Hi Henri..Crankshaft flywheel sets were rationalised around a single set of forgings around 1949.
These were machined to suit the various engines for which they were used to give the correct stroke, balance etc.
Engines covered by this set of forgings were M20, M21, M33 and B33.
The forgings were numbered 66-635 (as yours are) so you will need to measure them to determine the stroke. Measure from the center of the big end to the center of the mainshaft. An M21 has a stroke of 112mm and the measurement described will be half of this....so for an M21 that will be 56mm.
M20 and M21 use the same conrod..these are listed as 66-480 in most parts lists but I know other numbers were used. To check, the rod should measure 8.250" (8 1/4") between centers....Ian
email (option): julie@wright52.plus.com
Hi Henri..look in the 'Technical Section' of the site under 'crankshaft specifications' for a detailed account of flywheel changes and numbers...Ian
email (option): julie@wright52.plus.com
Hi Henri...The M20 has a stroke of 94mm. Half of that would be 47mm. So it looks like the wheels you are measuring are M20. Yes, I am sure about the 56mm dimension for the M21....Ian
email (option): julie@wright52.plus.com
Hi Henri..Cast iron M21 flywheels were numbered 66-705 and 66-707. 66-635 wheels were steel with both halves having the same number.
Mixing flywheel halves is not recommended but is possible if everything trues up ok on assembly.
66-708 is not a number I am familiar with...could you check that you have read it correctly and that it is not 705 or 707?....Post a picture if possible..Ian
email (option): julie@wright52.plus.com
Saw your listing in the tech section last night. Thorough stuff and helpful.
Here are some pics.
I will go search in the storage area for a matched set. The second number is on the reverse side of the 708.
email (option): unpob@yahoo.com
Thanks Henri...I will have to go back to all my crank info. and make sure I haven't made a numbering mistake. It is clearly 708.
Hopefully I have...otherwise I have to track down that number!!....Ian
email (option): julie@wright52.plus.com
Hi Henri...well according to me 66-634 should go with 66-636 to make a pair of flywheels. Clearly things have got mixed up...but that still leaves the question as to where 66-708 came from!.I have checked my parts lists and cannot find any reference to it.
I don't have a 38 or 39 parts list though...maybe it appears there.
Oddly (as well) the parts lists show the conrod for both models as 66-480 but they are frequently marked 66-481 and again I haven't found that number yet in a parts list....very odd.
Does anyone have the 38 and 39 parts list to check this out?...Ian 
email (option): julie@wright52.plus.com
I don't have a 38 or 39 parts list though...maybe it appears there.
Oddly (as well) the parts lists show the conrod for both models as 66-480 but they are frequently marked 66-481 and again I haven't found that number yet in a parts list....very odd.
Does anyone have the 38 and 39 parts list to check this out?...Ian
The numbers being quoted here are BSA forging or casting numbers, not BSA part numbers. You will not find many forging/casting numbers in the parts books, for the simple reason that after being machined/finished, one forging/casting number can end up as several different part numbers.
I concede that some forging/casting numkbers may be identical to part numbers.
Many flywheel sets have identical forging/casting numbers, but wheels are actually different, ie oil drilling and keyway for crankpin (plus rivetted shafts, but these can be changed).
M21 piston has higher pin than M20 piston.
Stroke of flywheels and piston are only practical differences between M21 and M20.
All pre WW2 OHV 500cc have 94mm stroke.
All M20 SV 500cc have 94mm stroke.
Flywheel forging number of 1937 to early 1939 500cc OHV and 500-600cc SV production is 66-614, these have a distinctive "D" shaped counterweight opposite crankpin.
Mid-1939 and later KM20, KM22, KM23, KM24, 1940 WM20,- WM23 and postwar M/B33 used flywheels with forging number 66-635.
All authentic pre WW2 OHV 500cc flywheels appear to be stamped "OHV" near forging number. Presumably this was done when shafts were fitted, to quickly ID for final assembly.
CAUTION, do not assume any flywheel with 66-614 or 66-635 forging number is correct for pre WW2 OHV 500cc or M20 SV. Post WW2 B33 etc flywheels also use forging number 66-635 and some M20 and M21 flywheels have no numbers at all.
Some flywheels have had all numbers and marks polished off in the past by "tuners".
66-635 flywheels have the counterweight opposite crankpin extended up each side to be more "C" shaped.
350cc WB29, WB30 and postwar B31 flywheels have a distinctively different shape, deeper recess and narrow outer rim (on side) 350cc wheels are 88mm stroke. Some B29/B31 wheels have forging number 66-610. These 66-610 flywheels may also have been used on B24, B25, B26 etc, but this is not confirmed (shafts will almost certainly be different)
To identify flywheels of unknown origin the stroke and pinion shaft length must be measured, 1939 500cc OHV wheels are 94mm stroke and have a pinion shaft which projects approximately 85mm from bearing abutment to tip of thread.
Dimension from centre of mainshaft to centre of big end pin:
47mm = 94mm stroke;M20 SV 1939 to end and also M22, M23, M24 1937-1940 OHV 500cc.
44mm = 88mm stroke;WB29-B31 350cc OHV.(narrow edge rim, deeper recess)
44mm = 88mm stroke; B33-M33 500cc OHV.
56mm = 112mm stroke; M21 600cc SV.
