77 GS750 <> 82 GS650G Fork Lower Swap

[sam000lee]

I'm gathering parts for a twinpot brake upgrade on my '77 GS750 and am trying to find fork lowers with caliper brackets on both sides. Dual caliper GS750 forks are apparently hard to come by on ebay so I'm looking into forks from an '82 GS650G.

Comparing the two sets, the most obviously visual difference (besides the caliper mounting points), is that the GS650G right fork doesn't have the bolt on cap on the bottom, but operationally the GS650G forks also include the air system.

My main question is whether I can just take the internals from my GS750, including the sonic springs and along with the fork tubes, and put it all in the GS650G forks. The other option is to rebuild the GS650G forks with my sonic springs and swap the whole assembly in, but I'd rather not have the air assist component in the forks.

The fork lower part numbers are basically the same and the oil seal part number is also the same for both which is encouraging.

GS750 Right: 51130-45032
GS750 Left: 51140-45032

GS650G Right: 51130-34200
GS650G Left: 51140-34200

 

[sam000lee]

Alright so I figured I would just go for it and went ahead and bought the GS650G forks. I started taking them apart and am embarrassed to say I'm running into an issue on disassembly.

In the picture below, I have my disassembled GS750 fork. To get to this point, I broke the bottom hex bolt free, removed the springs and dampener, then the fork tube. At the bottom of the fork tube there's a cap insert (#6 in the diagram), pictured next to the dampener.

For the GS650G fork above, I've removed the hex bolt and dampener, and was expecting the tube to be free. The GS650G manual refers to a couple of specialty tools. The first I think is to just put pressure on the dampener to remove the hex bolt that I've already got out. [Q1] Is that correct?

[Q2] The second tool seems to be for removing the "damper rod securing bolt". I had originally assumed that this was the hex bolt that's present in both the GS650G and GS750 forks but now am thinking it's a different bolt. Is this the case? I can make a tool that will get down there and have seen some info searching around but there are some varying specs. What do I need to get the fork tube out?

Last one for now: the dampeners are slightly different lengths which isn't super surprising considering that the 650G was equipped with air valves so I imagine is set up differently. My original plan was to just swap all of the GS750 stuff into the fork lower of the GS650G. [Q3[ Any reason why this shouldn't work once I figure out removing the 650G fork tube?

 

[sam000lee]

Looking again with fresh eyes this morning, in the diagram for the GS650G, there's a collar that goes around the lower fork tube. Could that be getting stuck on the fork seal that I haven't removed yet?

 

[sam000lee]

Alright I got it out. The 650 fork tube has a sleeve with a larger OD then the tube itself. To remove the fork tube, you have to pull the oil seal spring clip and retainer and then basically use the whole fork tube as a slide hammer to bash out the seal with the sleeve.

As for the fork tube lowers, they are machined a little bit differently at the top, I'll take pictures later for future reference but I'll be re-assembling everything with a combination of seals from the two models.