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Thread: CB250G5

  1. #1
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    CB250G5

    Hi All,
    Just introducing myself here. I’m a few weeks into a total rebuild of a 250g5. Have attached some photos or many parts in boxes and shelves - pretty much the way I bought it several years ago for about £200. Just getting around to starting it now. Every part is a project no matter how small so will be happy share the process. The basket case came with a spare engine which I’m concentrating on at the moment. Mostly in preparation for the primary engine rebuild. As I’m sure you’ll all know there are many aspects to these bikes from finding original parts to finding closest paint match etc. these photos are just a flavour of the parts stashed in every nook and cranny

    but will do my best to keep you posted on progress. Any comments, tips, anything else very happily received. I’ve got a few replacement parts in boxes like rear mudguard, silencers etc which I’m sourced from US, Germany which are nice and clean so very happy with them. Anyway, onwards and upwards. Thanks again.

















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    Last edited by ancientdad; 11.05.22 at 7:52 PM.

  2. #2
    Benevolent Dictator ancientdad's Avatar
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    That is quite the project you have there. We're in it for the long haul so we'll be here as you sort things and complete each challenge along the way.
    (move along, nothing to see here)

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    Hello asbobs,
    That CB250G5 UK model reminds me quite a bit of my 1975' CB360T;do you know both the 250 and the 360 share the same crankshaft stroke of 50.6mm ?
    I would be interested to see if the cylinders/pistons(67mm bore)of the 360 along with the 360 cyl. head will bolt-up to your 250 engine . The valves on your 250 may already be the same size as the 360.
    I would do it if it was me over here with the CB250G because of the Interstate hywys. here.
    I imagine your UK motorcycle inspectors would view the large '356cc' cast into the cylinders and fail the inspection.. a way around that would be to install CB360 cylinder sleeves to your 250 cylinder block.
    I know that would take away from the originality of your bike though.

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    I imagine your UK motorcycle inspectors would view the large '356cc' cast into the cylinders and fail the inspection
    Once the bike is registered, as has to be done with imported bikes, it becomes an historic vehicle ( more than 40 years old ), and never needs to be looked at by the man ever again, MOT exempt. If the bike was a UK model, with a valid V5 registration document, better still.

    The only folks who might raise an issue are the insurers, but if you tell them the bike is a CB360 there'd not be an issue. Anyway, Dremels work quite well on cast alloy numbers ..

    I had a blue CB250G5 back in the day, never really gelled with it.
    1972 CL175K7
    1970 CB174K4, 'upgraded' to a K6 alike
    1971 SL175, with a few non standard parts !
    1998 CB600 Hornet

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    Hi all

    Thanks for the comments. I’ve got the v5 which is great. I’m going to keep to a faithful restoration trying to retain as much of the original as possible. As I got the bike so cheap I’m happy to spend some cash to do a good job. The main issue is the chrome work. I don’t mind a few specks hear and there but things like the forks are pretty bad as well as the rear fender but eBay has been good for original parts in original-ish condition with little or no rust. Also managed to get a good set of silencers from the states for about £250 so was please about that as new ones are difficult to find and used ones are often is poor condition. I’ll end up getting some parts rechromed but I’ve found good used parts would ultimately be cheaper than chroming.

    Engine paint has been an issue - which is one of the reasons I joined the forum. Duplicolor seems to be a popular choice in the states but it’s hard to find over hear. VHT is also proving difficult to find, at least in the ‘cast aluminium’ colour which I hear is a good match. Have bought some Motip engine paint to see what the colour is like. It has a gloss so shouldn’t need a clear coat but will see when it arrives.

    I’m most nervous about the fuel tank as that’s past a polish so will need a respray but as it blends from one orange into another I’m worried whoever does it won’t get it right. Will leave that until later anyway.

    Will post some photos as the number 2 engine rebuilt progresses.

    Thanks again


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    Hi all

    Forgive me if I should be starting a new topic elsewhere on the forum but I was thinking last night about bilbikek411’s comment on putting on a 360 head. As I’ve said, I’ve got a spare 250g5 engine that I had intended selling but now I’m thinking of keeping it and putting it in something else - possibly with the 360 head.

