Hardcore Biker
I was the tinkerer who originally fitted an XT350 piston into a '79XR250 motor.
I had the XR piston reliefs machined into the XT piston, thats all. Cylinder was re-sleeved to accept the 12mm oversive piston, and case halves machined also. I used an XR500 head gasket as the 500piston is 89mm!! XR500 carb and manifold bolts straight on. Would have benefited from a mild cam and free-er exhaust, but went really well (Pretty sure it still goes!!) Muzz,0211320763.
Hardcore Biker
"""I was the tinkerer....."" Well, I still AM that bloke!
Forum whore
Cheers Muzz -- once I get the sodding thing back from Holeshot, might give you call. What do you mean by the XR piston reliefs? For valves? I don't remember the piston having any dishes to make room for valves -- my RS piston has a couple, but that's because the camchain jumped off, not because it was made that way
Hardcore Biker
Not sure about RS pistons, but XR/XL ones have a raised dome with valve reliefs machined into them. Just had the positions,angles,and depth replicated in the XT's flat-top. Meybe a TT piston has a raised dome for a higher compression??
Scooter boy
Hi guys, first of all let me introduce myself: I am a middle-aged German (52), who - incidently - is working on the same topic: getting the Yamaha350-piston into the Honda 250RS.
My wife is riding the Honda and I own XT + TT350 Yamahas and thus the idea as I has both pistons in my hands already.
If you are still looking for a Yamaha piston, let me know: "second-hands" I have some, but merely for getting an idea, not for using....
Sometimes there are also XT350 pistons on ebay (NOS)....and should I get two, I would be happy to post one to you.
Will stay connected via this forum to hear about your results and will post mine as soon as I get forward.
Cheers for now
jester
Forum whore
Hey cheers for the post mate. Will be very interested to hear how you progress -- if you do end up with an extra one on your hands, it could be a good idea to buy it off you. You Germans seem to like big-boring CB250RSs -- a while ago there was a German-built ~280cc RS on Youtube, but now it's disappeared.
Right now the first task I have to do is rebuild my Super Cub so my sister can ride it to university -- then I can get stuck into the CB350RS
Scooter boy
280cc is fairly straight-forward: just slot in a 78mm piston and its done.
335cc requires a bit more of work: this 86mm Yamaha piston needs another sleeve as the Honda sleeve won`t take the big-bore.....so start to lock for a Yamaha XT350 cylinder in time. This sleeve has to be pressed out and shortened. Next: the crankcase won`t accept the big sleeve!!....I am myself looking for a crankcase to do the machining there...the engine of my wife`s RS will stay untouched!
Please let me know more about your XR-crankcase: whick model is it exactly?As there are basically no XR`s around here, which XL model will do the trick also (exact Model would be helpful - RS is MC02 for example)...
Hardcore Biker
http://www.bikebandit.com/houseofmot...m8347sch227992
That the one you were thinking of last month?
Forum whore
So you're going to use a sleeve from an XT350? I was thinking about getting one machined up from nothing.Originally Posted by jester_parks
If you're referring to me with the XR crankcase, I don't have one -- I have three RS engines in bits. All of them are MC02. As far as I can see (I have one crankcase split open), it just needs the `entrance' ground out to be a bit wider (6mm all round). Unsure what to use for a base gasket, perhaps have to have one made up as well -- the normal base gasket isn't much more than a centimetre wide anyway.
The XL250S (between 1978 and 1982 I think) and the XL250R between 1978 and 1982 (actually I can only see information on a 1982 XL250R, so perhaps that was the first year they made them) are the same engines as the CB250RS. Differences in piston compression, valve size, header size, and later on with the XR250R, only one balancer and 6 speeds. The crankcases should be the same though -- apart from the CB250RSD MC02E, which was electric start. I can't help you with the proper engine numbers, can't find information on that unfortunately.
That's what we're looking for!
Scooter boy
According to the Wiseco-list, the XL250 in the years 1979 - 82 and the XR250 up to 1983 should be identical to our RS-engines: 74 x 57.8 mm and 19mm piston-pin. Later on they had 75 x 56.5mm and 17mm piston-pin.
No idea about the gears and balancers though, but fancy the idea of 1 balancer shaft only "cog-driven"!!
Will keep an eye on that!
Base gasket I will use the original one and cut it to size.
Head gasket - I think it was mentioned earlier - XR500??
Forum whore
That would be the XR250R -- the last model they made before the move to 75mm piston (RFVC engine). 6 speeds, single gear driven balancer. A member on here converted a CB250RS engine to use the gearbox and balancer; he used an XR250R crankshaft as well. I have the 6 speed gearbox and balancer at home; will convert a CB250RS crankshaft to suit, as the XR250R crank I have has massive big-end float. I will just change the sprocket to a gear.
Is there enough meat in the base gasket to cut out? They look rather narrow, but I haven't done the measurements yet.
Hardcore Biker
sounds like a great project
you could just get some gasket paper and make a base new base gasket take 5mins its a magical mysterious product
dealers can be surprisingly helpful
the xr and xl pistons are quite different bigger dome on the xr and bigger again on the weisco
whats the bore size of the 500? maybe is sleive would work
"The world is a strange sad place. Ride as often as possible and try not to think about it".
Forum whore
Scooter boy
trust me: as soon as I`ve got my set (piston & sleeve, that is), I will get another set for you.
Yesterday my (japanese) wife translated an article about a japanese RS into english, which was quite interesting: this guy used a 84mm XR350 piston and in order to fit it to the small-end, made the conrod himself....
apperently his engive is revving to 10000 and would do more....
My Keihin FCR is ordered in Japan and things are falling into its place!
Forum whore
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
Reply With Quote
Bookmarks