Hi all, just picked up a 76 dt400. It ran when I got it, but clearly had issues. I pulled the head and jug to find a badly scored piston skirt, and some shoddy repair work. The cylinder looks good after a hone, but looking closer at the cylinder there is a (rather large) passage between the exhaust port, and the decompression valve. 
 is this normal? Or something that burned through do to the piston damage?
any help in deciding where to go would be helpful. I'd be fine buying a new jug, but not sure what to trust from Ebay, and the like. 
thanks. 

Mine has that too.  It is for the starting compression reducer that is connected by a cable to the kickstart mechanism at the rear of the engine.

I understand the function of the valve, but I didnt notice the hole leading down to the exhaust port. The one to the cylinder wall looks good, but the the other has me wondering. I'll try and post some pics. 
thank you!

 

The last photo makes sence, the first two are what I'm questioning. 

To me, the first photo looks normal with some carbon buildup in the hole that allows the pressure to be reduced when kicking the engine over.  The passage also allows pressure to bypass to the exhaust on the downward power stroke.

The second one looks normal if you removed the cable operated valve.

The last picture looks like you did a nice job honing the cylinder.  It's hard to tell if the transfer and exhaust ports are chamfered properly.

Jayb, thank you for your help!
I was kinda worried about that transfer hole when I saw it, but what you say makes sence. 
Thanks for the kind words about the hone job, it was in pretty good shape after letting the acid eat off the aluminum from the piston. 
I think I'll just get a new piston and put it back together. 

Another question about the cylinder repairs. I have two studs that are stripped out. Any thoughts on drilling them a touch larger and retapping them for an oversized bolt/stud?

I am not a qualified machinist or welder.

However, there needs to be enough metal around the new larger threads to support the load or the aluminum will crack.

I had an AT1 with a stripped cylinder stud hole in the case that was repaired with a helicoil before I got the machine.  One of the cylinder head bolts would not stay torqued down.  The lower case cracked where it was thinned to install the helicoil.  The repair was to remove the helicoil and have a welder fill in the hole as best he could with a MIG welder.  Then we redrilled and tapped the hole for the original stud.  I was lucky to be working in a machine shop at the time with skilled welders and machinists.  It worked well and held. 

If you are talking a cylinder base stud, I would not go with a larger stud.  Hopefully someone more knowledgeable will chime in.

The studs in question are the ones to hold the head to the cylinder. When I removed the head, someone had already tried to repair them with a helicoil, and failed miserably(imho)  
the insert was sticking proud of the mating surface by almost 1/8"! When I tried to file it down flush, it pretty much disintegrated. 
Just wonding how much I have to lose trying to tap it out to a larger size? If it fails,  I'm not really in any worse of a position then I already am, except maybe being stuck on the road somewhere. 
there seems to be plenty of meat around the stud hole, and plenty to enlarge the hole in the head for clearance of a larger bolt. 

The decomp bleeds down to the port outlet when open so 'that' hole has to be clean or at least not blocked fully. Wouldn't matter how big the hole is down to there as its metered at the start in the cylinder wall. Nothing happens when the valve is closed of course. Just softens the kick, & the kick back but shouldn't kick back on those as electronically retard for starting.