RM-250-A barrels

[huskibul]

  hey guy's i have an rm250-A project that needs a barrel and head,just wondering if anyone's done the number's on what if any other model barrels are interchangable ,B/C-PE  ?or is it A only ,any info much appreciated

[huskibul]

  Could someone please give me bore and stroke spec's on above barrels?,cant find it anywhere cheers

[lukeb1961]

70x64  RM250A
67x70  RM250B / PE250B


http://cid-407d7ef0965d3991.skydrive.live.com/browse.aspx/Racing%20Model/RM250A

[DR]

So far as I can tell the RM250A is a unique 1 off model bore and stroke (ala RM125A) I've never owned an RM250 to know for sure but I feel from what I found that the only cylinder to fit the A will be the A model cylinder. Someone please correct me if I am wrong

Bore and stroke

RM250A  70x64

RM250B/C/C2 67x70

PE250B/C/N 67x70

you beat me to it Luke The TS250C/N/T/X ran 70x64 also and there may well be a chance the TS cylinder would fit the A bottom end as both use the same base gaskets and share the same bore and stroke. Again, I'm not 100% certain but it all seems to point to this being a distinct possibility

[JC]

Trouble is, TS250B/N/T etc barrels are centre port exhaust, while RM-A is side-port. Could be modified tho

I also believe RM250-A barrel is unique to that mdel only

[lukeb1961]

chance the TS cylinder would fit the A bottom end as both use the same base gaskets and share the same bore and stroke. Again, I'm not 100% certain but it all seems to point to this being a distinct possibility

It's much more interesting - that base gasket (11241-41100) is common to RM250A/B/C/C2 and the TS250B/C/N/T/X and the PE250B/C/N and the DS250T.

We pondered this as a potential 270cc PE250B a while back. I never got the time/chance to confirm it though.
Luke

[DR]

Spot on JC ..the TS250's post '76 are centre port..I never thought of that

Just on a side note, the A, B, C and C2 RM250's also share the same rod which points to the A having a different crank..makes me sort of wonder if the crank from a B/C would work in an A if the matching B/C cylinder was fitted also

[JC]

That sounds like the better way to go Doc

[huskibul]

  iv'e been down that road a little with the 125-A which has an 87mm crank compared to a 90 on the B/C ,hence wont fit in the different case's, not sure if the 250's are the same,the 250 B/C must have more offset on the crankpin to get the stroke , would anybody have a B/C crank i could compare diameter's?  the PE would they have different taper on shaft(external flywheel etc?) thanks for the info

[LWC82PE]

There was a thread on this  about 2 weeks ago, probably in Tech, bike talk or suzuki boards, you will find the same answers there.
Here http://ozvmx.com/community/index.php?topic=12659.0
But yeah to answer the different strokes but same rod scenario has to be that the offset of the pin.
What you want to be doing is measuring/comparing part numbers for left and right bearings. Dont worry about different crank tapers and what ever, that can easily be fixed and would not be a drama.

[huskibul]

  pe, the N/T both rm and pe (if their anything like the 125's) are a complete different animals iam not expecting any of it to fit,thats my reason for new thread, the base gasket,s may be the same but that seems to be all, the a-b/c mains are the same,but the physical size of the crankshaft (wheels) have different #'s(alpha)and I have  measured 125 A-B/C-and T crankwheels and they all step up in size 87-90-93.7mm(they just dont fit in case's) iam guessing the 250's may be the same ,the only way is to measure a B/C crank and put up against my A crank,if that doesn't pan out i maybe need to go to A barrel only? ,limiting availability on lot's of gear . anbody got a B/C crank they could measure?cheers

[JohnnyO]

There are always A model barrels on ebay in the US, it will save a lot of headaches.

[huskibul]

  Yeah no worries johnny,theres plenty of A and b/c barrels come up in US , but  I   and ia'm sure others would like to know once and for all if it's possible ,also someone told me that the 125-A only has 2 o/s pistons (the B has eight) available, haven't checked the 250's out yet ,one step at a time,surely their's some B cranks(shot or otherwise) out there,that we could take 1 measurement off (crankwheel diameter) , when it's done its done one way or the other.  cheers

[crs-and-rms]

just measured a and b crank same dia, same lenth on the primary drive side ,same lenth on the stator side the only diffrence is the location of the crank pin thats how they make the stroke diffrent  so a B or C crank will fit in an A crank case but there are lots of a barrells on us ebay as some one else has allready stated

[LWC82PE]

If the cranks are different and the later crank with the different stroke wont fit in, you can always move the pin on the A crank. Specialist machine shops can do it for you, its not scary task. but as a few others have said, just get a A barrel off ebay or what ever and save the hassle. if you can only get one on max bore you can get them re-sleeved aswell.

Now i am 99% sure that the TS250 70mm piston will fit in the RM250 A. I did have a NOS RM250 A piston a while back and compared it to my TS250 piston. I wanted to see if i could use the RM250 A 70mm piston in my PE250. it wasnt an advantage over the TS250 piston i already knew would fit correctly so i gave the RM piston to a guy in USA. From memory the dimensions were the same when comparing RM250A piston to the TS250 Piston. The only major difference was that the cut outs in the side of the piston skirt were angled to the front or rear. The TS250 doesnt have that and the cut outs in the side is an even rectangle cut out. i dont know if that would effect things??? If not then you instantly have a source of pistons from 70-72mm. In wiseco you have 70, 70.5, 71, 71.5 and 72. There are other brands available too. Im pretty sure that ProX goes up in .25mm increments instead of .5 but they still only go to 72mm max.

So in summary, if you can not get a 70mm stroke crank to fit in or dont want to modify the 64mm stroke crank to 70mm, then fitting the 77/78 67MM barrel is only going to reduce your capacity to 220cc at best and i believe there would be some port timing issues aswell, that would be the main propblem i reckon as the piston wont have a long enough stroke, to reach TDC with the 67mm barrel. I reckon. Thats only my theory so it could be wrong, it really needs to worked out with real parts in hand. I could measure the heights of some barrels and that would reveal more.