #9 on: 13.07. 2017 23:14 »
Crank, cam, thick flange barrels are all interchangable the only thing that is a challenge is fitting an alloy head to the plunger due to frame clearance on the carb. As for gearing on a plunger to the best of my knowledge there is only one size engine sprocket (duplex primary chain) and the gearing is changed by the rear sprocket not the engine sprocket.

The big journal crank was introduced when BSA upped the compression ratio for the rocket models as were the thick flange barrels.


It also has a sludge trap tube which if I remember rightly the small journal crank never had

Online KiwiGF

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Re: Late model A10 cranks in early style A10 cases???
« Reply #10 on: 14.07. 2017 01:10 »

It also has a sludge trap tube which if I remember rightly the small journal crank never had

I cleaned out what I thought was a sludge trap on my SJ crank, there was a small space under each end cap on the cap filled with gunk, on mine the small tube joining the big ends was also nearly blocked with gunk.

I can't be 100% sure BSA intended the crank to have a sludge trap, but I'm fairly sure  *conf* maybe there is a BSA document to state one way or the other? Either way I'd recommend cleaning out the spaces under the crank end caps of a SJ crank during any bottom end rebuild.
New Zealand

Last had an A10 in 1976, in 2011 it was time for my 2nd one.

1956 Flash Frame EA7-168x Eng. CA10 913x, left BSA together for Liverpool, 5th Dec 1955.

B31 “hot rod” (yeah right)

Armstrong Siddeley Whitley for rainy days (with wife).

GL1800 Goldwing not sure why yet

Online Sluggo

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Re: Late model A10 cranks in early style A10 cases???
« Reply #11 on: 14.07. 2017 02:00 »
I KNOW I have a couple of the late cranks and rods,, (Have to hide them from some of my friends)
so I think I have that covered, but not sure how many I have.  I have a late A10 (63) build on the horizon as well.
But I have a lot of BSA cranks in storage and some might be the early small journals.  I sometimes assume "oh!,, I got plenty of those or those" and then it turns out it was not what I thought.  BSA has so many variables!  Never assume!

I sold one of my BSA Hornets projects a few years ago. (Got a lot of Unit A65 and unit singles)
Assumed i Had plenty of axles, Apparently that year (66) Hornet used a special axle?  We played hell trying to find a correct one in my stuff.  Of all the axles at the time we found one with the right shoulder and spacing.  Many other examples,, who knew unit oil tanks were not all the same?   *dunno*

Ill be pestering all of you for ID of many parts, I know enough to be dangerous on Triumph, Norton, Buell and Harley,, but much to learn about BSA stuff.  Especially the early stuff.  Great forum to be able to get help on this.
 *help*
Remember that any advice received on a free internet forum is generally worth about 1/2 of what you paid for it.
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Offline bikerboy

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Re: Late model A10 cranks in early style A10 cases???
« Reply #12 on: 14.07. 2017 21:18 »
The LJ and SJ cranks are very easy to tell apart. The SJ has the bolts for the flywheel inside the flywheel where the LJ has them on the outside of the flywheel in case you are not aware

Offline bikerboy

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Re: Late model A10 cranks in early style A10 cases???
« Reply #13 on: 19.07. 2017 00:26 »
As requested sluggo. This is how I setup one head which is in my swinging arm bike at the moment but has been on the plunger

I will also take a pic when I get a minute of a manifold I made to fit the RR head that has the detachable manifold and I am at the moment working on a manifold to take the mark 2 amals on my plunger but thats proving challenging due to the lack of space.

It also depends if you are going to use a standard pentrol tank or not because the taps get right in the way of my bolt on manifold

Offline bikerboy

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Re: Late model A10 cranks in early style A10 cases???
« Reply #14 on: 19.07. 2017 00:29 »
BTW

On that pic I machined the head down and drilled and tapped direct into the alloy for the inlet spacers which I think come off a (dare I say it) triumph

Before the purists complain I picked that head up cheap at an auto jumble because it had already been butchered before I got it  *smile*

I will try to take some better pics at the weekend when I get out into the garage