1973 GT750 – Sods and Odds

Still plugging away at different bits and pieces of the 1973 GT750 project bike – I was side tracked with doing some video conversions from some twenty year old 8mm video tapes I had that I wanted saved to hard drive, and as well I’ve been playing with Windows Vista in preparation for the arrival of a new EPSON V700 scanner – but more on that later.

It seems hard to believe that it was August of last year that I first cracked the cases on the 1973 GT750 project bike engine, and discovered that things were a bit of a mess ! Just to recap, since then the frame and other bits and pieces were powder coated, the barrels had new threads inserted and the gauges have been rebuilt. I have a load of items in to be re-chromed which shold arrive back by month end. The crankshaft was rebuilt by Greg Miguez at his shop Greg’s Cycle here in Calgary, and the barrels have been bored to first over by Joe Haseloh at RPM Services in De Winton which is just south of Calgary.  I have been slowly sorting out parts and bits – keeping or repairing what I can and picking up new or NOS parts when I have found them. The good part is that as there is no deadline, I haven’t really felt pressured to get it finished, but that is also bad news. As is true of most people, without some sort of deadline I find that not too much happens ! Of late, the days have been getting longer, and we have had a few above zero days so I suppose that spring may actually be arriving yet again, and I’d like to be riding this bike once the warm weather gets here. Bottom line then is  I do need to pick up the pace  !

One thing I wanted to do was modify the upper engine case to include a small change to make future water pump cartridge removal easier, if it should be required.  This modification was first proposed by Allan Tucker on the Sundial GT750 board with input and tweaking from several board members. This and other removal tips are available at this link. What happens is that over time, the GT750 water pump cartridge gets seized into its enclosure, and if you then need to replace a seal or o-ring, you have to remove the engine from the frame and split the cases to get at the top of the water pump shaft so you can tap it out of the case. With this simple modification, all that’s required is to remove a small grub screw and then you can insert a punch and tap the cartridge out of the case with the engine in the frame.

In the photo to the upper right (just ‘click’ for a larger image), the screw driver points to the location of the 6mm x .1.0 grub screw. You need something that will sit flush with the upper surface of the engine case otherwise the starter motor has clearance issues – and don’t forget the Loctite ! Detailed instructions about where to drill and tap the hole are on the Sundial board web site, and as well a copy of Allan’s instructions is available for download here (just right-click and select ‘Save Link As’). Of course, in my instance, as I had the engine apart, all I had to do was flip the case over and drill a hole up through the end where the upper water pump shaft bearing rides. I’ve included a photo of what this looks like to the left (again, at the end of the screw driver).

Hopefully I’ll never need to use this, but it is a handy trick to have up your sleeve !

The engine cases are pretty much ready to go now – cleaned up and painted. As well, I’ve also changed out a few of the engine studs, and naturally the first one just twisted off flush with the upper engine case. This caused me an additional half day’s effort to drill out the broken bit of stud,  and re-tap the bolt hole to just clean up the threads. Quite annoying ! The only reason I even touched them was that the barrel locator shoulders on the two outboard studs had either completely corroded away, or had never been there in the first place – looking at the photo to the right the shoulder on a new stud is clearly visible on the stud to the right of the photo. The one I removed and which I think may have originally had a locator shoulder is on the left, but really its just a guess. There was no sign at all on any of the studs of a shoulder, and as well per Joe at RPM who bored the barrels to first over for me, the barrels were worn slightly crookedly which makes me wonder if the locator shoulders were ever there at all.  No way to know now of course,  but following the re-bore the cylinder bores are now square to the bottom gasket surface, so presumably all will be well in the future now that the locator studs have been replaced.

Over the next few days I’ll get the engine rebuilt and then perhaps think about putting it back into the frame – I do still have some repair work to finish on the tank and fork ears before sending them out for paint so there are still lots of things to be done.

March 15th Update:

Some additional research reveals that in the 1972 and 1973 model years (J and K models) there were no locating shoulders on the cylinder studs and all eleven studs were the same part number (09108-10006) . The shouldered studs (part number 09108-10012) were introduced late in the 1974 L model year and fully implemented in the 1975 M series, in conjunction with other changes to the crankshaft and pistons as detailed in service bulletin GT-6.  In theory then, any engine after number 38060 should have the new style crankshaft, pistons and studs, so the reason I couldn’t find the shoulders on my 1973 engine, was because they were not there to be found !

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