Looking for some advice here. I found a running RD250 for really short money. I originally picked it up because I want to put a 350 top end on it, along with changing the primary gearing but other than that what else should I consider to make it work?
RD350 cylinders are hard to come by. The one set I have has a broken skirt on one side.
I read through the Kveldwulf thread and TBH I'm not afraid to try it, I can't imagine the quality has gotten worse and I don't mind having them bored to whatever. Can I run actual RD pistons in one of those cylinders?
Full disclosure, my intention is to put this on the track, and just sell the whole 250 chassis off without a motor because I have a 350 roller I'll use.
leave the 250 gearing in it will spin up quicker. ive run the kvekdewulf cyls for almost 4? years now no probs. yes you can bore them and run whatever you want
I run Kveldwulf jugs with Banshee Pro-X pistons on my 250 turned 350, and I use the 250 primary.
A tip with the Kveldwulfs - make SURE you are getting the US/UK spec. I ordered what was listed as US/UK, and was sent India-HT spec. The same seller on ebay sells both styles. Measuring the port heights is an easy check.
What's the us/uk port height spec for the exhaust? that's the only spec i ever see mentioned.
this one specifically https://www.ebay.com/itm/284796177040
Quote from: m in sc on June 01, 2026, 05:51:55 PMthis one specifically https://www.ebay.com/itm/284796177040
Everything (cylinders, pistons, rings, etc) for $169 and free shipping?
And they are decent?
There are even some similar kits out there for half that!
Wow!
i have a post on here about those in particular. look in the tech library for 'cheap cyls' post. the ones half that, i cant say for certain but 170 is suspiciously cheap. 80 is very very suspiciously cheap. there are different versions for sure.
I have some experience with the Indian spec cylinders. The biggest problem apart from the ports is that they are manufactured in such a way that the liners can slip down inside the casing in use. The head gasket then fails.
This can be fixed but my advice is to avoid them if possible.
It would be good if to know if the new US/UK spec cylinder liners have a flange or some other feature that stops the liners slipping. An X ray (aluminium is radiolucent) of the cylinders would show any flanges in the liner.
ive heard of those issues. these def arent them
is there an India spec cylinders by Kveldwulf ? I doubt. He has one type which has exhaust port at 29.5mm.India spec is at 32mm. India spec original Escorts Yamaha cylinders were as good as Yamaha ones, if the liner is loose that means the liners are replaced by someone
there are chinese copies of the indian lt ones, those are the ones to avoid, and that seller does not carry them
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?
conventional is to change them. me, i run the, cut the tab in the center off or radius it back, it needs to be addressed.
I measured the pistons and they seem really tight. IIRC it's supposed to be .5mm clearance?
It's nice to be precise, but these old 2 strokes are very forgiving and will run under pretty bad conditions. If the piston slides up and down through the bore, I'd run it and just break it in super carefully.
That said, I'd only run Pro-X pistons in my bike, after checking the bore. :dawg:
Right I get that, but this is going directly to the track. I would like to have some sort of baseline to start with.
Quote from: Tommo on June 25, 2026, 06:14:56 PMI measured the pistons and they seem really tight. IIRC it's supposed to be .5mm clearance?
.0025-.003
this is a very interesting discussion to follow :hifive: 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 :twocents:
I think you mean .05mm clearance, .002"
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
Quote from: RDnuTZ on June 26, 2026, 10:24:43 AMthis is a very interesting discussion to follow :hifive: 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 :twocents:
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.
Quote from: Tommo on June 27, 2026, 01:13:00 PMQuote from: RDnuTZ on June 26, 2026, 10:24:43 AMthis is a very interesting discussion to follow :hifive: 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 :twocents:
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 :vroom:
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.