A gear question.

[Dave Plowman (News)]

One for the engineers here.

The original Lucas starter motor on my old Rover has a 9 tooth pinion measuring 25mm diameter, top of tooth to top of tooth.

Have bought an aftermarket replacement.

That is clearly marked as being for the correct make and model, but has an

11 tooth pinion 29mm diameter. And a note saying that improves the performance. Ie, not to worry about it being different.

Not fitted the new starter yet, but curious how different pinions could mate OK with the same ring gear. The drive in all the motors is central - no offset to the centre of the pinion possible.

It's confusing my little grey cells. ;-)

[Tim+]

Whilst the drive may be central isn?t there still the possibility that the axis of the new motor is 2mm further away from the flywheel axis? If not, are the teeth on the flywheel and starter pinion deep enough to accommodate meshing more deeply (by 2mm)?

Otherwise, I?m stumped too. ;-)

Tim

[Dave Plowman (News)]

No. It is concentric in the fitting ring which locates in the hole in the engine. They also give instructions on how to alter the alignment of the motor unit to the fitting plate if needed - say for a kit car using that engine.

I suppose it must be that. Obviously the two won't be in as tight mesh as in a gearbox, etc.

Quite. Doing my head in. ;-)

[newshound]

At first I agreed with the "spacing" idea. But, normally the bell-housing has a circular seating for the motor, and the matching flange on the motor is usually co-axial with the bearings, although of course it does not have to be. So, I'd start by looking to see whether or not they look coaxial.

One of the clever things about the involute profile is that it still meshes OK even if actual distance between centres is not quite right (although you get a bit more sliding and hence wear).

So you can "squeeze in" extra teeth on the same centres, obviously at least one set has to be a little bit thinner to avoid interference.

I'm not familiar enough with gear design to know whether you could get away with it with this 22% difference, which sounds a lot. But, starter gears are typically pretty sloppy to accomodate debris, they don't do very much "mileage", and they still work after a lot of wear.

My guess is that the aftermarket starter has thinner teeth. They could easily get away with that in "strength" terms by using a slightly higher spec and therefore stronger material.

[Dave Liquorice]

This is a virtual gear simulator, might help?

[newshound]

As an afterthought, the increased diameter of the new gear rather suggests that the motor axis must be moved, otherwise the new gear would surely "bottom" in the starter ring.

[Tricky Dicky]

Is one of the holes in the starter motor elongated as that would give you a bit of wiggle room to mesh the gear correctly?

Richard

[Richard]

[newshound]

It's not about the bolt holes, there's a fairly big "separating force" between the gears when the starter operates, so the starter motor body needs to be clamped securely to the bell-housing to prevent movement. Hence a flange with a spigot.

[newshound]

Excellent link, that explains everything!

[Dave Plowman (News)]

The mounting has a circular raised part with locates accurately in a same sized machined hole in the engine. So all the fixing bolts do is prevent fore and aft movement - they don't locate it side to side. And the starter shaft is concentric to the entire mounting plate. The mounting plate is made by the UK firm to adapt the Denso starter to an engine they don't supply one for themselves. Like lots of classic cars. Basically the same starter motor with a custom made mounting plate and the correct drive pinion can fit a vast range of vehicles, old and new.

Yes - the teeth have a thinner profile. Other difference is the 'far' end of the pinion has a flange which is part of the teeth. Which I suppose would strengthen them.

Other thing is more teeth means the engine spins faster for a given motor speed. Odd, since it is already a gear reduction type.

[Dave Plowman (News)]

Thanks, Richard. All that does is confuse me. A minor difference I'd likely not have noticed. A different number of teeth and a larger diameter difficult to miss. And there is no offset on this unit.

[harry]

The teeth must be the same size in order to mesh. So, the gear must be bigger. So, the centre of the motor must be further way from the axis of the crankshaft. Should be easy enough to check.

[newshound]

But you mentioned a "mounting plate" in your other post. Seems likely in this case that the "offset" is there.

[Michael Chare]

If the replacement fits on the car and can start it then the pinion must be further from the crankshaft axis by 2mm. You could measure the distance between the centre of the pinion and the mounting bolt holes on both motors. They must be different though as the difference is small it will be difficult to measure accurately.

[nightjar]

It is not impossible for two gears to have the same effective diameter (measured where the gears actually drive each other) and a different major diameter. The real test will be to fit it and see if it works. :-)

[Dave Plowman (News)]

Keep on saying, no. According to the makers data they simply changed the pinion. Nothing else.

[Dave Plowman (News)]

Obviously if you increase the number of teeth you'd need to increase the diameter too - to keep the distance between the teeth the same, where they touch. ISTR the ring gear teeth don't have the same profile anyway. So provided they mesh and the tips don't bottom in the valleys it might just work. We're not talking about the sort of super accurate gears you'd want inside a gearbox.

[Roger Hayter]

Have you acttually checked for offset between the pinion and locating ring at various radial points with a vernier? It wouldn't have to be very much.

[Fredxx]

A different number of teeth implies the centre of the starter gear has moved.

Is there a way you can compare the two wrt to mounting holes, or an appropriate point?

If the centre is further away from the flywheel by 4-5mm I would say all is ok.

4mm in 25 is ~ 1/6, which is near enough to 2 teeth in 11.