Dimension from outer face of pinion shaft bearing abutment to outer face of unmodified flywheels (straightedge across wheel):
Approximately 0.035” = all 1939 OHV 500cc -1960 OHV B31-B33 etc
Approximately 3/16” = all 1939-1960 sidevalve M20-M21
Dimension from outer face of pinion shaft bearing abutment to outer tip of pinion nut thread:
Approximately 70mm = 1937-1938 500cc OHV.
Approximately 85mm = 1939-1940 500cc OHV & 1939-1963 500cc & 350cc OHV.
Approximately 102mm = 1939-1963 500cc & 600cc SV.
M20 1939 and later measures 47mm between centres of mainshaft and crankpin and has a pinion shaft approximately 102mm long.
M21 1939 and later measures 56mm between centres of mainshaft and crankpin and has a pinion shaft approximately 102mm long.
No idea what 66-708 signifies; possibly a material (cast iron wheels?) or shape change.
Identifying post war Goldstar flywheels is an entirely different subject and is covered in various publications. But if you find some flywheels which ID as B31 or B32 but don't tally up in other ways, suspect GS.
I have researched and assembled all this data for my own use restoring pre war OHV M23 & M24.
If no-one strongly disagrees with my data I will tidy it up some more and send to Henk as "word" doc so he can add to website tech data.
"First, do no harm"
Neale.
email (option): n.gentner@bigpond.com
phew!! neale what a genius it went over my head after the first sentence
Hi Neale...Interesting stuff. It seems to me though that there are problems now in writing up a definitive list of flywheel numbers.
With all the parts lists available it should be possible to make a list of all the part numbers for the different models. However, if those numbers are not on the flywheels themselves identification outside of the engine becomes complicated.
For example a B31 and a B29/B30 share the same forging numbers, are the same stroke and share the same shaft dimensions but are not physically the same, as the finished wheels are machined differently.
Also there are the material differences in the M Series types with some being cast iron and some steel...an important difference if you were to consider extensive lightening when tuning an engine for example.
Thus in a practical sense a full list of PART numbers has little value..you can't order new ones and you can't identify the ones you have using that information.
Similarly, a full list of forging numbers, if it existed, wouldn't identify the flywheels either.(as in the case of the B29/30/31.)
It appears that the original piece I entered into the 'technical section' is not a valid document and would best be removed to avoid confusion.
The origin of this posting was Henri's problem in identifying flywheels that were not fitted into an engine.
Stroke, shaft dimensions etc. etc. appear to be the only guide in this situation and familiarity with the physical differences between cast and forged flywheels the only way to identify material type visually.
In the case of the B29/30/31 you would have to actually know the difference between the two types to identify them once separated from their crankcases unless a full set of dimensions were available.
To summarise it therefore appears that detailed dimensional information may be the only way to go to resolve the problem of correct model identification, leaving the question of determining material type still unresolved..unless this can be done with forging numbers..Ian
email (option): julie@wright52.plus.com
For example a B31 and a B29/B30 share the same forging numbers, are the same stroke and share the same shaft dimensions but are not physically the same, as the finished wheels are machined differently.
Also there are the material differences in the M Series types with some being cast iron and some steel...an important difference if you were to consider extensive lightening when tuning an engine for example.
It appears that the original piece I entered into the 'technical section' is not a valid document and would best be removed to avoid confusion.
The origin of this posting was Henri's problem in identifying flywheels that were not fitted into an engine.
Stroke, shaft dimensions etc. etc. appear to be the only guide in this situation and familiarity with the physical differences between cast and forged flywheels the only way to identify material type visually.
To summarise it therefore appears that detailed dimensional information may be the only way to go to resolve the problem of correct model identification, leaving the question of determining material type still unresolved..unless this can be done with forging numbers..Ian
Hi Ian, I have just re-visited your tech piece.
You have gone into the part numbers much deeper than I have, we both may be right.
It stands to reason that BSA would need to be able to identify their own products and would not introduce a change of material for flywheels without a definite means of identifying them later in life.
By combining our two different approaches to the same subject, I now suspect, and perhaps you or others on this forum can confirm that:
600 range part numbers and forging numbers may denote steel flywheels?
700 range part numbers and casting numbers may denote cast iron flywheels?
If this proves to be the case I would have no problem being a co-author to a revised definitive tech article on flywheels.
"First, do no harm"
Neale.
email (option): n.gentner@bigpond.com
I'm being careful now Neale and any comments I make are subject to confirmation. The later 66-635 wheels used for both M20,21 and M33 and B33 are steel forgings and to my knowledge B series motors were never fitted with cast iron flywheels...So, it may well be that 600 (forging/casting) numbers denote steel flywheels and 700 cast.
We know both the M20 and M21 were fitted with flywheels of either material at different times.
I have to say I am not as familiar with the pre war engines as you are...how does that statement relate to the M23 and earlier M20/21?
I think you are right..somewhere in amongst all the information is probably a clear and logical story.
I have a lot of flywheels from the wartime and post war periods and can 'spot the difference' between cast and steel types visually..really I need to go through these noting numbers, types and materials.
I have no pre war flywheels, either SV or OHV so cannot make any physical observations regarding these.
Also Henri's 708 numbered flywheels are new to me..though it is possible I may have some in the pile and not have noticed them !.The only 'non numbered' types I recall were all cast iron ones, but as I say I feel I need to revisit everything I have in store.
I will try to allocate a day to this task and will note material, model type and any numbering present. I would like to get to the bottom of this subject!!...Ian
email (option): julie@wright52.plus.com