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Hi all,

I’ve been thinking a lot about primary gears recently, as I need a 1275 pre A+ one (24 teeth), but only have smallbore pre A+ ones in my stock of parts. As far as I can think of it, the only difference is the size of the front bushing, which is in smallbore primary gears the same size as the rear/tophat bushing and in 1275 primary gears somewhat larger.

The question I’ve got in my mind now is: Can you take one of the smallbore gears and machine it to fit the larger bushing for the bigbore engines?
I know how to do the work holding and how to bore it out so that the bore stays concentric. But I reckon that the material of the primary gear is hardened and wonder how good or bad it is machinable?

Has someone here machined one in the past and can give me a hint to what tool is best to do the boring? What insert type works? Carbide? Or hard turning with PCD or CBN? Or does it need internal cylindrical grinding?

Cheers, Mike

Back in the late 80’s when I was a poor student I installed a 1275 A+ Metro block into my 1979 Mini 1000 and came across the same mismatch problem.

I ground out the soft bronze / diva lining from the 998’s primary gear using a hand file, and assembled it straight back onto the 1275 crankshaft.

The rear edge of the crank where the shoulder is for the primary gear thrust spacer didn’t quite align with the step inside the 998 primary gear. But the endfloat was close enough to work, and I ran it like that for well over a year.

Yeah, it was steel on steel, but both are hardened surfaces and apart for a little bluing from friction it really wasn’t a big deal. The clutch didn’t judder or show any signs of a problem engaging/disengaging.

Would I do that today? Hell no... but it did work

I have been routinely boring out the small bore Primary Gears to convert them to 1275 types, since the Deva Bushes became NLA.

It's fairly easy to do, you'll need to clock up the Gear in a 4 Jaw chuck to get it on center, then bore away. I use Carbide tipped tools with a bigger radius on the nose, just be aware that these tools don't like light cuts and that the gear will get fairly warm while doing this, so be sure to let it cool to room temp again before taking measurements.

Finished size for the bush is 1.6550 - 1.6555"

I have been doing it for years, a fairly simple process, if you have the ability and access to a decent lathe. Make an arbour the grabs/locates on the seal surface of the gear rather than the splines. Then you can use a large 3 jaw chuck rather than screwing around with a 4 jaw.

I have wondered about this for a long time. It seems our antipodean friends are ahead of us. I have lots of A+ 1000 primaries but a complete lack 1275 ones. Thanks for the advice.