Gearbox shimming

[Jeff t]

Just fitted a new layshaft and gearbox 4 speed, I have shimmed up with a 1.9 shim and the gap was less than 0.04 but still spun freely. Ordered a 1.8mm shim assembled everything including the clutch nut and the gap is still only just 0.04 so the feeler gauge just nips up. The box is really free and I can select all gears by hand. So do i buy a 1.7 shim or will this be ok. I know the recommended is 0.07 but read some stuff that says if it’s free then a smaller gap is ok but Instinct tells me to re-shim but thought I’d get other views. Top end is Imola,

[Tractorman]

If the gap was .04 and you put a thinner shim in but the gap remained the same perhaps the new shim the same thickness as the old one. Some of these aftermarket shims leave a lot to be desired, MB sell excellent shims that are correct.

[Jeff t]

It was a Rimini shim and i compared the two by running my nail over it, the gap is bigger now but not 0.1 bigger, my question Is more would 0.04 be ok or re-shim with a 1.7 and increase the gap to 0.14.

[Tractorman]

If the tolerance is .07 then .04 is not enough.

[Jeff t]

Thanks I’ve ordered the 1.7 shim

[Warkton Tornado No.1]

Just fitted a new layshaft and gearbox 4 speed, I have shimmed up with a 1.9 shim and the gap was less than 0.04 but still spun freely. Ordered a 1.8mm shim assembled everything including the clutch nut and the gap is still only just 0.04 so the feeler gauge just nips up. The box is really free and I can select all gears by hand. So do i buy a 1.7 shim or will this be ok. I know the recommended is 0.07 but read some stuff that says if it’s free then a smaller gap is ok but Instinct tells me to re-shim but thought I’d get other views. Top end is Imola,

Something is amiss if varying the big shims sees no change in axial clearance.... It's almost as if there is only one feeler gauge available.

In any case, 0.04 ( 0.0016") is nearly just half of the recommended minimum. Three thou' (0.003") is the optimum IMHO. Any less & you run the risk of locking up the gearbox & not even your left 'trigger' hand will save you by pulling the clutch There are one or two old scooter racers that can verify what gearbox lock-up is like.

Gearboxes can take a lot of time & patience to set up. Over time, you may find the need to swap layshafts, gearbox end plates (including the bushes pressed in) & amass dozens of shims, some ground flat again after use, ideally measured up & engraved with sizes to make selection easier.

[Jeff t]

Thanks for the advice I am pretty certain there’s nothing a miss had the scooter 20 years just decided to take the Indian box out and fit a casa 4 speed, all assembled and the gap was spot on, however I used a MB selector spring and it was a pig to change gears so decided to strip down and at the same time upgraded the layshaft to a casa one. I have already ordered the smaller shim so I’ll see how that pans out. Oh and the feeler gauge is the smallest I have hence why I refer to it twice.
Cheers

[tim_mason]

my experience with Rimini shims is that you need to measure each one as the size etched on are generally incorrect ,

[Warkton Tornado No.1]

my experience with Rimini shims is that you need to measure each one as the size etched on are generally incorrect ,

EVERYTHING needs checking, whether shims or petrol taps.

I have shims from many sources, but the engraved sizes are often wrong. That includes AFR, MB & SR's.

I automatically measure & engrave the size upon arrival of any shims

[Jeff t]

All sorted now 1.7 shim used, good tip on the measuring although not being used I had a 1.9 shim that measured over 2mm. Also for info the Two Rimini shims are bang on as they are engraved. Thanks

[Fast n Furious]

If you shim to 3 thou (0.075mm) or less, then you may experience difficulty selecting first gear on some boxes "from a standing start" until the gear oil has reached optimum temperature and viscosity.
I find that 5-6 thou (0.125mm - 0.150mm) to work best. Not always easy to do when the stock shims jump in multiple of 4 thou unless you have access to a surface grinder. .Therefore, 4-8 though is the best compromise.
12 thou can sometimes make the gearbox rattle a bit after a few thousand miles.