The engine of the MK I version was unique in many ways:
There are threaded drain plugs installed in the bottom of each crank chamber in the lower engine case casting, which were removed with the introduction of the 1976 A model.
Japanese manufacturers also switched over from JIS to ISO thread and bolt standards in 1968. Note this is generally true - there are exceptions. Early cases were actually stamped as being 'ISO' standard, and this was dropped from the castings by the introduction of the A model (possibly earlier).
The bearing material was also changed - the brassy looking insert in the photo to the left is from a MK I case. Other cases I have are A's and B's and the insert appears to be steel.
There is no oil splash plate in the upper case of the early MK I engines as seen in the upper right photo, nor is there provision for one. I actually tore down an engine I'd just buttoned up, as I noticed I had one of these plates left over in the tub I was pulling parts from for the build. It turned out the plate was from a later engine (lower right) and not the one I was actually working on !
When the cylinder drain plugs were removed, the casting was updated to include the change in the weir height in the gear box, and the '1200cc' cast into the case by the oil filler hole was changed to '1400cc' . This change was originally detailed in service bulletin T-3 (available here) in which a rubber dam was added to L engines to raise the oil level by 200cc (1200cc to 1400cc). When I did mine, I made a dam from aluminium and had it tack welded in place - you can see the difference in height to the right.
Regrettably, Suzuki never did change the check oil indicator bolt hole location on the right side cover, and so when the 1400cc decal supplied with the rubber dam eventually disappeared on the L and M machines, or if using the check oil hole on A and B models and not measuring carefully, then owners would under fill their gearboxes by 200cc. To illustrate the difference in oil level, take a look at the lower photo to the right.
The clutch basket was an 18 tooth/finger design until the end of M production - the GT A/B models used a 12 tooth design. They are interchangeable (as a unit - obviously the plates can't be swapped between the different baskets) and Suzuki used parts up before switching to newer style ones, so an early GT500 may have a T500 clutch basket.
Early MK I clutch adjuster covers did not have a grease fitting.
Early MK I clutch rods were a different length than later MK I models changing at engine T500-15914, meaning the clutch push piece was also a different length. The short one in the photo (46mm) is early - the long one (55mm) is late.
