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Seat, Tank and Rear Fender

  1. Overview
  2. August 22nd - Mockup
  3. September 13th Update - back from the welders
  4. October 2 Update - Caswell Tank Sealant
  5. October 11 Update - Mini-Roll out

As mentioned earlier there are a few snags resulting from having decided to do the rear disc conversion using the GS swing arm. Although both models have about the same overall length (87.2 inches or 221.5 cm for the GT750 versus 87.6 inches or 222.5 cm for the GS750) the GS750 swing arm is roughly 3 inches (or about 8 cm) longer than the swing arm on the GT750, and has just over an inch (3 cm) of additional wheelbase . I suspect this is a good thing, as it should improve the tracking and overall handling, and also there is room now to perhaps install a larger rear tire. The downside is that the arc that the rear wheel travels doesn't look right with the stock GT750 rear fender, and as well it also means there is no easy way to use a stock GT750 exhaust system as the indents for the rear axel nuts are in the wrong place (too far forward). I had planned to use a rear deck and fender off a 1978 GS750, and luckily I had a set of aftermarket Gibson pipes I could use in place of the stock exhaust so I did not initially think this would be a major problem, however I was wrong. In the end, it did result in a number of frame modifications being required for the rear fender mount, the seat and the tank, as well as of the tank itself.

If you compare the photo of a GT750 (right) with a GS750 (left), you can see that the GS tank is longer than the GT tank by about 3 inches, as it extends to just over the side cover, whereas on the GT750, the tank stops roughly even with the front of the air filter box (red line in both photos). Also note that although the top shock mount is still in roughly the same location on both frames relative to the steering head, you can see the effect of the longer swing arm in the increased angle of the shock itself (green line).

The problem then, is having started as I did from the rear of the bike to try and get the rear fender to look right with the arc of new swing arm, and extending the frame to allow the rear deck to be mounted in the same manner as the GS750 stock frame, once I have the seat mounted there is a 3 to 4 inch gap to be filled in some way, either between the front edge of the seat and the rear of the tank, or between the rear of the seat and the rear deck being mounted on the fender. You can actually see it clearly in the photo below as being the gap (yellow lines) between the scrap rear fender off a GS750 that I'm using for clearance checks as I fab up the frame mount extensions, and the original inner fender that come stock with the GT750.

I'm tending toward modifying the tank and have mocked up what it might look like in the photo below. Lengthening the GT750 tank actually makes it about the same length as a GS750 tank, and together with the rear deck makes it appear a bit more modern.. At the moment, I'm waiting for a few parts to arrive before finalising the changes, but I expect to make a decision in about two weeks.

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August 22, 2008 Update

I have finalised the design, and have arranged to take the frame in to The Welding Shop here in Calgary - the tank will have to be steamed out before welding, but this isn't too difficult to arrange. I have cut off all the tabs for the GT750 inner fender mount, as well as the old seat hinge mounts and have either created new mounting tabs based on the GS750E measurements, or have adapted/modified GS750E mounts to suit. At the moment, I'm just waiting for Cactus Machine to finish the work on the new brake lever as they have the sleeve which is to be welded in for the rear brake actuator.

The frame with the tank extension and the mounting tabs has been sandblasted (you can read about that here) and I've included a photo to the left of how it looks currently. The masking tape that you can see marks the location of the new tabs and mounting points for the relocated inner and rear fenders, rear deck, seat and new master cylinder for the new rear disc brake.

I plan to start work on the engine this coming week.

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September 13th, 2008 Update

The frame and tank are back from The Welding Shop, and as well I have finished off the shaping, filling and priming of the tank so it is now ready for the next couple of steps - these being sealing, and then finish coat painting. As well, I have double checked the location of the mounting tabs for the seat hinges and rear deck and will need to make a couple of minor changes. Over all, I wasn't happy with how the GS750 seat sat on the GT750 frame - it was a bit too high, so I've modified the rubber bushes that fit into the underside of the seat and which rest on the frame members. This has let me drop the seat about 3/8th of an inch, and overall I think it looks better as a result. I also have decided to raise the rear deck slightly at the rear which means I'll need to relocate the mounting tabs from where I had originally planned, either by welding new ones, or more probably by just fabbing up a couple of small brackets.

