RD250 to 350 questions,

[Tommo]

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

[m in sc]

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

.0025-.003

[RDnuTZ]

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 

[1976RD400C]

I think you mean .05mm clearance, .002"

[m in sc]

i forgot to put the " in my call out.

like ive said, ive run mine for years and beat te snot out of the pistons no issues

[Tommo]

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.

[RDnuTZ]

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.

well that certainly adds a lot of context and straighter path to what you are after: a known good set of cylinders & quality piston kits and everything fresh within proper tolerances for peace of mind. Why not look for a used set of OEM RD350 cylinders that are at least perfect externally (no chipped/broken fins, etc.) and depending on how much sleeve left, bore to whatever size allowable + match new piston kits of your choice? If sleeves are too far gone, those can be replaced as well to accomplish what you want and donor cyls probably cheaper b/c of condition. Good Luck 

[Tommo]

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.