    Can I ask opinions on options for this engine in terms of what frame it might fit etc? I’m in Scotland so want desperately to avoid a Q plate so I’d love to hear any suggestions. It may be that any bike with a V5 would either have a same or better engine and any smaller frames (125, 175) may need fettling to fit so, again, could end up with either a Q plate or an unregisterable bike.

    Or should I just sell the engine?

    Many thanks.

    Asbobs


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    The 360 (and 250 variant) engine will only fit in a 360 or 250G5 frame. It can probably be made to fit other things but a fair to significant amount of modifications would be necessary to do so. Honda didn't build them, or intend for them, to be capable of larger engine swaps.
    (move along, nothing to see here)

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    asbobs,I recall member 'crazypj' having done this swap already and was hoping to hear from him on your thread.
    I hope he will get back to you if you send him a private message(sometimes it takes a while)asking about the details of installing a 360 top-end on your 250G5.

    I was just looking on a Google search about this and there's a certain member I'm familiar with who confirmed to some others that the cylinder head,camshaft,valves are the same between the CB250G5 and the CB360.
    I realize that 'just because it's on the internet,doesn't mean it's always correct' which I've found true many times.
    I just sent a PM to 'crazypj' asking his assistance to post to your thread.

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    Many thanks Bilbike, I’ll do that.
    Thanks again


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    Quote Originally Posted by ancientdad View Post
    The 360 (and 250 variant) engine will only fit in a 360 or 250G5 frame. It can probably be made to fit other things but a fair to significant amount of modifications would be necessary to do so. Honda didn't build them, or intend for them, to be capable of larger engine swaps.
    I'm probably wrong, but I had the idea the original 400/4 frame was basically the same as the 250/360 part.
    1972 CL175K7
    1970 CB174K4, 'upgraded' to a K6 alike
    1971 SL175, with a few non standard parts !
    1998 CB600 Hornet

  11. #11
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    Quote Originally Posted by Richard Pitman View Post
    I'm probably wrong, but I had the idea the original 400/4 frame was basically the same as the 250/360 part.
    I've never compared the two, but I'd be surprised if Honda used the basis for a twin frame with a much heavier 4 cylinder engine. Now that I compare the two I do see the similarities, most motor mounts are different but overall the main frame tubes and backbone are surprisingly similar though there would be a fair amount of revisions to the motor mounts necessary.

    honda-cb360tk0-1975-usa-frame_bighu0044f7125_20c6.jpg MjAxNjQzNQ-3eb5bfb0.png
    (move along, nothing to see here)

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    Quote Originally Posted by ancientdad View Post
    I've never compared the two, but I'd be surprised if Honda used the basis for a twin frame with a much heavier 4 cylinder engine. Now that I compare the two I do see the similarities, most motor mounts are different but overall the main frame tubes and backbone are surprisingly similar though there would be a fair amount of revisions to the motor mounts necessary.

    honda-cb360tk0-1975-usa-frame_bighu0044f7125_20c6.jpg MjAxNjQzNQ-3eb5bfb0.png
    Someone once told me on another forum that the Honda 650 Vigor single cylinder had essentially the same frame as the CB600 Hornet, four cylinder. Box section steel back bone frame connecting headstock to swing arm pivot, engine(s) hung underneath.

    EDIT Clearly wrong advice, having looked at the CNMSL fiches ....
    1972 CL175K7
    1970 CB174K4, 'upgraded' to a K6 alike
    1971 SL175, with a few non standard parts !
    1998 CB600 Hornet

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    asbobs,I've been looking quite a bit online,especially a member here(a decent,honest person who looked up the part numbers for the cylinder heads/parts on both engines)posted to another forum a while back and everything I see about the swap from CB250G5 up to CB360 cylinder bores says you can install a set of CB360 cylinders,cylinder base gasket,cylinder head gasket and pistons and then use your CB250G5 cylinder head,camshaft,etc. which will bring your engine up to CB360 specs.
    I see quite a number of posts from different places that the CB250G5 has the same cylinder spigot holes in the top crankcases as the CB360 and that the CB250G5 was made into a 250(down from 360)by using smaller cylinders/pistons for the purpose of regulations;actually a detuned 360 to satisfy engine size requirements in certain countries.

