A10 cranks

[David Tinsley]

I often see reference to small journal cranks and wonder what that means and what difference it makes, models the fit etc. I have a 55 A10 that I have just taken the crank out of and wonder how to identify it?

Same question is for thick flange or thin flange barrels, wondering which one my bike has. It has the original engine in it according to the BSA factory dispatch records

[limeyrob]

I'll make a stab at answering.....
Thick flange barrel - this will be a swing arm style barrel with the larger fins.  The base flange to the crankcase will be 1/2" thick.  It will have 67-1210 on the flange at the front drive side.
Small journal crank - the crank is bolted across under the flywheel rim.
Big journal crank - the flywheel has 3 radial bolts holding it to the crank.
Although the big journal crank is a later design and one piece and the journals are bigger, I don't think there's any evidence that the the small journal crank is not up to the job on a standard engine.

[David Tinsley]

Thanks for that info.

I have the bolted flywheel, 6 bolts, so the earlier small journal one
My barrel measures 1/4" at the flange, not 1/2", so must be the earlier one. Both incorrect for my year and model.

Interesting as the dispatch records show my bike with the engine and frame numbers that I actually have. I guess someone has been inside it at one point in its life and used the original crankcases.

[limeyrob]

I think the thin flange barrel and small journal crank may be correct for '55.  The 3 radial bolts replaced the 6 cross bolts from 1958 so your crank is correct. I'm not sure when the thick flange barrel was introduced.  What CR are you running?
The sludge trap plugs are smaller on the earlier crank so be sure to get the correct parts.

[David Tinsley]

I am guessing you mean Conrod by CR?
67/270 R3R1Z is the number on the conrod

[limeyrob]

Sorry, compression ratio .  A '55 Golden Flash could be 7:1 which would not stress the barrel or crank.

[JulianS]

The thick flange barrel was standard for the DA10 series Golden Flash engines but they are interchangable with the thin flange one.

The Rockets got it earlier from engine CA10R 4182.

The A10 Road Rocket got the early version of the large journal crank for 1955, this one, part 67 1149, retained the 6 bolts securing the flywheel to the crank and the small oil hole plugs.

The later large journal crank with the 3 radial flywheel bolts large oil hole plugs and removable sludge trap was introduced in 1957 for the DA10 engine from 101, the Road Rocket from CA10R 5021 and the first export only Super Rocket from CA10R 4650.

The 2 crank versions will interchange.

[Swarfcut]

 The real weakness of the small journal crank is the poor oil supply to the drive side big end journal. The oilway between the two big ends is simply a small cross drilling, and any sludge settling here produces the inevitable expensive calamity. The big journal crank with its  bigger diameter labyrinth oilway, in effect a tube within a tube, addresses this, but again any sludge decreases the oil flow.

  Big End Journal sizes are used to define the crank, small journal is early type, larger journals and improved sludge trap comes later and the later conrods  also look and feel more substantial and have easier access to the big end nuts. The surface loading on the bearings are comparable, big journal bearings are narrower....

 Conrods.
       A10 small journal are marked 67 270. Big journal 67 1160.    Any other following letters and numbers relate to production forging identity.

 Early A10 rods use a reduced hex flanged castle nut with split pin on the big end bolt. Later versions have a larger hex. The part number changes to reflect this, which from experience indicates a different machined profile to the big end cap to improve access to the larger hex nut. The actual conrod forgings appear to be identical.

 All A7 cranks are of the same small journal diameter, even mainstream Longstroke versions using steel conrods.

 I reckon thin flange barrels are thicker than 1/4". We're talking the thickness of the barrel mounting flange to the crankcase mounting face, not the cylinder lower spigots' wall thickness into the crankcase.

 Swarfy.

[David Tinsley]

Thanks for that info swarfy,

Got my measurement of the barrel flange wrong earlier. It is closer to 0.44" on my cheapo digital caliper.
looks then like my engine is all as it should be. now I just have to keep working on the getting the last castellated nut off the conrod big end, what a PITA that is proving.
On the plus side both sludge trap plugs came out fairly easy with an impact driver once I relieved the peening marks.
had enough for one day and the beer in my fridge is calling

[berger]

A7 barrel base flange is 3/8ths ,

[limeyrob]

So that's 3/8 for A7, 7/16 and 1/2 for A10.  Makes sense.

[KiwiGF]

I often see reference to small journal cranks and wonder what that means and what difference it makes, models the fit etc. I have a 55 A10 that I have just taken the crank out of and wonder how to identify it?

Same question is for thick flange or thin flange barrels, wondering which one my bike has. It has the original engine in it according to the BSA factory dispatch records

The unfortunate reality is that the only original engine part might be the drive side crankcase half, with the engine number on . 

Crankcase halves have a stamping to show if they are a matching pair or not.

[David Tinsley]

I often see reference to small journal cranks and wonder what that means and what difference it makes, models the fit etc. I have a 55 A10 that I have just taken the crank out of and wonder how to identify it?

Same question is for thick flange or thin flange barrels, wondering which one my bike has. It has the original engine in it according to the BSA factory dispatch records

The unfortunate reality is that the only original engine part might be the drive side crankcase half, with the engine number on . 

Crankcase halves have a stamping to show if they are a matching pair or not.

I cannot see any stamping on either of my crankcase halves and no evidence of them being ground off.

[Swarfcut]

  Matching halves are stamped with the same number on the front, usual place is the centre lug under the dynamo cradle. Sump plate and crankcase mouth joins will be perfectly  level, also there are sometimes circular stamp marks on the joins from quality inspection in production. Oddball cases can be machined to match, line boring cam bushes and timing bush etc, but this all adds to costs.

 If a camshaft turns freely in a pair of cases, that's a quick test that they are a good set.

 Swarfy.

[Radlan2]

      The Roland Pike development engineers memoirs says the change to LJ cranks was because they would snap when run at over 5,000 RPM for a sustained period of around 20mins due to the harmonic frequency of the vibration at that speed, the larger journal stopped this problem. He also states it could be overcome by polishing the grinding fillet rads in the corners of the big end journals using a ball bearing pressed at high pressure against that radius.
       I doubt if many owners today hold their machines at sustained high RPM and those that do are likely to have the large journal crank anyway, it does seem strange that BSA did not upgrade the revvier 500. This was why I used an A65 crank in my track bike instead of a 60yr old small journal crank.
       Chris