OzVMX Forum
Clubroom => Bike Talk => Topic started by: albrid-3 on May 14, 2009, 06:05:16 pm
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Why hasn`t this been done, has anybody made or turned a honda 73 - 74 250 Elsinore into a big bore. 300cc or 360 cc could it be possible to do.
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It possible because Honda was racing them in the US in 74 as a RC450. I've got some photos of the bike at home, it was basically a CR250M with a HUGE barrell and head on it.
But apart from making it as a garage queen it would not be a worthwhile project in my opinion. The 250's gearbox can bearly handle standard HP let alone the torque a big bore would put out.
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I doubt that honda used a CR250m bottom end for their factory 450. They made totally new works bikes every year and hardly anything on them resembled production stuff. A 250 bottom end would need a massive piston to take it to 450cc and it wouldn't be very nice to ride with such a short stroke. It might be possible to bore a 250m to 300cc using the piston from something else.
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Over a certain point, usually about the 285cc mark transfer duct entry starts to get compromised, then only a new cylinder with more efficient ports can take advantage of the bigger bore.
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COULD YOU BORE IT TO 263 WITH OUT A PROBLEM, THEN THAT WOULD LEGAL FOR THE BIG BORE CLASS.
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I doubt that honda used a CR250m bottom end for their factory 450. They made totally new works bikes every year and hardly anything on them resembled production stuff. A 250 bottom end would need a massive piston to take it to 450cc and it wouldn't be very nice to ride with such a short stroke. It might be possible to bore a 250m to 300cc using the piston from something else.
All of the factory bikes in 73 were identical to and based on the standard CR250M. The only specials they ran in 73 were at Daytona which "Honda had put a little more capacity into than the AMA would have liked" ;D as per Gary Chaplin. For the rest of the season the factory team ran stockers.
The RC-450-73 obviously had a longer stroke and different Barrell but the bottom end was all CR250M. Even in 74 the "Trick" RC-74 250 was just a tarted up proddie bike which is probably why they were so far behind the other brands in 74. they didn't start making the full factory specials full of unobtainium until 75 with the RC-250-75 and the RC-400-75 (the RC125 was still similar to the proddie bike)
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has anybody got any photo`s of the 73 - 74 model big bores
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A crank with a long enough stroke to make 450cc's wouldn't fit into 250m cases, the crank wheels would be too big. I don't think honda had a 450 in '73, isn't that why gary jones rode a maico in the trans ama series?
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Gary Jones rode the Maico because he got sacked along with Don & Dewayne by Honda prior to the Trans-AMA series.
Gary Chaplin rode the RC-450-73 in the 73 Trans-AMA, didn't do anything special though, probably because it was just a converted 250 not a ground up open class bike like the other brands.
Even though it was called a 450 no one really knew what capacity it was, may have only been a 280 or 300 etc. Just as with the later RC-400-75 wasn't a 400, during the season it was raced as a 360,380,400,420,460,480. Honda did some weird stuff back then as they were finding their feet with 2 strokes
Dave I've got some photos of the RC-450-73 and most of the factory Hondas up to 1980 (including my favorite, the RC380-79 VLRS) but they don't scan very well, I'll try the scanner at work. They are classic photos, lots of porno mo's & body shirts in the pits back then ;D If not i will bring them along to Campbelltown so you can have a look
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Thanks mick, would be happy to see them. dav
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way back when Ray Ryan and another WA guy who's name I can't place built what I think was a CR360 based on the 250M. I had an article that went into detail somewhere but typically I can't find it when I want it ::)
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Why hasn`t this been done, has anybody made or turned a honda 73 - 74 250 Elsinore into a big bore. 300cc or 360 cc could it be possible to do.
Dave..The factory actually tested a 360 (or maybe it was a 320, I can't remember based on a bored and stroked 250 but the gearbox had enough trouble putting up with 250cc, making a big 'un was out of the question without a major gearbox revamp or switch to 4 speed which was knocked back by the bean counters at head office.
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ok, got my facts wrong as usual. Ray Ryan did the report and the bike was built by Rick Gill ;)
(http://nvr0jq.bay.livefilestore.com/y1pi5c7NfXQnnUP12oDYpAHRi8k_cLlVW1DS9AClkVZXASaked9H4PxGOHB0j4kri9hawvQxj3naQvghsNUz7xZ2aIHgp_JBQuT/Honda%20CR360M.jpg)
Full size here
http://nvr0jq.bay.livefilestore.com/y1pi5c7NfXQnnUP12oDYpAHRi8k_cLlVW1DS9AClkVZXASaked9H4PxGOHB0j4kri9hawvQxj3naQvghsNUz7xZ2aIHgp_JBQuT/Honda%20CR360M.jpg (http://nvr0jq.bay.livefilestore.com/y1pi5c7NfXQnnUP12oDYpAHRi8k_cLlVW1DS9AClkVZXASaked9H4PxGOHB0j4kri9hawvQxj3naQvghsNUz7xZ2aIHgp_JBQuT/Honda%20CR360M.jpg)
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Nice job ! Rick is still about, and still has a Honda shop at the same address...although these days, he races....grimace.... a car... :-[
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Thanks guys, you are a wealth of information, in that documant does it explain in detail the dimensions of the bike.
