LCGB Forums

I have an Indian GP 'gearbox'. It's a robust setup but I don't like the big gap between 2nd and 3rd gear.

I've been told that it is ok to replace the 38-tooth third gear with a 39-tooth third gear from an Italian gearbox. Without the need to swap any other parts for example the gear cluster.

Can someone with more knowledge then me please confirm?

Thanks

Hi.

I haven't heard of any interchange-ability but that is a fantastic question!

It made me grab my bible (Sticky’s) & check the data. Certainly, the OD’s are within a nats. So, that is a very helpful indicator but not absolute proof.

However, if the PCD’s of the loose gears (38 or 39) are identical & the PCD’s of the cluster third gears (16 or 15) are identical, then, presuming that the pressure angle of the gear faces are identical (which I would bet they are) they would be interchangeable.

Perhaps somebody else will add to this response, but if they don’t, I will be more scientific & check further, but that will take some time.

Best Regards

watching with interest, a question I had asked myself.
Might be linked, at the Euro there were several DRT "Special gears" as raffle prizes appeared to be single loose gears.
I never found out what they were but maybe the missing link.

http://www.drtdenis.com/news/lambretta.pdf
sort of answered my own question.

The origin of the question is a brief conversation I had with a well-known Lambretta engine tuner who said he had a supply of loose gears for this purpose. He is a hard guy to get hold of, doesn't retail parts and in any case it would be good to corroborate that a simple loose gear swap will work.

Derek, you might be onto the source - I have emailed an enquiry to DRT (in English!) - we'll see if they reply.

Perhaps there's a UK-based dealer who can let us know more...

Hi Warkton Tornado, did you get anywhere with your research on this?!

No reply yet from DRT re their loose gears.

I checked another 'bible' - Dave Webster's Tuning Manual - he doesn't seem to rule this gear swap either in or out.

I can't believe no one else has tried this swap as it's such an obvious one to do to fix the 3rd gear issue...

Hi.

Sorry for the delay in response.

I really need to measure up to verify the theory, but I'm still optimistic.

Give me a day or two....

Top Man )

Ive got all the bits try this on the bench.
if I get a chance ill try this week.

Sorry for the delay in responding, but I needed a bit of space to think this through!

As far as I know, nobody has ever stated how the gears in a Lambretta are derived, as in the tooth form & size. It would be interesting to know, as although I am by no means an expert, I cannot fathom out the premise on which the specification of the gears is based.

Initially, I thought that they were based upon the modular concept. Some are very close to 2.5 MOD. But others are massively different.

So, for now, I give up on that part of the “Understanding Lambretta Gearboxes”

I wanted to give an even more scientific response, but cannot, as I don’t know the Innocenti theory.

Back to the question, my initial excitement that Stuart-L asked:

“I have an Indian GP 'gearbox'. It's a robust setup but I don't like the big gap between 2nd and 3rd gear.

I've been told that it is ok to replace the 38-tooth third gear with a 39-tooth third gear from an Italian gearbox”

Unfortunately, you cannot do so. Whatever the PCD’s for Italian or Indian are (& they must be very similar) the meshing is specific: 39 with 15 & 38 with 16. The individual pitches (thickness) of the teeth are different.

What threw me was that the outside diameters are the same, or similar.

Logically, SIL should have decreased the size of third gear thus allowing the cluster third to be bigger.

I hope this helps, even though it’s not the response I would have liked.

I use a GP150 Indian Vijai super gearbox and I dont find 2nd to 3rd a problem.The first 3 gears are exactly the same as the Indian GP200,the 4th gear is taller ( 19x35 as opposed to the GP200 which has 18x36 ) and gives more speed for less revs.
Paul

The thing is on the Italian GP gearbox the gear cluster has a different number of teeth to the Indian cluster, I can`t see the Italian 3rd gear meshing with the Indian cluster

be careful with your mix n' matches maties, as the Indian 200 gearbox comes in 2 variants , they have the same teeth on all loose gears & xmas tree, but there's a difference in the o/dia of the 1st (kickstart) gears & therefore a difference in the xmas trees. I haven't found anything wrote down anywhere regarding this but Lambretts Restore of Donny pointed this out to me last year when I was struggling to get a gear match.

Thanks for all the feedback lads, there's some good knowledge here.

So it's clear that interchanging the Innocenti and SIL loose gears won't work.

Which leaves the rumour from more than one source that there are newly made 'special' loose gears on the market. If anyone has more info on these parts, it would be good to hear.

Meanwhile I will check back with the tuner I had the initial discussion with and report back

Theoretically, a 'bastard' gear could be made with the desired number of teeth, in your case, 39.

I'm not sure that is the best gearbox to alter, though, unless they are plentiful, free/cheap & strong.

Good Luck.