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MG Midget and Sprite Technical - Finding 7.5 degrees advance

Here's a challenge for the mathematicians on this BB. I want to set the static timing on my 1275 midget before starting it for the first time. The pully does not have the usual notch denoting TDC (to set against the pointers at the bottom of the timing chain case). I do know when the engine is at TDC as I have a pointer at the top of the timing chain case and TDC marked on the pulley (so I came set the timing with my strobe without crawling underneath in future).

My question is this: how far round the diameter of the pulley represents 5 or 7.5 degrees of advance? I presume it is 5/360ths (or 7.5/360ths) of the diameter of the pulley. Is that right and does anyone know what that is in mm. I think this is a standard pulley despite the lack of a notch (which, believe me, I have searched for!).

I ask this because I would like the engine to start approx immediately and I don't want then the engine to be idling on first start up while while I set the dynamic timing with the stobe (bearing mind recent threads on this BB about initial start up and bedding camshafts/followers).

Chris (why didn't I pay attention in maths class) Hasluck
Chris H (1970 Midget 1275)

Chris, to find the length around the circle of the pulley (the arc length as it is technically known) you need to do the following quick sum:

The circumference (the length of the outside of the circle) = Pi x Diameter. Therefore you just need to find a fraction of this. Also, Pi = 3.147 roughly. Therfore the distance around the outside of the pulley is:

3.147 x Pully diameter x No of degrees advance / 360

the diameter (if you don't know is the distance across the pulley).

You could then measure the distance you calculate by wrapping a flexible tape measure round the pulley.

Hope this makes sense and helps.

I also hope this is correct, as a design appraisal engineer it will be embarassing if this is wrong!

Malcolm
M Le Chevalier

Chris,
I did much the same as you with mine, having mounted a pointer on the top of the timing chain cover where it can be seen.
I measured the pulley diameter = D
Circumference is then D X 3.141592. One degree of advance is this divided by 360. So you can measure and mark whatever advance you want, 5, 7.5, 10 ....

Do be sure about you tdc mark, taking measurements before and after tdc and then finding t he mid point. Piston travel close to tdc is minimal and difficult to measure accurately.

Just think a bit about engine rotation and make sure you make advance, not retard marks!

Guy

Guy
Guy

yes, that is correct

divide the pulley diameter by 360, and that is degrees for you.


Norm
Norm Kerr

and if you need clarification search wikipedia for 'circle' as it better explains what the diameter, circumference and arc of a circle are (because they can use diagrams!)
M Le Chevalier

Thanks for the quick responses. In my question I should have said circumference when I said diameter. Apart from that I wasn't too far out but your comments confirmed my thinking (I'd also forgotten about Pi - school was long time ago). TDC was marked by the engine builder (Peter May) so I am sure it is correct.
Chris H (1970 Midget 1275)

WOW
we all replied at once!

":o)

Quick: which direction for an A series: CW or CCW is advance? I was looking for a reference that clearly defined this but don't seem to be able to find it this morning...



Norm
Norm Kerr

edit... yes guy is correct, 3.142 not 3.147, not that you will be able to measure to that accuracy!

Don't get diameter and circumference mixed up, they are different things! :-)

Malcolm
Those that can't do, teach!
M Le Chevalier

The advance mark needs to be to the right of the tdc mark when viewed from the front. Then as the pulley rotates clockwise the mark arrives at the pointer before the tdc mark does.
Guy

I'm a bit of a lazy dude at this
I simply wrap a flexy tape right around the outside of the pulley to get the circumfrence and divide (actually the calculator does it for me) the circ. by 36 and this gives 10 deg. and mark it on the pully around clockwise from the TDC mark. looking from the front of the engine Then you can mark 20 and 30 and the 5's in between If you are marking on the timing cover it will be anti clockwise for your advance markings Willy
William Revit

Norm... the way I'd work this out is:

- from the front, the pulley turns clock-wise when the engine is running

- fix the TDC mark appropriately

- then advance is acw from this

HTH

A
Anthony Cutler

Chris, to actually answer the question you asked:
5.2 or 7.9 mm

assuming you have the standard pulley.
David Smith

"Chris, to actually answer the question you asked: 5.2 or 7.9 mm"

alright smart-arse, I don't happen to have the pulley off an old A-series engine lying around at work to actually measure it! :-p

I am joking btw (I know it's often hard to tell on such things!) :-)

Malcolm
M Le Chevalier

Anthony,
If "acw" means anti-clockwise, are you sure? I had just advised the opposite! Surely a mark on the pulley anti-clockwise from tdc would arrive at the fixed marker later, ie. after tdc?

I thought I had the answer, but now I am confused!

Guy
Guy

Chris,

I scanned a degree wheel and then print it to the size I need to fit the pulley on the motor. Tape it on and you don't have to calculate anything!


Tim Michnay

Very difficult thing to describe.

I assume Ant means you would need to turn the pulley acw to advance, although the mark will be c/w from TDC.
Dave O'Neill 2

Dave,
Thanks, If that is what Ant meant, then we are saying the same thing! Phew!

Guy
Guy

Chris, you are getting much too complicated here, all you need to do is get is somewhere near right to get the engine started, then get the timing light on to get the timing where you want it!
Cheers John
HALL JOHN

John, Chris said in opening post he doesn't want to run engine at idle and strobe it, he wants it to be right enough to break the cam in from initial fire-up, usually this means run at 2000 for 20 mins.

Malcolm I'm working at home today :-)

Guy / Dave / Tim the direction confuses me completely so I'm not going there ;-)
David Smith

>>>>3.147 x Pully diameter x No of degrees advance / 360<<<<


Yeah ... but you didnt account for the thermo expansion from the engine heat up, nor the metal expansion from the molecular structure being streatched from being ran at higher RPMs...seriously, can you trust the data as is...LOL


Prop
Prop

David,
my first remark:
<< Just think a bit about engine rotation and make sure you make advance, not retard marks! >>

Was meant as a jest. But it is rather important!
Guy

I know David, but as you know, none of these engines are the same, and complicated vernier equipment is needed to get it set accurately, It only takes seconds to set it once it's running.
Cheers John
HALL JOHN

Thanks for all the comments and to David Smith for providing the precise answer!! I came to the same numbers, but only after spending 20 minutes struggling to get a tape measure round the pulley on the installed engine before thinking "hey, why don't I measure the one on the engine I took out and which is on the bench" (dooooohhhhh!).

This certainly seemed to spark of an interesting debate.
Chris H (1970 Midget 1275)

This thread was discussed on 25/02/2011

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