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dencos are basically impossible to find,  give up on that. i actually am in possession of a set, took almost a decade to get. i plan on measuring thrm up so they can be reproduced, but they are, from what ive seen, the rarest street pipes for rds ive ever seen that weren't one offs

Back in 2011 I sold an RD that had Dencos on it, should have just sold the bike and kept the pipes. Anyway I know there's at least one more set out there in the wild.

Also bumping the thread.  I'm hoping to make the last round this season with the Rd350 and will be needing a set of chambers.  Years ago I had a really nice set of pipes on a 350 and should have just kept the pipes.
NOticed the Jemco site doesn't work.  anyone know what's up with that

So that was it, I had a couple things going on.  The R5 plates are definitely thicker, and I also used a plain steel one where I shouldn't have on either end of the stack. The ball at the adjuster end isn't 5/16" either, I believe it's one from the steering head bearing. 

Yeah about that. I took the steel plates out of the RD motor, but had a new stack of friction plates. Some of the steels in the RD clutch had wear I wasn't crazy about so I swapped them out with ones from the R5.  At least one of those was noticeably thicker than the RD ones.  Anyway, I ended up stacking these like the manual shows as best I could tell.  So first in was a steel, then a friction plate, alternating as such until I used up the seven friction plates, then capped it all off with a steel. So, that's 8 steels and 7 friction plates. The drawing is nearly impossible to discern what the actual count is supposed to be and I had a surplus of steel plates. Just seemed to make sense to me. Wouldn't surprise me if this was not correct.

Okay cool. 
Next question, I have everything in there now, both balls, etc and I ran into an issue where when I squeeze the clutch lever, the cable can't actuate the clutch, in that I don't see the plates separating in the basket. I'm squeezing as tight as I dare.  So when assembling the clutch basket the six screws holding the springs are run in all the way, correct?

Reassembling the motor, have a few questions.  It seems like the clutch pushrod is either too short or the clutch adjustment screw is too short, I'm not sure which.  The clutch adjuster is from an R5 I had sitting around and the RD motor came to me without that cover.  So I just took the adjuster off and put it on the RD engine cover I got off Ebay. The adjuster screw disappears all the way into the mechanism before it touches the pushrod, so the nut can't be used.  I don't think the clutch itself is somehow out further, I'm just trying to figure out what's going on here. The 5/16 ball is in there, and looking at the shop manual it looks like there are two 5/16" balls, one on either end of the rod.  When I took the motor apart there was only one.  Anyway, even if I put a second one in there the adjuster screw would still be way down inside the adjuster, no way I'm getting a nut on there.

I had been looking since last fall and TBH I couldn't find a set that I really wanted so I punted on the Chinese ones, figured why not. I really wanted to have the bike ready for this season but it's not looking like I'm going to make it, having to travel for work more often than usual.  So, timing is everything.  Be nice to get it running by the end of the season but actually making it to the track is going to be a next-year thing. 
Still need a set of pipes, a tank, seat, ignition, need to deal with the rear suspension.  Might explore the idea of using an R3 swingarm but IDK, that's too far away right now.

^this.  Also you get more accurate readings if you hook it up to a battery the bike isn't using.  Don't ask me how I know this, it's just something I remember from owning an old Triumph fitted with an electronic ignition. 

this is a very interesting discussion to follow  Seems like there are 2 people with competing visions, goals and budgets for this project.

The opening post seems focused on putting together a budget track ready racer who buys a cheap runner 250 and is willing to go with budget top ends, etc. vs. later comments focusing on precision like measuring weights of pistons and bore tolerance and contemplating upgrading to known quality pistons like Pro-X without even installing the kit and trying it out? Also wanting a baseline to work with.

Curious to see which side wins the day here, but I suspect the vision of closer tolerances, and upgrading pistons immediately might not be satisfied without a complete tear down of the motor to verify everything is in good usable condition for a baseline before getting in too deep 

Yes, it is a bit confusing.  I had a change of heart with the 250, it's a really nice bike (complete less the tank and everything on it works) and I decided to save it (the blue is really pretty), and now I'm using a set of 350 cases and gearbox I had with a crank from an R5. The 350 barrels that came with the cases and gearbox were trashed, one had a broken liner and the other liner was badly pitted. The crank was also way out of spec and had been welded up after it was presumably rebuilt, so it's now serving as a doorstop. I have the carbs off the 250 and that's all I'm taking off of it.
So yes, i did a complete teardown of two motors to get where I am now and I don't want to go through all this trouble to have the top end be my single point failure of a long project first time out.
Things are going great though.  The 350 already had the R5 primary gears in it, good thing because the R5 motor I had had a broken outer clutch basket.  Probably would have worked, presumably it was working, but that bike came to me as a roller with no carbs and some stupid front end off a Honda.
The 350 engine I have came from a guy into RZ's, said he knew nothing about it (ha!), and I probably paid too much for it.  Looked like it had been stored under a chicken coop.  One head had a crack in it that spanned the width of the head gasket and that's probably what killed it.  It had been raced, it had the R5 primary gear and the drain plug in the bottom was drilled for safety wire. I would've been able to save more of it if it hadn't been stored the way it was but whatever.  Being in Maine it's hard to find things like this, we're kind of at the end of the road in a lot of ways as far as that goes.
This is more a labor of love for me, I know an RD350 isn't as competitive anymore as more modern bikes in the same class, I normally race an SV650 and I do okay.  There is one guy at the track I go to who campaigns an RD and because I used to own one years ago I thought it would be fun to race one for real instead of just buzzing around the local roads. So that's what I'm up to.

Right I get that, but this is going directly to the track. I would like to have some sort of baseline to start with.

I measured the pistons and they seem really tight.  IIRC it's supposed to be .5mm clearance? 

Just a follow up with a couple questions.  Joined the Kveldvulf club the other day, cylinders arrived with pistons as advertised, found one piston was .75g heavier than the other, does that even matter, I don't know what sort of tolerance I should accept.
I read through a lot of threads about these cylinders and could've sworn that it's okay to run them with the pistons provided but couldn't find that particular comment. The porting is US/UK spec, but all the edges are wicked sharp so I'll spend some time on those.
So is the conventional wisdom to run the provided pistons or ProX?  If I get other pistons should I send them out with the cylinders to get them to fit perfectly and if so is it going to be such that I just by the first oversize ones or stick with stock bore?

Ahh, sweet.  I'm pretty far from needing it at the moment but thanks.
Still looking for a set of Denco pipes and some other doodads.

Is there a lighter weight chain conversion for the RD's?  530 chain seems a little heavy for these bikes; I see the newer 400's at the track with lighter chains on them, they look almost like bicycle chains, and those are a higher HP bike, so I was just wondering.

What are the forks from originally?
I've done a number of emulators in various types of forks, so need to know what your starting with.

The forks are from an rd250.  The shop manual doesn't show all the little parts that are on the damper rod as far as I can tell.  The damper rod has a conical thing at the bottom, and the open top thing on top where the spring rests, with a roll pin holding it on, but between these two there are a couple of baffles with holes, separated by a spring washer and a spacer, and a cylindrical spacer that fits around the conical thing at the bottom.  Not quite sure what to make of it because all this is below the spring, it seems like any rebound damping would be controlled by the gold valve, so why keep all the baffles etc?