The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: Servodyne on 18.11. 2018 20:30

Title: Strange A10 Crankshaft
Post by: Servodyne on 18.11. 2018 20:30
Hello everybody,
I've recently bought a large journal A10 crankshaft off ebay, but I'm now wondering what I've actually bought as it is totally different to any that I've seen before. The flywheel isn't attached with the usual 3 retaining bolts but appears to be 'shrunk' on with 3 retaining threaded studs that are flush with the outer surface of the flywheel. The question that concerns me is will it be compatible with a 1957 swinging arm A10?
Cheers Jim
Title: Re: Strange A10 Crankshaft
Post by: berger on 18.11. 2018 22:44
well knock me over with a feather, I aint never ever seen one "O" them there cranks likey that one *conf2* maybe if you measured the flywheel diameter and compared it to one with the three radial bolts you might find its a skimmed down cranky and they've centre popped what's left of the bolts to stop them damn darned suckers flying out and messing up ya engine  :-\ *dunno2* edit could it be a racing crankshaft?
Title: Re: Strange A10 Crankshaft
Post by: Swarfcut on 19.11. 2018 09:47
  Jim..... Could be another Unicorn Part, Heard Of But Never Seen.  There are rumours of a One Piece Big Journal Crank, which precedes the more familiar three radial bolt type. I would consider it impossible to manufacture and machine this design of crank from a single forging, so on initial examination it may appear to be in one piece, because of the hidden bolts and hence the myth that is made in one piece.

 As long as the journal sizes are within the accepted dimensions, and the throw is correct, then it should fit fine.  Do not neglect the usual cleaning procedure for the oilways.

  No doubt the respected contributor JulianS will be along soon to offer further enlightenment.

  Good thought there berger, another possible explanation.

  Swarfy.
Title: Re: Strange A10 Crankshaft
Post by: JulianS on 19.11. 2018 12:09
This is an early Road Rocket crank, changed in 1957 to the version with a removable sludge trap and radial bolts.
Title: Re: Strange A10 Crankshaft
Post by: Swarfcut on 19.11. 2018 12:54
  Julian.. Thanks for the possible answer to the mystery. Looks to be a sort of halfway house in the design, larger big end journals but still retaining the small sludge trap plugs and oilway.  Together with a change in design for flywheel attachment. Alas my screen is small and the part numbers were difficult to read so that's how I see it.

   Rumour or a myth, this proves the existence of a relatively short lived and rare part. Another one for the rivet counters in the quest for ultimate originality.

 Swarfy.
Title: Re: Strange A10 Crankshaft
Post by: RichardL on 19.11. 2018 13:03
Always great to see obscure docs from Julian's treasure trove. One thing that hit me was the "SR" in the engine number for "Scrambles Rocket." I don't think I've seen that before (but that could just be me).

Richard L.
Title: Re: Strange A10 Crankshaft
Post by: JulianS on 19.11. 2018 13:59
The "SR" used on Spitfire scramber engines, starting with the CA10SR in 1957 then DA10SR for 1060 season until end of 1961 season when the DA10R engine was used.
Title: Re: Strange A10 Crankshaft
Post by: muskrat on 19.11. 2018 18:57
G'day Jim.
Quote from Rocket Racer "My understanding is that the 56/57 road rockets originally had what is often called either a transitional or heavy crank: this has a very small sludge trap arrangement and the flywheel is not removeable. the webs are also a different shape and it is solid through the centre. This is also a large journal."
I think the key word is "heavy" so should give good torque.
Cheers
Title: Re: Strange A10 Crankshaft
Post by: Servodyne on 02.02. 2019 18:23
Thanks all for the replies. Some very useful information.
Cheers  *smile*
Title: Re: Strange A10 Crankshaft
Post by: Whipstrut on 05.02. 2019 00:35
Well folks, there are at least two of these cranks in existence, as I have one too. Always wondered why there wasn't a sludge trap tube, and it weighs 26 lbs.
Title: Re: Strange A10 Crankshaft
Post by: trevinoz on 05.02. 2019 20:08
There's more than two in existence.
A friend of mine has at least three.
Title: Re: Strange A10 Crankshaft
Post by: Servodyne on 05.02. 2019 22:28
G'day Jim.
Quote from Rocket Racer "this has a very small sludge trap arrangement and the flywheel is not removeable.
I think the key word is "heavy" so should give good torque.
Cheers

Hi
I've just removed the timing side sludge trap plug just to see what the set up is. The crank's on standard size so the plug has never been removed before. Wow, no sludge trap tube, just a 1/4" hole which was virtually clogged with hardened sludge. I certainly wasn't expecting the sludge trap to be such a small diameter hole, but I should be able to clean it out without removing the drive side plug. Next job is to take it for a regrind.
It should make a nice smooth motor with plenty of torque, but I think you would definitely need to run it with a cartridge oil filter.. 
 
 
Title: Re: Strange A10 Crankshaft
Post by: worntorn on 06.02. 2019 02:58
I don't think you can get extra torque output from a crank whether it's heavy or light, as in measured continuos torque output on a dyno.
The heavy flywheel does give extra momentum, which makes starting off without stalling easier. The downside is they are a little slower to rev up.
I once had an Ossa race bike that came with three external flywheels, two aluminium in different weights and a heavy bronze.
It had the bronze flywheel fitted, so I switched for the lighter al. job. It revved noticeably quicker, which was fun for awhile, but I soon tired of kickstarting it after multiple stallouts when used in stop start off roading. The bronze wheel went back on.

