2. Engine

Engine Issues
Starter Clutch
Gudgeon Pin
Electronic Ignition
GT550 Project Menu

Engine Issues

As mentioned previously, this bike came to me with the engine stuck. As a part of the deal, I also was given two donor bikes of the same model, but different years. As is often the case, these bikes had been left outside with the carbs and exhaust pipes removed, so water, snow and whatever was blowing by had made their way into the engine cylinders and they were solidly stuck and - as per the photo to the left - not looking very good at all. After letting them soak for several weeks with a liberal amount of penetrating oil in the barrels, I pulled them apart to see what could be saved, Interestingly, both donor engines appear to have failed due to holed pistons, so naturally I was curious whether the project GT550 had the same problem. As well, I had been told that this model of Suzuki had a fragile starter, but when I pulled these apart both donor bikes showed no sign at all of starter problems. One of them even still had its starter motor fitted, and when I'd pulled it out, I tried it with a power source, and it spun freely.

At the end of the day, I had about one and a half engines with serviceable bits and pieces suitable as spares - one actually had a set of first over pistons which are becoming rare to find now. The barrels were in rough shape, but at least could possibly be bored out, and one set of engine side covers were worth keeping.

The next step was to strip down the project bike and see what its problem was. As it turned out, it had more than one. Once the head was off I was initially relieved to not see any holes in the pistons, however the centre cylinder had a lot of small metal particles on the top of the piston as well as in the exhaust port, and once the barrels had been pulled off it was clear where they had come from per the photos below. The centre piston's gudgeon pin had snapped, and then the piston had essentially self destructed. The bits of broken aluminium alloy that had fallen down had lodged between the crank throws and the crankcase which was why it wouldn't turn over. As well, when I pulled off the clutch basket, the starter clutch assembly basically fall apart as the centre boss had come completely apart. In the photo below to the right, you can see the broken one to the left, and what it's supposed to look like to the right.

Suzuki completely redesigned the starter clutch assembly in 1975, the year after my bike was built and obviously the updated starter clutch version had never been installed. The service bulletin is GT-26, dated August 15, 1975 and a copy can be found here. I have two 'good' old style starter clutch assemblies from the donor bikes as neither of them had been updated either, but I wasn't too happy with using these as I could reasonably expect the same failure mode again at some point - and as the original plan was to put this bike on the road for my wife to use, I doubted she would be impressed if it quit working !

Starter Clutch

The new style clutch basket and starter drive assembly arrived in due course, together with most of the seals, gaskets and other bits and pieces I needed. I managed to track the starter clutch assembly down in the UK from Suzuki-Dave who is a member of the UK Kettle Club, and from whom I've bought other items previously. Tracking down all the other seals I needed has taken some time, as some have not been available from Suzuki for quite a while - for example the oil seal that goes behind the drive sprocket ( 09283-25050 ) I finally sourced from a dealer in New Zealand. Most of the crankshaft bearings and the connecting rods are no longer available from Suzuki either, so I'm hoping that the crankshaft I have will be OK.

I spent a very pleasant day up at Fred's earlier this week washing the engine cases in his parts washer, and they now look much better. I also rinsed out the crankshaft assembly just to try and flush any remaining small bits of aluminium off the big end bearing etc. and that all looks pretty good as well. As my wife is out of town this week, I very seriously thought about washing the crankcase halves in our dishwasher - this has been done by a number of folks and supposedly works well, however my guess is that they are all single (or divorced) so in the end I decided that I'd better not push my luck with SWMBO'd.

As per Suzuki Service Bulletin #GT-26 of August 15, 1975, the new starter drive assembly ( 21200-34820 ) was introduced with engine number GT550-63692 and is quite different from the older style, as can be seen from the photo below - the new style is on the left with the new starter clutch gear above it. These assemblies come as matched sets and per the bulletin must be replaced as a unit. Of course none of these parts are still available from Suzuki, so anyone restoring these engines will likely have to take their chances.

Gudgeon Pin

It seems luck is in short supply, as I am still waiting for a couple of seals to arrive. In the meantime, I have assembled most of the engine - while doing so, I noticed that in addition to revamping the clutch assembly Suzuki had also changed the gudgeon pin design. The piston failure in the centre cylinder seems to actually have started with the gudgeon pin cracking in half, and so I was pleased to see that the new pins had a much thicker wall, as can be seen in the photo below. Presumably then, the failure my engine had was not an isolated event !!

Electronic Ignition

As I did for my GT/GS750 project , I installed the Boyer Bransden electronic ignition system on this rebuild which I bought from Walridge Motors who are the Canadian distributors. Other systems are available, some of which are probably better suited for racing and high performance, but for this bike and the sort of riding it will get I think that the Boyer Bransden system is more than good enough. Below to the left is a photo of how I've spliced it into the wiring harness - this photo is actually of the GT750 install, but the wiring is exactly the same on the GT550. The control box is mounted under the top frame tube. To the right is a photo of the pickups and the mounting plate as installed on the GT550. So far at least I am really happy with the setup - both bikes start easily and run smoothly.