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MG Midget and Sprite Technical - starter pinions

hello Chaps...here's today's conundrum...input from wise heads sought..
My '66 1275 is pretty much as nature and MG intended and is driven more or less everyday. I seem to be jinxed with starter motors..having had one where an internal washer failed inside the barrel and more recently another pinion failing as shown in the picture. I have now confused myself totally about the number of teeth that the pinion should have...both 9 and 10 teeth versions are available. My elementary grasp of geometry now kicks in with the thought that the angle between teeth cannot be the same if the ring gear on the pinion has the same diameter, Which should be fitted to an M35G starter motor??
Many thanks to anyone who chips in with a comment/answer....although I am getting very quick at changing crash starters I'd rather spend more time driving ....



David Cox

David

A very good question, to which I don't have a definitive answer, although I'm sure that someone will (hopefully).

However, my own experience is as follows.

On one of my race midgets, I found that if the engine stopped when hot, the starter motor wouldn't turn the engine over, although it was fine when cold.

I swapped out the motor (10 tooth pinion) for a spare that I had, which had 9 teeth and never had the problem again.

Whether that was because the starter was faulty and heat-soak from the hot engine caused it to malfunction, or heat caused the various parts to expand and jam, I don't know.

After that, I only ever used starters with 9-tooth pinions.
Dave O'Neill 2

Thanks for that, Dave. Should have said that I have a nine tooth pinion fitted.
so...let's see what the collective thoughts are...2-0 for 9 tooth at the moment.
This is a query which could help everyone..so let's here from you "out there"..
Dave
David Cox

Don't know about the Sprite/Midget issue
but back on old 1500Cortinas they had a 9 tooth and the starters would jam every now and then and you would either have to wind them out with the square on the end of the starter shaft or put the car in gear and rock it till the starter banged out
If you fitted a 10 tooth pinion they were slightly bigger in diameter to accept the extra tooth and that was enough to stop them locking up
William Revit

I've had that problem with a Mk1V Cortina, I put a pre-engaged starter motor on it in the end. Think I have said this before; I put a new starter ring on the Sprite, with the engine on the bench the starter motor side screw was fouling the lip on the sump so the motor wouldn't fit flat on the bellhousing. It was the correct motor, I had to file the screw down.
L Langley

David Cox,

I can't help with the 9/10 tooth question but have in the past run into a starter jamming and swapping from one to the other sorted it but can't now remember which was the solution. Regarding gear tooth angle I have discussed this with a gear maker in the past and it is complex, IIRC these gears are involute and as the ring gear has a large number of teeth the teeth are nearly a straight taper, the pinion is anything but and is pretty much at the minimum number of teeth so the tooth profile is much more curved and due to being below a certain number of teeth it gets undercut so the tooth is narrower at the bottom than the middle in order to function which compromises strength but is OK if up to the job. Regarding 9/10 teeth engaging with the same ring gear that is done by subtle alterations to the tooth profile to compensate, the tooth profile may not be proper involute for the size but is adapted to work OK and as it only works intermittently is good for the job.
David Billington

thanks, all. Just as I thought: witchcraft meets can of worms. I might just get a 10 tooth pinion and next time the SM fails [as it will....] I will switch over and try the pinion with more teeth.
cheers, Dave
David Cox

quick note to those still interested! A 10-tooth pinion will give a slower cranking speed but transfer a higher torque to the engine. Since the SM has to get all the moving parts of the engine going then more torque is needed. So I'm told. Dave
David Cox

Hmm

"quick note to those still interested! A 10-tooth pinion will give a slower cranking speed but transfer a higher torque to the engine. Since the SM has to get all the moving parts of the engine going then more torque is needed. So I'm told. Dave "

Round the other way I would think, it would be the 9 tooth that delivers more torque / less speed to the flywheel

If the battery / starter are up to scratch the 10 tooth will spin it over faster
William Revit

yep...you're right Willy...meant to write "9-tooth"...

gear reduction speeds and torque transfer operate in reverse directions so to speak...rpm speed gets divided by this GR ratio, torque is multiplied.....should have paid more attention in my physics classes!
Dave
David Cox

Or,by observation, the 9 pin is bound to take more revolutions to travel around a given flywheel.
Bill Bretherton

I have puzzled over this before. There is only one ring gear specified, and the spacing between the starter motor axis and the ring gear is also a fixed dimension. So the diameter of both the 9 tooth and the 10 tooth pinions must be the same as well. The only variable can be in the tooth size which means the 10 tooth pinion must have smaller teeth.

The mesh of the gears must be pretty imprecise to allow for the variation in pinion teeth size.
GuyW

But if there is a design fault and the bendix is not meshing squarely with the ring!
L Langley

seems like we're not the only ones pondering this:

https://www.mmoc.org.uk/Messageboard/viewtopic.php?t=68975



Dave
David Cox

Guy,

It's not that the meshing of the gears is imprecise, the tooth profiles are adjusted to get correct mesh even though one or both pinion gears is not to the normal diameter dictated by the gear tooth DP choice. DP=Diametrical Pitch most likely to be used in inch designs. If you have the pinion diameters to hand and the diameter of the ring gear and its tooth count then could likely work out the DP size and which of the pinions is standard and which is not.
David Billington

Over my head I am afraid, David!
It just seems to me if one pinion, say the 9 tooth one, has teeth which mesh with the tooth pitch of the ring gear, then the other (10 tooth one) cannot also fit accurately as the teeth must be smaller, since the pinion circumference is the same.

No doubt at all that there is some mysterious (to me) geometry involved in the shape of the tooth tips where they first make contact that mitigates this apparent mis-match though!
GuyW

Morning :) You are earlier riser than me :)

So, to answer questions :
- I got the silver module
- My (LMS4) coil is 1.5 Ohm
- My external ballast is 1.5 Ohm

=>Should i just get a 3.0 Ohms coil, keep the module + Optronic "points", and remove the LMS4 + ballast ?
=>Should the new module be a 12volts ?

Ta,
CH Hamon

Cedric,
wrong thread.
Nigel Atkins

Prop incarnate.
Dave O'Neill 2

This thread was discussed between 30/07/2019 and 04/08/2019

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