Glen
Title: Re: Strange A10 Crankshaft
Post by: Greybeard on 06.02. 2019 09:08
Quote

I've just removed the timing side sludge trap plug just to see what the set up is. The crank's on standard size so the plug has never been removed before. Wow, no sludge trap tube, just a 1/4" hole which was virtually clogged with hardened sludge. I certainly wasn't expecting the sludge trap to be such a small diameter hole, but I should be able to clean it out without removing the drive side plug. Next job is to take it for a regrind.
It should make a nice smooth motor with plenty of torque, but I think you would definitely need to run it with a cartridge oil filter.. 
The early flywheels did not have  a tube in the sludge trap. I feel you should remove the other plug and leave it out until you've had the crank ground.
Title: Re: Strange A10 Crankshaft
Post by: morris on 06.02. 2019 11:23
There's more than two in existence.
A friend of mine has at least three.
Makes me wonder what an enormous quantity of NOS parts will flood the market after we're all dead and gone...  *doubt* *smile*
Title: Re: Strange A10 Crankshaft
Post by: Swarfcut on 06.02. 2019 12:02
 Now you know what a small journal crank looks like inside.  As GB says, get that other plug out, the cavity behind it will also be well clogged up.
   The plugs were never (or rarely) removed for the oilway to be cleaned out, back in the day. The crank grinder took your money, and in a matter of a few short weeks there was a good chance you would be back again for another grind. In the Service Sheet, 206, Engine Dismantling, it gets just two short sentences, a matter of four lines of print.  Most owners, including me, looked at the plugs, reckoned they were in there for life, and just flushed it through with paraffin, little realising what was inside.

  As it is a rare crank I would only have it ground if the journals are worn or scored. If still within size and not worn oval (there is a suggested 2 Thou ovality limit) you may get away with just a  journal polish, which is the final stage in grinding a crank. Essential to ensure no sharp edge to the journal oil supply holes, a sharp edge will gouge a groove your new shells. Lots of information on the Glyco Bearings Website.

 Same with the timing side journal.  Only grind off the minimum to clean it up and go for a one piece custom bush, line bored to fit the crank.

 Swarfy.
Title: Re: Strange A10 Crankshaft
Post by: cyclobutch on 06.02. 2019 12:38
I don't think you can get extra torque output from a crank whether it's heavy or light, as in measured continuos torque output on a dyno.
The heavy flywheel does give extra momentum, which makes starting off without stalling easier. The downside is they are a little slower to rev up.
I once had an Ossa race bike that came with three external flywheels, two aluminium in different weights and a heavy bronze.
It had the bronze flywheel fitted, so I switched for the lighter al. job. It revved noticeably quicker, which was fun for awhile, but I soon tired of kickstarting it after multiple stallouts when used in stop start off roading. The bronze wheel went back on.

Glen

I also enjoy the extra engine braking of a lighter flywheel. Toying with having the flywheel lightened on my T3 when I pull it out in a couple of weeks time - and that's to speed up the gear change a little.
Title: Re: Strange A10 Crankshaft
Post by: berger on 06.02. 2019 16:16
well for anyone interested an A7 crank -10 thou weighs 14lbs and its flywheel without the 6 nuts and bolts is 8lbs- just thought I would mention it because no pub until friday *beer* A10 cranks are bigger too *smile*
Title: Re: Strange A10 Crankshaft
Post by: muskrat on 06.02. 2019 18:50
I lightened my A7SS racer crank by about 3 Lb. The flywheel looked like Swiss cheese. Revved out to 7500rpm very quickly. Needed a fist full to get off the line.
Cheers
Title: Re: Strange A10 Crankshaft
Post by: Servodyne on 06.02. 2019 19:34
As GB says, get that other plug out, the cavity behind it will also be well clogged up.

Ok, You and GB are obviously correct, it's not worth the risk in trying to save any effort in removing the drive side plug. It's just that the timing side was such a pig to get out, hopefully the drive side won't be any more difficult.
The crank will need a regrind unfortunately, as it's around a thou under bottom tolerance on all journals. I'm just amazed that sufficient oil was able to get through at all and that the journals are as good as they are.
My plan for the crank was to replace the later one that's fitted to my 57 Spitfire even though it's post engine no CA10 SR.231 and has the correct crank fitted. I was going to upgrade it with billet conrods and raise the compression ratio from 8.25:1 to 9:1 with some new pistons that I've recently acquired, but after reading some of the comments re the heavy crank I might have a rethink.
Thanks for all the replies.
Jim
Title: Re: Strange A10 Crankshaft
Post by: Swarfcut on 06.02. 2019 20:28
The remaining plug knows what's coming.....you will have no problem. If you sacrifice it, replacements are readily available.

 Swarfy.
Title: Re: Strange A10 Crankshaft
Post by: Greybeard on 06.02. 2019 21:47
... replacements are readily available.
With hex sockets
Title: Re: Strange A10 Crankshaft
Post by: Servodyne on 07.02. 2019 12:57
As GB says, get that other plug out, the cavity behind it will also be well clogged up.
  Swarfy.
I'm really glad I listened to you two. I had to get a bit medieval with the drive side plug but it was well worth the effort. I had thought that if it was 1/4" all the way through, I should be able to clean it out good with a long 1/4" drill and hole brush. I didn't expect it to be 9/16 in diameter all the way to the flywheel from the drive side, so I would never have been able to clean it out properly. 
Good advice
Cheers Jim

Title: Re: Strange A10 Crankshaft
Post by: Greybeard on 07.02. 2019 17:22
 *smile*