    I would like to ask you if the OEM mufflers you purchased for the bike say '360' stamped into them?
    The other way to tell is look for the manufacturer's muffler model number.
    They usually have an HM_ _ _ number stamped into the cases of the mufflers.
    I really like the upswept style of the OEM standard mufflers on the CB360T.

    The upgrade in engine size we're speaking about also reminds me of a motorcycle I own which is a 1981 Honda CB125S;this year/model is the first CB125S USA spec bike to have CDi ignition rather than ignition points.
    I was looking-up different part numbers using a Honda dealer website Partzilla.com and noticed the crankcases are the exact same part number(center-code number;there are a couple different digits in parts of the suffix of the number because the factory has stamped a different engine number)between my 81' CB125S and the 1979-82' Honda XL185S.
    I was able to install an XL185S crankshaft,complete top-end,etc. and just get myself another stock exhaust(CB350 right muffler)that handled the extra flow of the bigger engine.
    The bike now feels similar to a lightweight version of a 250.

  14. #14
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    Quote Originally Posted by bilbikek411 View Post
    I see quite a number of posts from different places that the CB250G5 has the same cylinder spigot holes in the top crankcases as the CB360 and that the CB250G5 was made into a 250(down from 360)by using smaller cylinders/pistons for the purpose of regulations;actually a detuned 360 to satisfy engine size requirements in certain countries.
    I think the same is true of the UK CB250 K0-K4, just the original 325cc engine with a smaller bore, for a period in the UK learner market when a 250 could be ridden on 'L' plates. 64mm bore in the 325, 56mm bore 250, both 50mm stroke. All other parts the same, AFAIK, apart from jets.
    1972 CL175K7
    1970 CB174K4, 'upgraded' to a K6 alike
    1971 SL175, with a few non standard parts !
    1998 CB600 Hornet

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    Quote Originally Posted by bilbikek411 View Post
    asbobs,I've been looking quite a bit online,especially a member here(a decent,honest person who looked up the part numbers for the cylinder heads/parts on both engines)posted to another forum a while back and everything I see about the swap from CB250G5 up to CB360 cylinder bores says you can install a set of CB360 cylinders,cylinder base gasket,cylinder head gasket and pistons and then use your CB250G5 cylinder head,camshaft,etc. which will bring your engine up to CB360 specs.
    I see quite a number of posts from different places that the CB250G5 has the same cylinder spigot holes in the top crankcases as the CB360 and that the CB250G5 was made into a 250(down from 360)by using smaller cylinders/pistons for the purpose of regulations;actually a detuned 360 to satisfy engine size requirements in certain countries.

    I would like to ask you if the OEM mufflers you purchased for the bike say '360' stamped into them?
    The other way to tell is look for the manufacturer's muffler model number.
    They usually have an HM_ _ _ number stamped into the cases of the mufflers.
    I really like the upswept style of the OEM standard mufflers on the CB360T.

    The upgrade in engine size we're speaking about also reminds me of a motorcycle I own which is a 1981 Honda CB125S;this year/model is the first CB125S USA spec bike to have CDi ignition rather than ignition points.
    I was looking-up different part numbers using a Honda dealer website Partzilla.com and noticed the crankcases are the exact same part number(center-code number;there are a couple different digits in parts of the suffix of the number because the factory has stamped a different engine number)between my 81' CB125S and the 1979-82' Honda XL185S.
    I was able to install an XL185S crankshaft,complete top-end,etc. and just get myself another stock exhaust(CB350 right muffler)that handled the extra flow of the bigger engine.
    The bike now feels similar to a lightweight version of a 250.
    Apologies for delay in replying particularly after your really helpful answer, so many thanks for that. I think 360 is the way I’ll go then.

    My silencers are HM369.


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    Thanks to all for your such detailed responses. And apologies for my delay in thanking you all. It’s very much appreciated as well as incredibly interesting to read!


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    The HM369 silencers are the correct ones.

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