When it "locks the shafts," high loads are introduced into the gear/shaft/bearing/case network.
 

"My method is much simpler and faster though."  

My method introduces the simplest loads into the gear/bearing/shaft/case network, and they are zero.  I think it is worth doing on our old cars with parts becoming more dear and rare with each day.  I don't see how it is any faster!

"VERY few of us are going to have those spline hex-adapters lying around..."  

We could fix that...  

The reality is that, when assembling or disassembling the mainshaft nuts, any means of locking, jamming, or simultaneous engagement of two gears introduces high static loads into the network of gears, bearings, shafts, and case.  While a few of us have experienced outright breakage, such loads can reduce the life of bearings with rolling elements, and the best practice is to reduce them, or if possible, eliminate them completely.  Just because you can drive the car out of the shop doesn't mean that the service procedure has not reduced the life of the transmission to some degree.

All the "lever moments" are taken into account in my model, and the load tables reflect this.

My first transmission rebuild was on an MG1100 unit that I had transplanted into a Mini, oh, 45 years ago.  It produced serious growling noises, and bits of bearing cage were clinging to the oil plug magnet during an oil change.  I resorted to the usual desperate measures to remove and replace the mainshaft nuts.

one more reason why I like straight cut drop gears




set a spare gear against the first motion gear and the idler, it locks the shafts.


For helicals, I just put the jaws of a large pipe wrench right over the gear teeth.  Start applying twist with a wrench on the nut, the pipe wrench jaws are pulled in, gets a nice grip over a large area at the top of the teeth.  Never broke a tooth.

Reasonable physics lesson, though the nut wasn't particularly tight - I've had one that started to distort the case and had to be cut off...

My method is much simpler and faster though.

How do you lock 1st and 2nd together? 1st and third or forth or 2nd and third or forth......................

Yep, that will happen, have had several red faced clients walk through my doors with wrecked 2nd gears and layshafts when using this combo. 1st and 4th gear is the way to go to prevent this. It's like a gun going off when the broken tooth makes it's exit so I'm told!

I've spoken with Keith and his analysis is absolutely correct... the mind of a Mechanical Engineer!  However, in the real world, I'm more like minimans.  VERY few of us are going to have those spline hex-adapters lying around, and those of us around these things have seen some incredibly 'wonky' repairs done by my British cousins and others... and 'gotten away' with it!  Torque wrenches and special jigs are wonderful, but how about an old leather work glove jammed into the gears and a chisel to torque the big nuts? Just about all of these that have spent time in England have chunks missing from the big nuts, courtesy of the chisel treatment!

I too have disassembled many units that have been put together questionably, to put it mildly!  I have found those nuts so overtightened that I've had to go bypass the regulator on the air compressor(allowing 180 PSI to the gun!!), crank up the 'NASCAR' Snap-On impact gun to max, and STAND on it for several minutes to just barely get them apart.  Never damaged a gear or a bearing knowingly (didn't inspect bearings I was throwing away anyway).  I put this kind of fun video on my YouTube channel for the home mechanic using basic hand tools:

I think I would have phrased that
"Wow, somebody sure had their nuts in a torque".
Entertaining, nonetheless. Thanks for sharing.  (and it gave me something to do while waiting for varnish to dry!)
.

I once fractured a gear tooth using 1st and 2nd locking method.

Wow, that's a mouthful just to torque up a pair of nuts! and I'm sure it's all very good info but in reality I've seen people jamb all sorts of stuff in to the gear clusters to lock the box and I've never seen any damage to the box. for the past 45 years I've engaged two gears and had at it with a very large torque wrench and never had a failure. Well I lie a little because in the early days it was lock two gears and have at it with the air gun! but again still no problems. and I can't really see that the loading to the bearings is a big problem, for a start its spread between both front and rear bearings and the lever moment from the gear teeth are very small, I would imagine that the load under drive would far exceed that of the torque wrench? yes I understand you are talking about a static load to the bearings but I just can't see it as being excessive at 150Ib's at the lock nuts?

In the classic Mini gearbox, the torque applied to remove or replace the first and third motion shaft nuts has to be reacted somewhere. The usual method to immobilize the rotating parts, as recommended by the various official and aftermarket service manuals, is to lock the transmission by engaging multiple gears at the same time. You then have to hold down the transmission case.  The applied 150 ft-lbs or so then produces large internal forces due to the tooth geometry and the lever ratios in the transmission, which in turn produce static forces in the rolling element bearings in the transmission.

Some of these forces are calculated here:

//www.wkeithadamspe.com/uploads/4/7/2/6/4726944/mini_gearbox_static_load_calcs_v05_20160427.pdf

Finding the loads in the first motion shaft bearings proved to be a problem. The roller bearing between the shaft and the flywheel/drop-gear case is not present during service, resulting in an incomplete bearing set, so the network is statically indeterminate. One would have to know more about the lash and elasticity of the various parts to estimate the loads. Additionally, the forces are in directions that the bearings are simply not designed to bear.

It is best to avoid such loads altogether, similar to the practice of pressing a roller element bearing into a bore through the outer race, or onto a shaft through the inner race. Do not apply static loads to rolling element bearings during service!

During assembly, the answer is simple. When reassembly has progressed to the point where it is time to torque the mainshaft nuts to specification, engage fourth gear only. This leaves the first and third motion shafts locked together, but free to rotate in the transmission case. The laygear does not even have to be installed. Hold one nut with a proper socket, and apply the specified torque to the other. Both nuts will be simultaneously drawn to the applied torque, with zero loads introduced to the bearings, and no harm done, with zero bearing life consumed during the assembly process.

Disassembly is a bit different, since the chance is about zero for both nuts to loosen simultaneously. Again, engage fourth gear only. Hold one nut with the proper socket, and loosen from the other. Remove whichever nut, and its corresponding gear, and place a hex-spline adapter, like the one in this topic's photo, onto the open spline. Now you can remove the remaining nut. Once again, the bearings have been exposed to no loads.

Here are four methods to support the "ground" end of the mainshaft during service:

1) A buddy, a beer, and some bruised knuckles
2) A big vise and a spare breaker bar
3) A bench mounted third hand
4) A portable torque trap, admittedly over the top!

//www.wkeithadamspe.com/uploads/4/7/2/6/4726944/mini_gearbox_benign_rnr_method_20160426.pdf

As the parts for our favorite little cars become more rare and more dear, I advocate doing all we can to "do no harm" during service.  My offer stands to any Ohio Mini club member to perform this benign tear-down or torque-up, like the demo at Hal's in April.  Good day!