SC500A spec sheet lists compression ratio at 7.1:1

It'll be getting the DT360A flywheel along with the Rex's ignition.  From what I'm seeing the SC500 used a similar, external rotor flywheel.

The decomp passage is cast in downstream of the valve.  Like the 360, it's pretty large.  Larger than necessary.  I do remember the 76-77 RD400 with that diagonally drilled decomp hole.  So small it had no real effect at running RPM.  It was a bit further down than these though.

I think I'll see how it kicks with the wee little hole and go from there.  I have run the DT360 without the decompressor hooked up.  Not terrible to start but you definitely feel the difference.  The 95mm piston has about 30% more crown area so as far as pounds per square inch goes, I don't see it starting with the stock kick start reduction ratio and no decompressor.
 

I did some back of napkin calculations using the port height map and guesses at specs until I got 7.1 to 1 compression using the screwy "after port closes" system... 

Using that total head/gasket/dome volume but measuring from BDC like is normally done the compression ratio is about 13.5:1 on the stock SC500 engine.  Which is about right because most enduros I've measured are around 12 to 12.5 to 1 going from BDC.

Using the "from port close" your lowering port takes you from 7.1 to 1 to about 7.6 to 1. 

I am very curious how youll like the big 500!!
I think you're on the right track with the port timing for a street engine
I would offer my help building a pipe but I am not sure I am able to at the moment, perhaps soon!
I would try the 5 speed transmission and at least 38mm carb if I were in your position.
I put an IT465 engine in a DT400 frame....but no decompression release on that engine so its a bitch to start! I have been meaning to try one of those husky button style decompressors in the cylinder head

My 465's & 49's i've had never have a comp release but ya gotta kick them hard from the top against compression. They're a long stroke motor compared to the SC500 70mm stroke so maybe that helps kicking. Seems hard case they went from a 360 to a 500 just by cylinder bore alone & no wonder they rev hard.

I went into this with the five speed transmission firmly in mind.  With the gearset on the bench though, they appear lightly constructed compared to the 500 gearset.  The top gear ratio is comparable in both transmissions and the gear spacing is fairly even in the four speed.  Even with the 16/44 gearing, I feel like first gear is fairly short and abrupt in the five speed.  I'm looking forward to a taller first gear with the expectation that it will be able to pull it with gusto.

What really chaps my ass about the pipe is that I apparently missed one on ebay back in May of this year.  Problem is that I filter my results to "used" condition only to cut out the vast majority of irrelevant garbage in the ebay search results.  This one I missed was new old stock, listed in the "new" category as it should have been.  $250 with free shipping.  I found that old listing today and it sure bummed me out.  That would have been perfect.  Not the most modern pipe but it was a brand new, through the frame SC500 pipe.  They're a few inches longer than the MX360 pipe and with different tuning.  In conjunction with the shortened exhaust timing I bet it would have been pretty much ideal for the kind of power I'm looking for.  The search shall continue.  If anybody knows where there is an SC500 sitting in a junkyard I would be inclined to fly out and pull the pipe off of it. 

If all else fails, I will weld up what is sure to be a fairly ugly but probably effective modified KX250 pipe.  Would stick out like a hernia and look very out of place but I can extend the head pipe and midsection to make power at low revs.  Looking for torque here, if it's wound out by 6500 and ready to pull the next gear I expect that would go a long way toward extending engine life.  With that goal in mind and also considering how tired I'm getting of spending money on this project, I will try running the same 35mm carb I have on my 360 to see how it does.  Can always go bigger, might not need to if I'm making greasy gobs of torque in the low mid. 

Another example on the subject of to decompress or not to decompress, my 1970 Kawasaki 350.  No decompressor, just a long kick lever and a more favorable gear ratio from the kicker to the crank. You have to be more careful in where you start the kick, it doesn't have as much throw.  Get it in the correct spot and it lights off easily and consistently.  The worst bike I've ever kick started would be the 79 Husky 390.  Left handed kicker, no decomp, very short throw and lots of compression.  An excellent example of a bike that chooses who is and is not allowed to ride it.

I guess the gears will look a bit lighter squeezing a 5 speed into where there was a 4 speed. My "83K YZ490 came with a 4 speed but still stripped the teeth of one gear. I put a 5 speed in from a 465. There's a story to 'that' i that i bought it off ebay at a yz465 box but had an annoying gap between 3rd & 4th & in the end i decided it was from an IT465. I raced a yz465H motor in an IT250K or L frame at the same time & that had a nice close box all the way. So i like to think you'd be right using an MX360 box in ya "Wild Thing".

Quote " The worst bike I've ever kick started would be the 79 Husky 390. Left handed kicker, no decomp, very short throw and lots of compression. An excellent example of a bike that chooses who is and is not allowed to ride it."

Yes,that applied to my Maico 450. That promptly fired back and wrecked my ankle two years ago and it's still not 100%.
That had a decompressor on it as well.
Maybe if i was a foot taller it would have been easier.

She's about done. Still got no pipe, not sure which way I'm going on that. 



The DT360 tach drive components install easily into the SC cases.  Everything under the clutch cover is DT360 parts other than the clutch. Using FZ6 plates and DT360 steels. Still looking for some suitable clutch springs. I can fit eight friction plates in there, that should help.

 

Deck height looks quite good. Might lower the port floors a little more.

  

Wiseco makes an ideal small end bearing, the B1071 if I remember correctly.  

 

Here we see a fundamental flaw shared between this era of the MX and DT360 as well as the SC500. The exhaust skirt shrouds the hell out of the main transfers at BDC.

 

Would be beneficial to reshape this area of the cases to avoid turbulence and restriction. Probably won't be necessary though. I'm not trying to make 55hp here.

 

I decided to ditch the reed stoppers on account of how tight the fit would be. The inside of the intake port will do the job just as well.