I've been looking around on Ebay (as you do):) and came across someone selling BSA WM20 Lightened Mag Pinion's plus another listing from the same seller Lightened timing gears .

I'm curious to know what advantage it is ,if any , and what effect this would have on the performance of the bike . Here's a link the the Ebay listing for the pinion .Sorry you'll have to copy and paste ............

https://www.ebay.co.uk/itm/BSA-B34-B31-B33-M33-MAGDYNO-PINION-LIGHTENED-M20-M21-ARIEL-NORTON/383334168466?ssPageName=STRK%3AMEBIDX%3AIT&

Dave

Gold Stars and other tuned engines would have these fitted along with lightened cranks, lightened rocker arms, clutches etc...

The purpose is to reduce the inertia created by the weight of the components as they are being accelerated from standstill when the throttle is opened...
The result is a quicker 'pick up' of engine speed/revs. and consequently quicker acceleration as the engine gets into the 'power band' more rapidly...
As with many engine tuning mods they don't, and aren't intended to, work in isolation but as part of a total tuning package...

I would imagine there would be no tangible effect on the performance of a standard M20 engine, though it would look trick when the timing cover's off! ... Ian

email (option): ian@wright52.plus.com

Thanks Ian , I was curious as to why and now can see through your explanation relating to an overall engine tuning package .I never thought of that, yet this is what the engine tuner does to my lads 2 stroke racing engines lightening , matching casings, polishing gears etc , should have realised ,I've alot to learn about these BSA's .

Thanks

Dave

Never seen a lightened BSA - Gold Star Mag pinion - the late ones I have seen had a peg in them which engaged with a timed breather. However, the idler pinion below the mag pinion was lightened by BSA with a number of holes being drilled in it and it might have been marginally thinner than that used on an M.20. Eddie Dow used to produce a replacement for a Gold idler pinion in alloy which reduced the weight even more.

Here's some parts I prepared for a previous tuned engine...As Kieth notes the Goldie mag pinions weren't lightened on standard engines...I did some work on an ex works factory engine and it was all done...Also barrel shaped solid pushrods to cut down on flexing....Ian

email (option): ian@wright52.plus.com

I note the lightened timing cover plate - the M.20 timing cover plate is quite a thick piece of steel carrying the bush and the cam spindles so I assume you lightened this. The late Gold Star timing cover plate that I have uses a much thinner sheet of steel with raised sections around the bush and cam spindles and the lightening holes I think are circular.

I'd like to see a photo of that...I haven't come across one in that format...The lightened timing plate (which I cut by hand) and the other components were part of a project to lighten the whole bike...Note the drilled through bolts and lightened bolt heads!...Ian

email (option): ian@wright52.plus.com

It will be a while before I get in the garage to get a photo of the Gold Star timing cover plate. I am surprised to see the standard sump plate cover. I fitted the finned alloy one complete with magnet that Eddie Dow used to sell to all my BSA singles.

And there is another thing to take into account.
Weight damps out vibrations.
To stop handlebars shaking violently you pack lead into the ends of the bars.
A lot of modern bikes have steel weights fitted as standard.

So when you lighten an engine, you can also make it a lot more shakey .
Careless removal of weight an also leave the engine grossely out of balance and render the bike unrideable.
Racres ill put up with this but on a street bike it can be fatal.

Side valves and in particular side valve singles are very much momentum engines.
They use the energy stored in all those parts to smooth out the power delivery. much like a pottery kick wheel.
Drilling out the cams used to be a "popular" mod down here and when taken to excess made the engine almost impossible to start.

email (option): bsansw1@tpg.com.au

Handlebar end weights alter the resonant frequency of the handlebars rather than actually 'damping out' vibration purely through the addition of more weight...The vibration will still be present but at a different, less annoying and more practical frequency..The resonant frequency itself is a function of the bars construction rather than any out of balance forces..

Reciprocating parts within the engine/ primary drive which are symmetrical (ie. do not have out of balance weight) are in balance and remain so if weight is removed in such a way that their symmetry is not affected..So drilling the gears radially and/or reducing their width evenly does not affect their balance...Likewise with removing mass from the clutch assembly...

This does not apply to the cams in the single cylinder BSA engines we are discussing as the cam lobes are out of balance anyway to a degree...Alterations to the weights of the crankshaft flywheels and the piston beyond certain limits, will necessitate rebalancing of the crank assembly...Material should be removed as evenly as possible from rocker arms and the arms themselves should be matched to each other for total weight after lightening...Timing gear plates (or any other component) should not be lightened to a point that affects the structural function of the part...Ian

email (option): ian@wright52.plus.com