Wife beat me to it, wasn't able to get the Rickman bodywork from Australia that I was begging for as a birthday gift, but she followed through on this pretty well!
Fairly snug tucked fit into frame, looks like the Rickman hanger and Yoshi hanger are both at or very near in agreement with each other. I need to get an engine in there to mock up chainline / gearing to see how steep of swingarm angle I can get away with. Then I will see if the Yoshi pipe really lines up perfect as it ls looking to do. Would have preferred the more radically upswept version but this thing is very cool, has awesome history, fits in great with the primarily vintage speed parts nature of the build, and fits in VERY WELL with the performance nature of every aspect of this build - this pipe is SUBSTANTIALLY LIGHTER than the Kerker System K setup I have as the alternate, and much more racing cred/lineage.
Uh oh, is this pipe going to work on my 894cc or 920cc GS750 engine? It was for a GS100!?!?!?!!!
I didn't think twice about fitting this thing on what would likely end up as a GS750/850 hybrid big bore, after comparing the difference in crankshaft stroke between that and a GS1000. 8.4mm longer stroke on the GS1000, so the engine likely is only 8.4mm taller, hence the exhaust hanging down less than a centimeter lower... I will definitely run with that if I choose a 750 big bore!
Looks like I will probably be getting some high end one off custom JE Pistons for the Rickman in 72mm, same as the MTC set I have in my GS750 workhorse. These will be 10.35:1 approximately as opposed to 10:1 on the MTC. That will allow me to put GS450 71mm bore sleeves into the GS425 (stock 67mm bore) block and achieve 489cc and around 10.85:1 and still have decent cylinder wall thickness as long as I am not boring all the way through the aluminum between the fins to fit these substantially oversized sleeves. I was leaning toward the 475cc 425 / 894cc GS750 Rickman for wall thickness and cooling reasons, but after getting the 920cc 750 in tune very well with stock cams and carbs this season and being able to spin the big 150 Avon RoadRider substantially, and also being able to lift the front end off the ground without any rider input other than rolling the throttle open a fair amount and somewhat rapidly... well the big cc's of the modded 750/850 on stock (but advanced slightly) cams was quite addictive. Lifting that big heavy 175lb 4 cylinder engine'd bike's front end off the ground substantially even with a 1.125" longer than stock swingarm is quite a feat, so that power was quite addictive.
On the little featherweight twin monster racer build, maxing out the potential of it is paramount, as I want it to be quite the little ripper as to not be sitting idle because I would rather ride the bigger bikes. jumping from 475cc to 489cc is one more thing to add up on that thing to make it a beast. .380" lift megacycle cams and possibly bigger valves and some minor porting work by Rapid Ray or Rockdale Cycles plus a japanese imported 2 into 1 exhaust and either custom twin cylinder racked VM29 smoothbore carbs or custom racked 2 cylinder RS34 carbs, I think this thing may be the best bet for the RS34's of all of my builds (largest displacement per cylinder of any of my bikes), and my buddies have a spare 4 cylinder set with one broken slide that is no longer available, and will make me a deal on them. Accelerator pump setup smoothbore flatslides that take the same main jets that all the VM carbs use. Less parts for me to stock is good. the 29's would allow me to stock even less as the rebuild parts are mostly similar to the vm26's etc oem carbs.
I think I can take the GS425E's claimed 40hp and easily push 57hp @ the crank if not 60hp. Some minor head porting and larger valves may be necessary to squeeze the most high rpm potential out of this. Maybe stock GS1000 size aftermarket valves may do the trick. I don't think the heads will need much porting as they are already designed for 34mm CV carbs (just recalled this when looking up hp/tq specs, RS34 it is!), so the head for 34 cv's has BIG intake ports. I don't want to increase the volume of the port much at all as I don't want to kill all low end response (bigger ports take longer to fill with air/fuel charge, so they make it a less fun street engine and only improve the fun factor at police-attracting exhaust decibels at high rpm operation)
So I'd have a 2 cylinder bike that weighs 330lbs or so (60% of a GS1000) and has half as many cylinders as a GS1000 but almost identical displacement per cylinder, and more horsepower per cylinder by a longshot over a "STOCK" GS1000. I think this combo equates to a screaming little twin that would be nearly as fast as a stock GS1000 up to 75 or 80mph until the aerodynamic drag pushed too hard on the lower torque output twin cylinder racer. This should be one of the fastest bikes to ride in the twisty roads for sure.
I am still slightly considering the other option - 80-87 GS450 engine, GS500 sleeves bored and pressed in, GR650 Tempter TDCC head and pistons. For $800 I can get the best pistons possibly ever made for this type of setup, 10.5:1 CP pistons made by Carillo. This would give 527cc's. It has been done with stock GR650 pistons before. the thread on GSR is deleted but the youtube video "GS527 Beast" exists still as evidence of this monstrous twin combination. This would lose my kickstart only and roller bearing crank bottom end, but would give me more readily available plain bearings and other more readily available engine parts (some 750 bottom and a few 400/425 bottom end parts are no longer available now). I like the press pin roller bearing crankshaft though, virtually indestructable. no worries of throwing a rod when the the rod does not have a bolt-together seam! If I get bored in the next year or two, I might attempt a GS450/527cc clone of my 489cc build and use stock GR650 cams (bigger lift than any factory GS cams ), and have a fun racer bike for the wife to tear around on. More budget oriented than mine with stock cams and GS500 forks and caliper with a single 310mm rotor, but very similar.