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That seems simple enough, a clever toolmaker / firey thinks !
A de-comp a la Maico etc would go a long way to keeping the kickstart in one piece. Had to laugh at their solution - no kickstart spindle !
So who is going to do one? And what a great way to recycle an old dunger. I'll go and warm up my milling M/C
Dave Mac :D
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That's a cool bike. Would it still be pre '78 legal if you were to forward mount the shocks like that bike?
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Very interesting. I often wondered if that supposed 450 in 73 was an oversize 250 nowhere near 450 in actual capacity.
Gary Jones raced it too - for a few laps at the 73USGP until it broke a frame. Did he get sacked before he moved on to Maico? My recollection is that, having sampled the honda 'big-bore' & knowing 1st hand how uncompetitive it was, he went & bought a Maico to ride the open class Trans-ama in 73 - sort of 'in yr face Honda, since you can't provide me a competitive big-bore'. Then Honda sacked him.
There were aftermarket kits available in US in early 70's to take the elsie out to 324cc using an 80mm Yam piston & new liner. (Also available for the Yam & Suz)
According to Motorcyclist Retro & Terry Goode, the 1st Elsie prototypes were 335s, but it's unclear whether that was their actual capapcity or just a code-name. Does anybody know? Interestingly 78x70mm makes 335cc.
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JC i tend to agree with you about gary jones riding the maico then leaving honda. Does anyone have a pic of a '73 450 honda? I'll check thru some old mxa mags and see what i can find.
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Actually, Gary was already in hot water with Honda prior to the open national. While he had ridden the big bore, one was not provided for him. The frames had broken earlier in the year-Don Jones Racing actually modifed production bikes with the works parts they wanted for the 250s.
Gary was in danger of not honoring his contract to ride all the nationals, so Don bought a 400 Maico -and Gary raced it-honoring his contract. There was some $$ on the line.
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Hmm, I just had a look at a pic of Gary Jones' 73 USGP 450 in an old MXA. Motor is definitely not std CR250 bottom end. It's only a small pic but it looks more akin to Karsmakers' RC400 in 74/75.
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what is the limit you go to on a standard bore with these honda cr 250, would one need to resleeve it, or could a change of barrel, like a barrel off another make be addapted. for example suzuki ts 400 barrel. or a later model cr barrel
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Anything can be done with enough time and money,increasing the stroke is a better option.
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Tried to get hold of our mechanic from 70s but think he moved to Auz for better sheep. we had a 74 CR bored to aprox 400cc with a triumph sports car piston,that bike won Beach racing national titles.
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Dave,
You'd need 72.25 bore (=264cc w 64.4 stroke) to get into Open class. Wiseco made up to 2mm o/s (ie 72mm) for most 250's of the era. You'd have to decide if 72 was the limit or if you could go slightly more. Thats if you can find a 72.25mm piston to suit. But would a 264 Elsie be competitive in the open class?
I have an MT crankcase in the shed & the spigot opening is 82.6mm dia. Wiseco/Serco listed blank flanged sleeve (from LA Sleeve) that is 82.5 OD & 76 ID which would slip in the cases & could be ported to match CR cylinder. If CR cases are the same that would give 292cc w'out machining the cases.
RM370 piston is 77mm = 300cc. Don't know what is 76mm - perhaps a watercraft/jetski.
Early 250 Canams (pre78) are 74mm so P8 Wiseco would be 76mm if you could find one. Deck ht & skirt length are close to CR/MT.
You'd need to enlarge the ports to compensate for more capacity of course or you may end up w a grunty slug. Remember Loz's comment above in reply #3.
Typically, when you big-bore a 2 stroke you lose height on the Ex & Trans ports & lose depth on the inlet - which is hardly ideal.
And you still have the gearbox/selector limitations to deal with.
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JC, THANK YOU SO MUCH FOR THAT INFORMATION, l WILL PRINT IT OFF. I WOULD LIKE TO MAKE THE CR LARGER, BUT IT WOULD NEED TO BE DONE BY AN ENGINEER.
CHEERS
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according the MOTOXFOX guide for CR's Chaplins 73 bike was a prototype.
the RC 125 only bared resemblance to a CR, up pipe ,centre port , split cradle??