I don't like to make a personal comment usually but I have to say it appears Panic is basing his comments largely on theory and not on practical experience...I totally disagree with much of what he says regarding the ACTUAL effects of crankshaft weight reduction in the M and B Series BSA engines. However, these misconceptions are not uncommon, one of the main worries my customers for tuned engines have is around potential effects on the engines broader 'characteristics'...
The postwar flywheels and conrod will fit OK into a wartime engine being dimensionally identical.
If you are contemplating removing material to improve performance check that the flywheels you have are steel, not cast iron. Both materials were used at different times. Steel wheels have a part number commencing with a '6'...iron wheels with a '7'...66-635(steel)..66-770(iron) for example....
Removing large amounts of weight from M20 flywheels is not really necessary unless you are radically altering the remainder of the engine...
In B Series engines particularly it is absolutely the key step to achieving substantial performance gains which won't be achieved without it...
An M20 flywheel set weighs 24lbs so taking a pound or two off it will help with pick up from a standstill but still leaves plenty of material to maintain flywheel effects at entirely practical levels and not lose any of the essential characteristics of the engine....Removing the weight largely from the flanks will help to maintain flywheel effect..Rebalancing will be necessary after the work is done....Ian
email (option): ian@wright52.plus.com
Ron...Cast iron is less able to take the loads exerted in those conditions when the cross sectional area is reduced and they are more prone to cracking.
It is also not unheard of for them to 'burst' at high revs due to centrifugal loadings...Cast iron was widely used for single cylinder flywheels as it was a cheaper material than steel and suited to 'cooking' engine applications.
Tuners of singles in the past, and no doubt currently in cases where alternatives are not available, turned the iron wheels right down and then shrank a steel ring onto them to stop them bursting under these high centrifugal loadings....
As a general rule it is wise to avoid cast iron flywheels if extensive lightening is planned. A more moderate reduction doesn't usually result in any problems though...
Darren...Apart from the flywheels you need to concentrate on the porting and internal head modifications. There have been numerous posts here on the subject so I would try searching the forum first...
If you do machine the crank and are having it rebalanced you will need to tell the company doing it that it should have a 60% 'balance factor'..
I don't recall your email....I'll go back through my inbox though, to make sure I haven't looked at it and then forgotten to do anything
....Ian
email (option): ian@wright52.plus.com
They probably are Ron...Like the M20 both materials were used at different times (whereas all B31/33 OHV flywheels are steel).
Some 'fleet users' and the war department specified steel flywheels and later engines (M20/21/M33/B33) used the 'unified' steel forgings part no. 66-635 which were machined with different crank throws to suit the application required....
However, at least in my experience, the M21 flywheels turn up far more frequently in cast iron than steel...I'll take a look to see what sprockets I have at the moment and give you a ring later....Ian
email (option): ian@wright52.plus.com
I'm not really prepared to describe engine specs in detail on a public forum whilst tuning engines is still part of my living...However, the effects you describe as a result of crankshaft weight reduction, such as difficult starting due to lack of inertia and clutch drag killing the motor are more likely to be apparent in a 4 stroke 450 Honda motocrosser or a KTM, both of which have virtually no flywheels, than they are in a BSA which has had its very heavy flywheels reduced in weight...
You mention a 10% weight reduction...That is nowhere near the amount of weight I remove from M20 flywheels when I machine them down to fit into tuned OHV motors as 'stroker' flywheels....
These motors will start first kick without any drama and will still tick over at 1000rpm (measured on an electronic rev counter)...Obviously they will not 'drive at slogging pace' in a high state of tune but surely that is not what you are after when you tune an engine?
In a sidevalve or less tuned OHV motor where you are seeking less effect and just want to 'pep it up' a little a 10% weight reduction and some mild port and headwork will achieve that without adversely affecting 'rideability'...Don't forget you would still have over 21lbs of metal to spin up after a 10% weight reduction.....
The 'misconception' that is common is that if you tune a single it will become 'peaky', lose all of its lower end and mid range abilities and become difficult to live with...This is simply not true at all in the case of moderately tuned examples and not too evident even when in a more highly tuned state...Ian
email (option): ian@wright52.plus.com
I've no idea what NIH disorder is...but I've a feeling it's not a compliment and it relates to the fact I don't know what I'm talking about on this subject...but you do...
...
If only general theory and practice fitted together so neatly....
Unfortunately, in the real world they frequently don't, as any development engineer will confirm....
I'm afraid in this case your conclusions regarding the effects of applying those theories to these engines are simply not correct.
I have built scores of these engines in multiple formats and specifications over a number of decades...and I know what they do..Ian
email (option): ian@wright52.plus.com