Scooterotica

I've been trying to finish an Italian gp150 engine. It was rebuilt before I acquired it with one of those "no oil" stickers on the chain case. I don't think it was ever run since. I put the washer and cone on the layshaft followed by the hub but when the hub nut gets tight on the layshaft (it's a closed nut if that makes sense) it's nowhere near tightening on the hub. At least not without a few washers. I'm presuming the hub belongs to the engine as it came in the box of bits with it but I can't be sure. Compared to the picture in Sticky's book, my layshaft seems longer at the hub end. Also the engine has a large gap between the gearbox backplate and first gear. The bearing isn't near flush with the backplate. Does anyone know what the problem might be?

Photos help a lot but from the sound of it this motor is missing the main shim? Without more information everyone is just guessing.


Rear hub could be the wrong hub/cone combination.

the "closed" nut could be bottoming out. probably worth trying another nut.
the cone may be the wrong angle and just not catching.

the layshaft should pull tight against the hub bearing, so I wouldn't have thought the issue is inside the engine.

Grumpy225 wrote:Photos help a lot but from the sound of it this motor is missing the main shim? Without more information everyone is just guessing.


Rear hub could be the wrong hub/cone combination.

Thanks Grumpy225 for the quick reply. I'll strip the engine tonight and check if it's missing the main shim. This is my first journey into the world of lambretta, if it was a vespa engine I'd know my way around it. It seems to me that the layshaft has travelled too far into the hub bearing and has left a big gap in the backplate but as I say, its the first lambretta engine I've worked on. Should there be a shim between the hub bearing and layshaft?

mick1 wrote:the "closed" nut could be bottoming out. probably worth trying another nut.
the cone may be the wrong angle and just not catching.

the layshaft should pull tight against the hub bearing, so I wouldn't have thought the issue is inside the engine.

Thank you mick1 for your reply. Your right about the closed nut bottoming out. As I said above, its as if the layshaft has travelled too far through the bearing and as a result has provided too much length for the closed hub nut to tighten against the hub. An open nut would work but I'd still have the issue with the layshaft being too far away from the backplate, not sitting fully into the backplate bearing and allowing loose gear movement. Is it common to find wrongly machined layshafts?

Hi . When the hub is fitted i assume the whole of the threads arnt protruding out through the hub ?

paul d wrote: It seems to me that the layshaft has travelled too far into the hub bearing and has left a big gap in the backplate but as I say, its the first lambretta engine I've worked on. Should there be a shim between the hub bearing and layshaft?

No, the lay shaft can only go so far into the bearing. Once you open it up you will see exactly what I mean. There isn't normally a shim between the layshaft and the bearing.

Stickys book spells it all out pretty nice.




The rear main bearing sits where the O ring groove is.

Photo from scootopia.co.uk

It seems a combination of a wrong hub nut followed by a dash of panic prevented clear thought! A friend called over tonight with an old gp150 engine just to compare them. Both layshafts were the same length from the casing and using the hub nut from his engine, I managed to secure the hub on mine. Relief! There's .5mm play between the gearbox shim and backplate and the layshaft does seem to be seated slightly deeper in the backplate than it should be but I've plenty of shims to make up the slack. Hopefully whoever rebuilt the engine maybe didn't set the backplate properly in the casing. Thanks Grumpy225, bazza3004 and mick1 for the help.

I like to break off the dome on the rear hub nut so that I know it's not bottoming out. On the cheap Indian ones you can use a punch from inside to knock out the dome as it must be in 2 parts.

Good thinking corrado. I bought my lambretta completely stripped with the hub in a box with various clips and nuts and all sorts. I think the hub nut that came with it has the capped part hammered in a bit. That's why it wouldn't tighten on the hub and had me scratching my head. Wow the lammer world is a complicated one!