(Updated: May 23, 2008)
One of the obstacles of trying to stuff an ST4 engine into an '88 Paso frame is the rear suspension. The swingarm mounting bosses on the cases are wider on the ST4 than the Pantah engine. If I wanted to use the stock swingarm, I'd have to get the engine cases machined. I opted for another option.
I decided that I didn't want to modify the engine too much, so I decided to use the swingarm from an ST2/4 for the project. I was originally thinking about modifying the frame to use the rear suspension from an ST2/4, also, but I didn't like the idea of cutting up a frame. Instead, I figured it may be feasible to modify an ST swingarm for the Paso. There are two problems with this: 1) the ST swingarm is considerably shorts (about 2"-3") and 2) the Paso's shock and exhaust have to go through the swingarm unlike the shock bolting directly to the swingarm on the ST bikes.
Here is a picture of the ST swingarm on the left and the stock Paso 750 swingarm on the right.
I was in luck with the fact that the guy I had bought the engine from is also a metal fabricator and builds custom motorcycles. Scott Frailey owns "Metal Visions by Scott" in Anderson, SC. He's located directly behind Timm's Harley-Davidson. I discussed the problem I was having, and Scott decided that instead of just a cut and extension, that it would be better to box that area of the swingarm. It was needing to have the hold opened up, and there had to be about a 2" stretch in the front. He took dimensions, made a jig, did the cutting and welding, and made it look really good. One important item to watch out for is the part of the swingarm where the linkage attaches. If the dimensions are off even a little bit, the rear suspension geometry can be vastly different. I talked him into taking some pictures of the process, but he hasn't given them to me, yet. As soon as Scott gets those pictures to me, I'll post them here. Here are a few pictures of the modified swingarm.
I need to buy some bearings and seals for the swingarm linkage before I attempt to install this. Scott had the tube for the linkage mounting point machined to the same dimensions as the original ones, so the bearings, seals, and hardware can be reused from the stock swingarm, if possible. I'll try to add more pictures and comments as things develop.
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