Horses for Courses,,,,,,,, But overall I agree with T20Racer,
Biggest REAL issue in my experience with many rebuilds of British engines, (Not many on A10s admittedly) is that the process of "Blueprinting" is the most important, followed by sensible time change parts replacements with basis of metallurgy and physics.
Blueprinting is the process of checking every surface and dimension and optimizing it to the best possible specs. Not easy, but in my experience (And I am certified in Aerospace inspection and repair (FAA-A&P as well as multiple ratings in NDT-Non Destructive testing) is that many of these old bikes were built to rather loose and sloppy tolerances. For their day they were pretty good and overall I think 1950s BSA stuff was some of the best on the market. But lets be honest. Every manufacturer builds things to a cost, not an engineering/machining ideal. Secondly after 70 some years, dimensions change, especially aluminum castings. In the BSA engines I have checked I have found significant dimensional changes that I am certain did not leave the factory with. But I have also found some really sloppy work as well that was "Good enough to go out the door" and many that WERE NOT good enough to be installed on a bike.
**Norton was probably the worst offender in that dept, and knowingly shipped junk that had to rectified at the dealer level, But all Brit manufacturers were guilty at times of this.
Something that Edward Turner begrudgingly acknowledged after returning from a trip to Japan.
But I wholehearted agree on aged rods and other bits. I bring it up often but in Stan Shentons book "Triumph Speed tuning", Triumphs engineers stated that the service life for a Triumph connecting rod was ONE,, thats 1,.... 500 mile race. Its simply physics that a alloy rod has a limited service life, Math is Brutal!
As to the specifics of a bush vs bearing, I have come to a middle of the road approach. I agree that a low stressed engine will run fine on a bushing if tolerances are kept in line. I also talked to a lot of racers who spent decades thrashing these old engines on their insights. I believe that the bearing conversion done properly is a sound modification.
Triumph ran the early 500 unit twins on bush but they were not without failures (same with the gooseneck frames prior to 67). As the 500 unit gained power it was necessary to upgrade all aspects of the engine.
(Gearbox too) So the castings were improved as well as the bearing change.
***CORRECTION: Ted Simons Triumph he rode around the world on was NOT a bushing model,, It was either a 72 or 73 Model which had many improvements over the early engines. So lets dispel that myth please ***
So, comparisons to cars always come up, and I have built a number of car engines and raced a few. But there is a BIG BIG difference and it comes down to design. The more cylinders you have the smoother the power pulses, which is why for example Jaguar ran a V12, A twin cylinder vertical twin without counterbalancers is a nasty enterprise, which is why Ducati and others ran a 270 deg crank. (Smooths out that nasty vibration) Only Matchless/AJS really got the design right and then failed in execution with their center main bearing. (Sloppy manuf tolerances). This is why the modern Norton 952-961 engine has a center main bearing as that crankshaft whip is extreme. I have the documentation on stress analysis equipment to prove this.
If you guys were to actually see how much these engines flex, whip and bend internally at speed you would never leave the driveway or garage. Some feel that the vertical twin engines of BSA-Norton-Triumph are purposefully weak and flexible by design and some argue that by reinforcing them you invite breakage. If there is any truth to that then you by extension have to acknowledge that parts that flex can only do so for so long and how much. (IE: Time change)
But a bushing in a main bearing application CAN last a very long time in a 4-6-8-10-12 cyl configuration if properly supported. Most of the small block chevy V8s I build are a high nickel cast iron block with 4 bolt main caps and substantial meat. They get line bored and I use seasoned blocks. They dont move or flex.
These old vertical twins dont have any of these benefits and the amount of flywheel flex and whip is extreme. But dynamic balancing is critical. Not the old static balance (Which is better than nothing) but true dynamic balancing is something I wont cut corners on. I have found piston and pin weight discrepancies of several grams out of the box, rods mismatched,Cranks with different strokes between cyls, and then there is rocking couple.
So a BSA A7-A10 puttering around under 4000rpm and intermittent usage will be fine on a bushing, but beyond that they need all the help they can get.