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After
doing some soul
searching, I've settled on a way forward. Basically as mentioned
previously the engines that
were included in the basketcase are the wrong ones for the model year,
and in any case are not in the best of shape - one has a holed piston,
and the other has had some sort of catastrophic mechanical failure
that has destroyed the cylinder barrel as seen in the photo to
the
left. I'm
assuming there are still bits of a connecting rod under the leaves at
the bottom of the barrel but given the general condition there doesn't
seem to be much there worth saving other then the engine side
covers and (perhaps)
the water pump. The second engine is slightly better in that it isn't
seized, and other than the holed piston there doesn't seem to a lot
more wrong with it but of course till I tear it down I won't know.  |
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A common problem with early 1970's Suzuki's is having the exhaust bolt thread strip that hold the exhaust flanges (and therefor the exhaust pipes) to the cylinder head. This is due to the soft aluminium alloy just wearing away over time with the repeated removal and installation of the exhaust pipes for cleaning or maintenance. An option to avoid this wear is to install studs to replace the bolts, and I know a few folks who have done that on their bikes, but I prefer the factory look. Being stripped is bad, but making it worse is that often what will happen once the original thread is gone, owners will open up the hole to the next bolt size (10x1.25mm) and just put in a larger bolt. Of course, once that one strips - and it's just a matter of time - then you are truly stuffed. In the past, your only real option at this point would have been to find someone who could TIG weld more metal back in, and then drill and tap a new threaded hole and to do this would generally require the engine be removed from the frame as the working space is quite limited.
For
the 'first time'
failure mode, installing a Heli-coil thread
repair insert is generally what most shops will do - this is usually an
easy, quick, long-lasting, relatively low-cost solution if you are
lucky - the 'luck' part mostly has to do with whether you can access
the thread to do the install without having to pull the engine - in
some cases on a motorcycle it just isn't possible.
.
The inserts themselves are about $1 each (in the US), and are made of a
hard stainless steel wire which has been coiled into a spring, and
which you just thread into a retapped hole.
But what about the case where the hole has already been opened up to a larger size and has now been stripped again ? Or for that matter is a stripped Heli-coil ? I suspect you have a better chance of winning the lottery, but I seem to be blessed with bad luck when it comes to exhaust bolts so I've included this as a possibility. For this issue TIG has been pretty much the only option - till the folks at Time-Sert came up with their Big-Sert product line. These are oversize thread inserts - plugs actually - and are really clever, but also more expensive than Heli-coils. Of course, if your other only option is to pay for someone to do some specialty aluminium welding, then it starts to look like a bargain !
In preparation for having my 1973 GT750 cylinders re-bored and bead blasted, I decided to first tidy up the exhaust ports which had a couple of stripped threads - one was suitable for a Heli-coil repair as it was just a damaged 8x1.25 mm thread and so fairly small. The second one was a small crater of a hole that you could almost drop a 10mm bolt through without it touching, so I decided to try the Big-Sert and see how well it worked. Ten minutes later, as can be seen in the lower right of the photo, I had a brand new, very nice looking 8x1.25 mm threaded hole and was very pleased with the result ! The inserts are available in various lengths - for this size of bolt between 8 and 20 mm in-depth. I had the 'stock' length ones of 11.7 mm, as well as some longer ones at 16.2 mm, and opted to install the longer ones just to give more contact area on the bolt given it was after all an exhaust port being used to secure the exhaust pipe, and so subject to extremes of temperature and vibration.
As with Heli-coils, you typically buy a kit for each size of thread you are repairing. The Big-Sert kits contain all the bits and pieces you need to do the installation: drill bit, tap, reamer, install tool and inserts (of course you have to supply your own electric drill and tap handles). Unlike Heli-coils, Big-Serts are threaded plugs with a cam-lock built-in to stop the insert from moving once in place. It's a nice design and ideal for this sort of repair - but they are about twice the price of Heli-coils (if you shop around). I bought mine from ToolsQwik in the US - very helpful and easy to deal with, good prices and fast delivery. After including the postage cost, and currency exchange (there is no duty charged to bring these into Canada - just GST which you'd have to pay anyway), I still saved about 30% over what I was quoted for the same items here in Calgary, so as always it does pay to shop around.
I will be dropping the cylinders off at a shop south of Calgary in De Winton called RPM Services next week after Christmas, to be bored to first over size. I already have the pistons, rings, gudgeon pins etc., so it now looks like I will be starting to re-assemble the engine sometime in January. In the meantime, I still have to finish the wiring and sort out the fuel tank and a few other odds and ends on the GT500 I'm also working on, so I'm not short of things to do !
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. The original thread is located here. 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 (which needs to be short - perhaps 5mm) 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, the screw driver points to the
location of the 6mm x .1.0 grub screw (in the US they are known as a
"set 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 o
ff
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, the owner of RPM Services, and 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.
