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Advice on lightening M21 flywheels

Hi
Am after some advice on M20 that i am building for beach racing here in Australia. I have M20 cases with steel M21 crank, 83mm bore, what i would like to do is lighten the crank and i am after some advice/ tips/ help on approx how much weight to remove?, remove it from OD or side of flywheels? etc.
Also will be running goldie scrambles cams, do also have touring cams, head is getting welded up to riccardo style combustion chamber, port work, possibly machining inlet tract out to 30mm (not so sure about this), running 30mm carb, possibly increase bore to 85mm and aiming for 6.5 to 7 compression. This is my 1st sidevalve build so any info would be appreciated.
Cheers Damien.

email (option): wilsondm98@gmail.com

Re: Advice on lightening M21 flywheels

Don't! But like all good advice it will be ignored.
Richard

Re: Advice on lightening M21 flywheels

I would get the flywheels and rod balanced and polished only,when working on inlet tract try to grind out the 90 deg turn up to valve aim for 45 deg also cut valve guide off flush.Fit larger inlet valve with double springs, radius cylinder bore edge on valve side.I used a 389 Amal flowed,300 main jet, when using scramble cams you may have to set valve gaps at 20thou or more depending on overlap otherwise NO COMPRESSION!and set timing at 5/16" to start with. When all done expect the inlet valve to hit the spark plug if you over rev( unless you fitting central plug. Have fun,I did:laughing:

email (option): barryrapley@sky.com

Re: Advice on lightening M21 flywheels

Hi Barry
Thanks for the great tips, have started polishing and looking to balance crank at 60%, was considering moving spark plug, will be doing that for sure now. Will concentrate mainly on head work, keep mods simple, will be racing against SV Harleys, not out to win but i do want to give them a hard time and mainly i just want to have some fun doing it. Thanks again, Cheers mate.

email (option): wilsondm98@gmail.com

Re: Advice on lightening M21 flywheels

Hi Damien...I have just returned from a holiday today and have noted your email to which I will reply over the next couple of days...If you are tuning your M20 for serious speed lightening the flywheels is an absolute requirement as the engine will not 'rev out' otherwise...This is the key to tuning all BSA M and B Series post war engines...Repositioning the plug is a good idea...Read what you can on tuning side valve cars and Harley Davidson KRs for general information but remember you can't just copy one specific engine spec. to a different engine...It all has to be considered and often adapted for particular applications...A 60% balance factor will be about right for the modified crank/piston assembly...Ian

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

Re: Advice on lightening M21 flywheels

I ground out the inlet valve guide flush with the inlet tract, radiused the top of the barrel to get better flow into the bore and had the head cast iron welded and then polished to the same contours as the SWD Big 4 as both machines run an 82mm bore. Run with an AMAL 389 Monoblock. A number of Gold Star cams were tried previously but no perceptible improvement in performance. Double valve springs used to both valves - I think the C.10 springs will sit easily inside the M.20 /21 springs.

Have not lightened the flywheels. Have run with a set of BSA MCR gearbox internals, lucas racing mag.

A lot of relatively inexpensive opportunities to improve the performance of what was considered to be grey porridge compared to fettling something exotic out of which you may not get much in the way of a reward.

Having said that didn't Harley Davidson say you cannot beat cubes?

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