This half baked, hare brained scheme is finally coming together. I got the idea shortly after I brought home my 74 DT360, as soon as I found out about the 496cc version of that motor that shares the same basic design. Too easy, I really had no choice but to put one together. 

The biggest problem was the top end. They're scarce and almost always clapped out. I picked up a core and sent it off to Rich Taylor for a resleeve. While that was in process I gathered up a NOS crank, a set of cases and a transmission. Today the cylinder came back, shiny and new.

 





 

The resleeve presented a valuable opportunity to correct what I consider to be flawed in the original porting layout. The stock exhaust port is 1mm higher than the MX360, which revs to 9000 RPM. It is a full 6mm higher than the mild mannered DT360. It comes from the factory with 196 degrees of exhaust duration, which is more than a modern 125cc two stroke.

Cylinder base machining is an option but when I need to subtract ten degrees from the exhaust port it would be a hell of a lot, and I would have to raise the transfers back up and make a large spacer for the top of the cylinder. Instead, I requested the port roof be lowered by 3mm to get the timing down into what I consider to be a reasonable range. This reduces exhaust port timing to about 185 degrees. Not exactly conservative but much more sensible than the original timing.

One thing I discovered with the DT360A is the unsuitability of the stock six petal reed block. It's too small for the soft spoken DT. It's certainly too small for the MX360 and it's wildly inappropriate for the SC500. Despite this, Yamaha used the exact same part on all three motors. I will be running the 500 with a modern 250cc reed block, which is about as wide as I can go within the stock casting. This is a four petal block that my 360 responded very well to. Should help the 500 bottom to top.

The goal is to build a reliable SC motor that makes good bottom end and pulls like a train through the mid-top. The stock SC comes on like a solid fuel rocket at about 4500 RPM and peaks at 6500. It'll rev higher but considering the size and weight of this piston, that seems like abuse. Power falls off past peak but it does hold onto 35 horsepower until 8000 RPM if you don't have the sense to shift by that point. Reducing the exhaust timing shouldn't hurt peak power and I expect a significant boost at low revs, below the RPM which the pipe is providing a beneficial effect.

These things were not well received by the magazines and it doesn't seem like they sold a whole lot of a lot of them. The potential is there though. 496cc's is enough to have fun, even with an antiquated air cooled design.

I will be finishing the porting over the next few days in my spare time. I asked Rich to leave it as is, in it's somewhat unfinished state. Wouldn't be as much fun if I didn't do it myself. As resleeves go, the ports actually match up very well. It doesn't need much but a little blending here and there will help.

Watching with interest & thanks for posting. If i recall correctly the base gaskets look the same or in stud pattern at least to the MX360-DT360 but something needs machining of the crankcase where it sticks out to the cylinder liner extension? of the 360 to take the 500. Got mixed memories of racing one of those monsters. Great on smooth rolling grassy circuits. I guess you've seen the video link i put up once from the NZ mx champs here in nz so i won't bore you again haha. Anyway looking forward to how your mods work.
Just to add--keep it rich & oily-'i think'. Lucky to find a piston.

The 360 base stud pattern is the same but you would have to hog a pile of aluminum out of the cases to get the 95mm top end on.  A whole set of SC500 cases was not expensive so I just went that way. 

I was considering using the DT 5 speed gearbox but when you have it in hand and you can see how lightly constructed those cogs are, I think most would have second thoughts.  The 4 speed gear set is thick and chunky.  Should have no problems with the 500 torque.

I have seen that old NZ moto footage.  Looks fast but it's hard to tell.  The dyno test in the '94 Dirt Bike magazine issue shows 39 hp at the rear wheel at 6500 RPM.  Not bad at all.  I don't know what to expect but it should be pretty stout.  Modern reeds will help tremendously.  The pipe is still up in the air too.  The MX360 pipe won't fit and I have not found a decent SC pipe.  I could weld up something but it's not going to fit like the original through the frame design, I'm just not that good.  It may well end up being a 2003 KX250 pipe with a lengthened head pipe and midsection. 

I will probably run the DT360 oil injection pump.  Been working great on the hopped up DT, always plenty of oil in the crankcase.  Seems excessive, would probably be just right for the 500.  This bike is no hill climber or drag racer, just an occasional ride around town bike.  Won't be run hard enough for long enough to be much of a challenge in terms of lubrication.

About done with porting: 

 

 

 

 

 

Quite the difference before and after on the exhaust port:

 

 

I have some extreme heat jb weld on the way to fill in behind the sleeve in the exhaust port. Hopefully that stuff holds up. It's not going to be directly in the exhaust gas stream, just right next to it.

Look forward to hearing how it runs.

I'm liking watching your build. I've owned two SC500's. Yes I'm crazy too!

wow your building a beast!
might kick back like a mule, are you planning for a compression release?
 

It will be using the standard auto decompressor.