Update: after some additional checking, I'm now convinced that on my engine the cylinder studs did not originally have a shoulder as these were introduced at engine number 38060
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.
Recall from above that when
I cracked
open the cases, I found a bit of a mess as it appears the oil had
leaked out and been replaced with water over the years, and things
inside were rusted solid. Other than the cases, I was unable
to
salvage much of anything in the engine case at all - even the
upper oil splash plate had rusted away and had to be replaced and the
starter motor was a solid lump of oxide. I was lucky in that I
basically had must of what I needed to replace all of the internals,
but I did not have a good gear shift selector shaft and so had to do a
bit of searching around to get one. The gear
selector shaft on the GT750 was changed sometime in 1974 to have a
longer rod on the left engine side. At the same time, the gear shift
selector arm design was also changed, and the new ones had much less
angle to them than the ones used in 1972, 1973 and much of 1974.
Although I had the correct, old style gear selector lever, the gear
selector shafts I had available to me had either corrosion or wear
right where the oil seal would ride and so were not useable.
After a bit of head scratching, I was very lucky (and thankful !) to
have one offered to me for the cost of the postage by Barry, who is a
fellow member on the Sundial Moto Sports Board, which
is a great resource if you are dealing with these old 2 strokes.
I previously highlighted a few of the other updates I made during the engine rebuild here and at last I had got to the point where I was ready to button up the cases, as can be seen to the left.
I had four GT750 injector oil pumps available to me - one was definitely suspect as on the bike I'd pulled it from, the oil injector tank was bone dry and missing its cap, which likely meant that water had been able to get into the pump itself. Although it turned, I didn't really trust it, and on the three other pumps I didn't have any history. A really good write-up on the Suzuki injector oil pumps is available on Richard's site here.
Having a working oil injector pump is a must - unlike other 2 strokes, the crankshaft design used on the GT750 Suzuki means you will likely burn out your main bearings if you try to run solely on pre-mix with out having the crankshaft re-designed. This sort of conversion is possible - Bill Bune's shop will do this for folks who need to run pre-mix for vintage racing applications for example, and I'm sure there are others. In my case though, it was going to be injector oil so I had to confirm I had a working injector oil pump ! To do this, I took a piece of flat steel stock, and drilled and taps mounting holes so I could bolt on the oil distribution loom and oil pump in a similar way to how it was mounted on the engine. The pump driven shaft protrudes though the bottom. I had a scrap oil pump drive shaft from one of my parts engines, and I drilled and tapped a hole in the bottom of it to take an 8mm bolt that I could drive with a 1/4 inch variable speed drill. The reason for doing this is that as the oil pump shaft is turned, it actually travels up and down about 4mm which meant I couldn't just connect the drill chuck directly onto the shaft - using the original drive shaft allows the pump shaft to both be turned and also allows it to move up and down
After testing the four pumps I had available, I was able to confirm that two worked fine, and that two were scrap so it was time well spent now rather than later after I'd seized up the new engine due to lack of oil ! In the photos below, you can see test rig mounted on the bench and the drive shaft on the bench beside one of the oil pumps. With the engine in the frame I can finally move onto other things !!
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I had the engine fully
assembled on the bench and so now was the time to
think about who to call to help lift it into the frame – these things
are damned heavy, awkward to manage and I’m not getting any younger.
Come to think of it, neither are any of my friends ! We (worldwide
GT750 folks) recently had a thread on this topic which can be
read here.
The normal method is to left the engine into the frame from the right
hand side and then somehow support it while attemptin
g to slide in a
couple of the engine mounting bolts. This can be a harrowing, possibly
dangerous adventure when doing it by yourself as the risk of injury is
high, and a male bonding experience if a friend helps you, as you get
to learn new, colourful and highly descriptive words that extend and
add versatility to your vocabulary ! It seemed like a good
opportunity to try another option.
A method I’ve used previously for removing an engine has been to lay the frame on its right side, unbolt everything, and then lift the frame off the bike. Many people have also done the reverse to re-install the engine, so I thought I’d give it a try, and I’m pleased to say that it worked like a charm ! With the engine blocked up securely so that it wouldn’t fall over and damage my lovely oil stained concrete floor, I just lifted my frame over the top and into position, slipped in a couple of bolts to secure it and then gently lowered it back so it sat upright on the motorcycle lift – job done ! The whole process took about 20 minutes start to finish with no back ache, no mashed fingers, no colourful language and there isn’t a scratch on the paint that I can see – to the right is a photo of the freshly re-united engine and frame. Doing all this work on the lift, had the advantage of being able to adjust the working height so it was comfortable, and so much easier on the back. Recommended.
From there is was a simple step to re-install the front forks, and then get the frame back up onto the centre stand so I could remount the swing arm etc.
