Hello all,

At the past, I remember that there was a discussion about “How many degrees are 7/16” BTDC”…
What was the conclusion? About 40°?

I am going to install a POWERDYNAMO magneto W/ alternator
(Don’t worry, it not an WM20, but a 1960 M21 W/ a modified chassis and a 712cc motor)
And there are various combination of switches to determine the auto advance pattern and rate.
There are 2 switch combinations applicable for 1 cylinder bikes, as shown In the table in the middle of this webpage at link below.

http://www.powerdynamo.biz/eng/systems/7197/719710inst.php

One option starts the advance at 1,250 rpm, 4° BTDC and reaches 40° BTDC at full advance- 2,500 rpm (36° total degrees advance)
Second option- start of advance is at 1,250 rpm, 2° BTDC and reaches 40° BTDC at full advance- 2,500 rpm (42° total degrees advance)
*I believe that the last figure of “42° total degrees advance” is an error, and should be 38°*

Any thoughts?

Thanks!
Noam.

email (option): noam10at gmail.com

Noam
I did some outline drawings on Autocad using the M20 stroke and conrod length.I found that 7/16" equates to 37 deg. (see attached)
I used a powerdynamo system on my Morini. It worked well, Just varnish the bare metal otherwise it will rust.
Let me know how it works.
Peter

email (option): pvlietstra at gmail.com

Better drawing

email (option): pvlietstra at gmail.com

Excellent Peter! Now can you do it again for Noam's M21 stroke Ron

email (option): ronpier@talk21.com

I'd also be interested in the degree value for the M21 crank.
Cheers
Pete

Right Ron, we are talking about an M21 here, not an M20!!!
I don't know the conrod length though. Can anyone conform it? See sketch attached. Am I correct to assume overall compression height is the same as the M20?

email (option): pvlietstra at gmail.com

M20 and M21 con rods are identical. So is the bore. The flywheels give the extra stroke and the piston gudgeon pin to crown hight is also different. Ron

email (option): ronpier@talk21.com

Right again Ron, checked the dragonfly catalog and confirm. Conrod length is 8.25", which equates to 209.55, not 209.8 as per my sketch.

email (option): pvlietstra at gmail.com

email (option): pvlietstra at gmail.com

Interesting!!
Any chance you could do a calculation for the XB33 with 88mm stroke and 187.3mm rod length
Keith

email (option): keithatkinson@hotmail.com

email (option): pvlietstra at gmail.com

Hmm...

Now this has got me thinking. Should I then on my M21 use a different ignition setting then the standard recommended for a M20 over in the tech-section?

/s

It's the same 7/16" (11.11mm) Simon.

Ron

email (option): ronpier@talk21.com

the dimensions which effect the crank-angle to 'distance from tdc' value, is the con-rod length and stroke - nothing else.

I will try and find my distance/angle software and post it.

Ok,

Thanx Ron!
Makes sense Nig!
This is the best place on all of internet!

/s

Note for the B33 the conrod length was reduced by 1/2" in 1954...From 7 3/8" to 6 7/8" which will affect the calculation....Ian

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

Thanks a lot Peter !

That is very interesting.

One question though:

The Stroke for the M20 is 94mm
The stroke for the M21 is 112mm
The difference is 18mm, and that means that the bigend on the M21
Is 9mm further away from center if compared to the M20,
But when piston is at 7/16 BTDC,
I see advance of 37deg', and only 33 deg' for the M21.

If the bigend is further out, doesn't it meant that the flywheel
Will be pushed more for the same stroke,
And therefore the advance in deg' would be grater for the M21?

Thanks,
Noam.

email (option): noam10 at gmail dot com

i use free software - earlier version.

Under the TOOL heading, the early version had a very useful 'crank-angle to piston position' and 'piston position to crank-angle' converter.

The latest version has a lot of litter, but it may still be there - if you send me a email i think i can send you the earlier version.

Also gives useful (for exhausts) cone development tools - print the development out on paper - stik it to the metal and cut out - used it for years on 2T exhausts.

Just found what you want on the above website under
you can download a zip. file.

Thanks nigP.

My email is noam10@gmail.com.

Sorry, I miss the address of the website you was referring to...

Cheers,
Noam.

email (option): noam10 at gmail dot com

No idea why internet address not shown?

Try #2; www.iwt.com.au

At the top - INFORMATION - then - DOWNLOADS - then, the 1st option - download as a zip.

I will send zip file direct to you as well.

Thanks nigP

Noam.

email (option): noam10atgmail.com

Hello Peter,

I calculated myself, not with exact numbers, but roughly,
To see again regarding the advance, and you are correct.
My theory was wrong.
For a fulcrum closer to the middle, the wheel will travel more
For the same stroke.

Best regards,
Noam.

email (option): noam10atgmail.com

Hi nigP

That software package is interesting. Like everything, when you start to think more you realise there's always a little more, like offset gudgeon pins, I'd forgotten all about those.

Seemed ok with the current version - but I had no prior expectation - here a copy and paste of the table I made, so M20 dimensions have 7/16 as about 36.6 Deg.

Crank Angle/Piston Displacement Values
Engine: M20
Stroke (mm): 94
Connecting Rod Length (mm): 209.8
Gudgeon Pin Offset (mm): 0
Angle Displacement
(deg) (mm)
35 10.24
35.1 10.29
35.2 10.35
35.25 10.38
35.3 10.41
35.4 10.46
35.5 10.52
35.6 10.58
35.7 10.63
35.75 10.66
35.8 10.69
35.9 10.75
36 10.80
36.1 10.86
36.2 10.92
36.25 10.95
36.3 10.97
36.4 11.03
36.5 11.09
36.6 11.15
36.7 11.21
36.75 11.23
36.8 11.26
36.9 11.32
37 11.38
37.1 11.44
37.2 11.50
37.25 11.53
37.3 11.55
37.4 11.61
37.5 11.67
37.6 11.73
37.7 11.79
37.75 11.82
37.8 11.85
37.9 11.91
38 11.97
38.1 12.03
38.2 12.09
38.25 12.12
38.3 12.15
38.4 12.21
38.5 12.27
38.6 12.33
38.7 12.39
38.75 12.42
38.8 12.45
38.9 12.51
39 12.57

What fun :-) now is there a use for the expansion chamber software on the same website.....

email (option): dickie.bobbie@hotmail.co.uk

This program is very good. http://www.torqsoft.net/piston-position.htm
Keith A

email (option): keithatkinson@hotmail.com

Hi Keith

It is, and again it has a 2-stroke theme, the site has all sorts of interesting areas.

Thanks

Richard

email (option): dickie.bobbie@hotmail.co.uk

Thanks everybody.

Cheers,
Noam.

email (option): noam10"at"gmail.com

somewhere in the expansion pipe calculations is a projection for a cone - i have used this many times for a Spanny-box's and very recently for a reverse cone mega for a B50.

Also useful for Silencers - only thing to watch for is, if the silencer has a flattened cross-section (think Brookland can and/or Velocette)you will need to trim the joint a bit - but the basic projection is good enough to work from.

a Brookland-Can i made a couple of years ago, i cut the projection out in thin card (on a stika machine) - trimmed it, taped it together with masking tape and offered it up to the bike (a 30's BSA sloper) and was able to mark out a cutaway for the pillion foot-rest and the mounting method.