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I have just started to assemble a SCCR 4-synchro remote gearbox and fallen at the first hurdle.
Third gear will not fit onto the 3rd motion shaft (without some force)
2nd gear has the same problem too.

To get to the bottom of the problem I have tried different roller bearings (from different suppliers) and the fit is just the same so they are eliminated from the investigation.
The shaft is NOS and I don't have another to try but can measure the OD with some accuracy and get it to be 29.61mm.
Problem is I don't have an internal micrometre to measure the ID of the gears.

Can someone advise or who has a spare 3rd motion shaft to measure tell me if mine seems OK so that I can decide where to go next?

the bore of the gears is 32.6mm (give or take) and they should just 'drop' onto the mainshaft (which has a shaft size of 29.5 (give or take) with no force required what so ever. once in place they should spin freely with little of no resistance (bar the friction from the oil)

if your having to force them on, something is wrong, either the gear hole is too small, the mainshaft is too big, or the races in the gear bearings are too fat.

none of which are problems ive ever come across!!!

Thanks Guru.

Just re-measured with a colleague's recently calibrated digital calliper and come up with similar measurements to yours!

Shaft OD 29.48mm, gear ID 32.72 (albeit not the best way of measuring an ID)

This is confusing. As you can see from the image the gear won't go on freely and I've tried several bearings from two different suppliers.

Am I missing something?

are they cheapo bearings, it may be that the rollers are too big, or they are sticking in the plastic cage.


it might be worth fitting the bearing to the shaft and just rubbing your thumb round to see if the bearings are all fully home

ive just tried a brand new one from stock, with a brand new third gear and it dropped on fine

you can spin the gear with one finger

Hi

I have met the same problem before....

And it was from the needle bearings.

As said guru, I had to reuse the original metal caged needles bearings.

It has been impossible to rebuilt the gearbox ( 4 synch remote) with plastic caged bearings. ( On any gears)

I would suggest you should try with original bearings

I hope it was any help.

Glen

Thanks Guys.

The bearing shown in the image came from Somerford and the alternative I tried from Minisport, they appear to be different but both plastic cage type.

Could these be metric sized bearings?

Does anyone know where you can get metal cage bearings?

On a more positive note, first gear runs nicely on its bearing! If I can extract enough torque from my 997 to go from 1st to 4th then who needs 2nd and 3rd anyway?

Maybe pop a roller out of it's plastic cage and measure it, then add the numbers.

Great advice Frogeye, you must have been reading my mind! Just been to do that.

Rollers measure 1.65mm

So 32.72-29.48 = 3.24. Divided by two = 1.62mm in other words I've got a light interference fit!

From what you guys are saying, I'm beginning to suspect that the bearings are metric copies of the originals that I guess would have been imperial.
If the rollers were designed to be 4/64ths that would be 1.5875mm

Any more thoughts?
Anyone know where I can get original bearings?

ive just measured a roller out of a brand new bagged minispares bearing....


1.59 - 1.60mm

measuring a roller out of a 30 year old plastic cased bearing I get the same.

what I said from the start.

cheapo bearings!

Thanks for the help and advice guys.

To close this one off, I tried another (Minispares) bearing (third time lucky?) but to no avail.
I bit the bullet and ordered another NOS shaft (thanks Tom at Minisport) and... success

There is virtually no clearance between these parts by design and I wonder if they were selectively assembled in the factory.
The first NOS shaft I have is probably in tolerance but won't fit my gears. I can hardly detect any difference in diameter between the two shafts but the image below tells the story.

Now that I can get on with the gearbox assembly I'll post more progress pics on my project topic soon.