Honda-SOHC
SOHC.co.uk Forums => SOHC Singles & Twins => Topic started by: Oggers on February 24, 2022, 11:28:07 AM
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Folks
Thinking about buying a 1981 (kisckstart only) CB250RS to recapture those heady days of my youth when I had one for a while. At the time, I thought it was a very good bike, but I never really took it apart to any great extent. The one I now have in mind has about 30K on the clock so I wouldn't mind delving into the innards of the engine to check things out. With that in mind, a few Qs if I may
How easy - or difficult! - is it to start exploring the innards of the engine. Is there anything in particular I need to be aware of?
What would you recommond replacing given the mileage - main bearings? crankshaft bearings? piston rings? cam chain? valve guides?
Anything else on the bike I should be mindful of?
Many thanks
Mark
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One of the forum has extensive experience on here (threads worth searching) of this bike that'll give you insight.
A beautiful rebuild to stunning condition along with trials and tribulations of engine work, I'm sure he'll contribute to help you soon.
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Hi Mark.
Here's a link to my CB250RSA restoration.
Considering the mileage of yours the items worth checking are the cam lobes and rockers, the cam bearings in the head and rocker cover (lack of oil changes or dirty oil will toast these leaving the head and rocker cover as scrap). The cam and rockers can be repaired by Newman Cams if they are not in bad condition. The rocker spindle pins can be a royal pia to remove, use plenty of heat and luck!
Renew the balance chain, CMSNL were charging £280 for a genuine Honda item but you can get a far better quality DID Go-Kart racing chain for £20, Andy the Chainman (he's always at Kempton Park Autojumble and has a website) got one for me and cut it to the correct length with a soft link.
If you need a rebore make sure the engineer doing the work is aware of the very tight clearance, don't do as I did and take it to an engineer used to old brit bikes and will overbore the barrel. I use SRS in Sutton for my engineering work or use Trigger on here as he's top notch and is used by forum members.
Good luck, post photos and there's lots of advice on here.
http://www.sohc.co.uk/index.php/topic,11929.msg91265.html#msg91265
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Another thread when the oil seal split resulting in engine strip to replace! Buy genuine seals if you can.
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http://www.sohc.co.uk/index.php/topic,15771.msg132065.html#msg132065
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Dave
Many thanks for this - much appreciated. If the cam bearings are gone are we talking new head? As for rebore I don't really want to go there as it simply is not worth the cost. If things have worn between rings and bore would it be worth simply replacing the rings?
Renewing balance chain - Was this easy or a pig?
I assume renewing cam chain and tensioner is easy enough?
Cheers
Mark
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Hi Mark.
If the cam bearing surface is badly worn in the head and rocker cover both are effectively fit for the bin. There was a guy in Carshalton who was developing a bronze bearing insert repair, I actually saw it but he went out of business. As long as the oil has been changed on a regular basis and kept topped up you should be fine but obviously you won't know until you remove the rocker cover.
The balance chain was easy to replace although you have to do it with the engine split. The cam chain is a hy-vo or Morse type and spares are available. The cam chain tensioner is a sprung blade type, mine had no wear at all despite 15k on the engine.
You could try putting new rings in if the wear is within limits but my thinking would be why bother when the engine is apart anyway, if the wear was on or near its limits I would go for a rebore every time. The new piston will have new rings in the kit, an IMD piston and ring kit is about £50, rings alone will be £25-£30 and you'll have to run them in. The biggest issue you will have is removing the rocker shaft pins if the rockers require replacement, they really are a pia!
These are great bikes, a real hoot to ride and the engine is really easy to work on.
Keep us informed of what you decide to do.
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Think I scanned these magazine articles on the CB250RS for Dave.
Useful reference
https://www.dropbox.com/s/318cabahyog0q9g/CB250RS_Strip_CMM_2011.pdf?dl=0
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Think I scanned these magazine articles on the CB250RS for Dave.
Useful reference
https://www.dropbox.com/s/318cabahyog0q9g/CB250RS_Strip_CMM_2011.pdf?dl=0
Yes you did Ash and they were a big help. I'm still looking for a FT500 engine if anyone has one lurking under a bench somewhere. I've just started back on restoring my Triton but the CB500RSA is something I hope to build next winter.
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Dave
Once again many thanks. All advice is useful I feel. I haven't acquired the bike just yet, but if I do, for sure I'll check it out as you suggest. My problem with a rebore is cost and who does it. Up here in the wilds of Aberdeenshire you may appreciate that such delicate machining is something of a rarity amongst the tractor fraternity and the lump hammer appoach taken by the oil boys' machinists is not particularly encouraging- I should know as I am one of them....I'll give it some thought when I look down the bore for those tell tale wear marks!
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Bought it.....pics when it lands from the courier.....
Dave - stand by for some questions over the weeks! Intention is to strip it fully, fit all the (new OEM) consumables that come with it, and fix whatever needs to be fixed. May fit SS fastenings just because I like the look of them. I also want to delve into the engine. Cam chain and balancer chain would seem due for replacement, and I want to check out any potential for worn bearings.
Has a round headlight - which I prefer, and a Motad exhaust - which may or may not survive my aesthetically critical eye.
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Dave
Once again many thanks. All advice is useful I feel. I haven't acquired the bike just yet, but if I do, for sure I'll check it out as you suggest. My problem with a rebore is cost and who does it. Up here in the wilds of Aberdeenshire you may appreciate that such delicate machining is something of a rarity amongst the tractor fraternity and the lump hammer appoach taken by the oil boys' machinists is not particularly encouraging- I should know as I am one of them....I'll give it some thought when I look down the bore for those tell tale wear marks!
Two good engineering shops in Dundee, Agra Engineering or Engineering Resource. A lot of the local guys use them. 👍
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Dragon
Appreciate that - thanks. I really don't want to spend the money though. Good bike though it is, a CB250RS is never going to justify the outlay on things like a rebore and subsequent new piston/rings as you would never recover the costs. I appreciate that may not be the point for some folk, but at present it is for me. When I get it, I'll take a proper look inside and think some more, but PO seems to believe it runs well with no smoke or loss of power, so perhaps a philosophy of if it ain't broke then don't fix it may be in order here. Reading Dave's posts, cams and rockers would seem to fall into the same difficult to justify pot. I'll take the risk if they are not too bad, and definitely clean the mesh and replace the oil. Replacing bearings may be worth it, as would most consumables and chains, but that would be about it for me at least.