MK3 strengthened areas

Chaps, please forgive the poor pics attached, I don't have access to the actual parts to examine, so only have a pic of my pre MK3 cases during assembly and a pic of a MK3 case currently for sale on eBay.

I was trying to see where the differences are, I understand the MK3 cases are stronger on the drive side, but when I look at the pics below it looks to me like the MK3 case has MORE material machined away.

If so, it must surely be weaker and not stronger ?

Is it an optical illusion?

Or am I missing something (again) ? (!)

If you look at the flat areas where the inner chaincases mate, you can see a much larger perimeter on the Mk III case, indicating more material before the annular radius of the case turns towards the joint.

Somewhere the Mk III cases have to make up for the increased width (bearing-to-bearing) of the Mk III crank, and it looks like this is where it shows. Is the timing side case also different, with some of the width taken up in the bearing seat or is it all on the driveside?

Ron Wood once said his flat tracker's cases always cracked when the rider would close the throttle at the end of the straights and not at max rpm. I'm sure he would have been using a Mk III bottom end in 1975, and was probably STILL welding a flat plate onto the driveside case for strength.

I recall reading that Herb Becker used a set of MK3 cases for Doug Mcrae's current race bike. Maney cases weren't in the budget so MK3 cases must have been the next best thing.

Glen

Danno said:

Is the timing side case also different, with some of the width taken up in the bearing seat or is it all on the driveside?

Click to expand...

The timing side is also different, including moving the main bearing out a bit to accommodate the wider crankshaft. That is why the MK3 timing side drive and intermediate gears are narrower than in pre-MK3 engines.

Ken

Danno said:

Ron Wood once said his flat tracker's cases always cracked when the rider would close the throttle at the end of the straights and not at max rpm. I'm sure he would have been using a Mk III bottom end in 1975, and was probably STILL welding a flat plate onto the driveside case for strength.

Click to expand...

I don't think he ever used the MK3 cases or crankshafts. By the time the MK3 cases were available, he was using one-piece Moldex crankshafts that he had made to his specs, and they were pre-MK3 width. The crankcases I saw in his shop were all pre-MK3.

Ken

A question was asked about life expectancy of his 750 crank cases.
I would imagine this is a very difficult question to answer as it will depend on state of tune, how the machine has been used over the 40 or so years and maintenance.
A lot of the modded stronger parts, such as cases from Steve Many have been produced for racing applications where the stresses are huge compared with a normal road user.
If your Norton was wheelied all the time, thrashed to 7000 revs with a pillion then I suspect you may have a problem.
I suspect the later parts are stronger and will last longer but if you are sensible with the way you ride and keep a regular check you should be fine.
I suspect there are many 750's running around still on the original crank cases.
I suspect a lot of Commando owners now are of a certain age and the machines are generally looked after much better than the first few years of their life.
If you intend thrashing it all the time, and are going to tune it then it may well be a good precaution to get some later cases.

T.C Christianson, the pilot of the dual Norton engine Hog Slayer drag bike experienced the typical horizontal split across drive side of his road going Commando. As I recall it is/was an 850 and I don't know what modification he had made but it was a cruiser.

Another engine case failure I had spotted was on John Gregory's Commando based road racer. John is the brains and wrench behind the Hog Slayer - he designed and built it. All around great guy. We were dicing back and forth at Road America in Elkhart Lake one year when in the paddock I spotted a fine crack forming from the right rear (drive side) engine case at the mount to the rear cradle. This is another notorious spot for failure to begin.

Some of these cracks can be repaired with welding and I have seen some gusset/weld in these areas as well as the main bearing plate addition.

It's difficult to say when something like an engine case failure will occur as so much depends on use, both past, present and future as well as maintenance (ex. was PO running with loose engine/cradle mounts).

Just my opinion here but for most street applications, the stock cases are plenty adequate but once you start screwing with the bike/engine, who knows.

In my opinion, for road racing, it's easy to make a business case for beefing up or running with heavy duty cases as the consequences of an engine case failure are almost always very costly and potentially fatal to a rider or his competitors (oiling track).