In the left photo below, you can see how I've stretched out the GT750 tank, as compared to the stock shape along side it. In the middle photo you can see how things line up when mounted - the rear deck is off a 1978 GS750E, the seat likewise and the tank is from 1976 GT750, and which has been modified. The overall line is very close to that in the photo of the GS750 at the top of this page. To the right is a photo of the tank lining kit I plan to use this week to seal the inside of the tank and ensure everything is leak proof and (hopefully) will stay rust free.

There are many products on the market for sealing gas tanks - and these days it is complicated somewhat by the introduction of ethanol into gasolines making the fuel more corrosive. Some tank lining products, used for example in aircraft and which have been used in some cases for motorcycles will work fine on pure gasoline/petrol fuels, but will actually dissolve if exposed for a prolonged period to newer bio-fuel mixes. There is some very good information available in the press and via the internet on the effects of ethanol on older equipment so do some homework. After reading what I could find, I went with the Caswell product (the American link is here and then just search the site for 'Gas Tank Sealer' for those folks from outside of Canada) and we'll see how it goes.

Update: Caswell Canada no longer supplies the product I used, although they do still sell it in the US. In Canada they now offer several products, all epoxy based. For my 1973 GT750 project, I used a local radiator shop which cost more, but also came with a guarantee.

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October 2nd, 2008 Update - Caswell Tank Sealant

I won't duplicate what you can find elsewhere on the web about the use of epoxy fuel tank liners - please see the previous note above - but I must say that I smiled to myself when I was mixing in the catalyst as all I could think about was my previous experience with two part resin/catalyst experiments in the Sandy household. When I was a teenager, my brother and I helped my Dad fiberglass a sailboat we were building in the basement of our house in Ontario - as we laid down the mixed resin and glass mats, invariably we would run out of time and the mixture would start to set, the container it was in would get too hot to hold, the smell was incredible (as I think back on it, I'm sure we were all higher than kites on the fumes) and my Dad would be rushing around at the last moment trying to apply rapidly solidifying gel that was turning into solid plastic so fast you had to be careful your hands didn't become permanent deck fixtures ! I have to report that my Caswell experience was not nearly as exciting, so just a very few comments:

Bottom line - from what I can see, I do now have a good coating of epoxy on the areas of the inside of the tank, so I'm finally ready to start painting !

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October 11 Update - Mini-Roll out

Today marked a small milestone as I fitted up the painted side covers, tank and tail piece to the frame to check the fit and clearances one last time. Of course I'm biased, but I like how it looks. Below to the far left is what a late 1978/early 1979 GS750 looked like, to the middle and right you see how the GT750 is shaping up.

Extending the GT750 frame has preserved the correct arc for the rear wheel, and stretching out the tank and changing the seat level actually results in quite a comfortable riding position. As well, the line through the front reflector to the rear tail light is a close match for that of the factory GS750 which is the look I was aiming for. An obvious big difference is the wheels - the 1977 GS750B and 1978 GS750C both had spoked wheels, but the 1978 GS750EC had the 'snowflake' pattern mag wheels - I may still decide to add a set to the project bike, as I do like the look of them.

I have a set of decals for the striping on the tank and rear tail piece coming from Reproduction Decals in eastern Canada which should be here sometime next week. I admit I find them a bit annoying to deal with as they are a Canadian company but price everything in US dollars so when the Canadian dollar is down, as it is this week, they are effectively making a bit of a windfall on both Canadian as well as US buyers. I will order the new tank badges and side cover badges from Badge Replica in Australia as the price is more reasonable and, the parts appear to be identical to what is offered by Reproduction Decals in Canada.

At the moment, I'm just waiting for a few more bits and pieces to arrive before I can get serious with the engine !

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