1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Help CB250RS (A) with XL250S head???

Discussion in 'Honda 250cc Singles' started by matthewb77, Aug 19, 2021.

  1. matthewb77

    matthewb77 Member

    Messages:
    7
    Likes Received:
    2
    Trophy Points:
    13
    Joined:
    Aug 19, 2021
    Gender:
    Male
    Location:
    Auckland NZ
    My Bike:
    1981 Honda CB250RS
    Hi
    I've recently bought a CB250RS mostly in bits and am putting it back together. It runs fine without any obviously terrible noises, but I haven't actually had it moving.
    I am trying to put the exhaust pipes on properly, however have run into an issue - the exhaust ports are too small diameter for the rings on the ends of the pipes to fit. 33mm port with a 38mm pipe end ring.
    There is also no screw hole for the tacho drive to be fixed in position with - it rotates freely currently.
    The paint on the head appears slightly different to the rest of the engine.
    This has led me to believe I have an XL250S head on a CB250RS motor - these bikes are supposed to be the same engine, but photos show this is not exactly the case, they are just very very similar. XL250S doesn't have a tacho so doesn't have a screw hole, and just has some sort of seal or blanking plate put over where the drive comes out of the head. The exhaust pipes also maybe look a bit smaller which would correlate with the small exhaust ports.

    So,
    1. Does this hypothesis seem likely?
    2. Is this going to cause problems in the long term? - the heads are supposed to be a bit fragile on these machines so putting the wrong one on doesn't seem like the greatest idea. But then again, it runs fine and these engines are supposed to be very similar.

    Thanks,
    Matthew
     
  2. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

    Messages:
    5,975
    Likes Received:
    4,476
    Trophy Points:
    1,148
    Joined:
    May 4, 2013
    Gender:
    Male
    Occupation:
    Retired
    Location:
    Tamworth, NSW
    My Bike:
    1937 Royal Enfield 250, CF Moto 250 V5, Honda's XL250, CBR250, VT250F2, FT500 plus a few others.
    The XL250s head just uses a rubber plug in the tacho drive hole, the XL250r uses the tacho drive, but I do not think it would matter to the running of the engine.
     
    • Like Like x 1
  3. gregt

    gregt Well-Known Member Premium Member

    Messages:
    276
    Likes Received:
    233
    Trophy Points:
    248
    Joined:
    Mar 26, 2016
    Gender:
    Male
    Location:
    Hororata NZ
    My Bike:
    many and varied
    Yes, that's what you've got. Short term the fix is to grind down the flange OD on the pipes.
    There's a thread here on CB250Rs's in which i note some of the head differences.
    If the screw holes in the rocker cover don't line up with all the head holes, you may have the RS cover, cam and rockers fitted.
    Which is possible but really needs a couple of holes tapped to do properly.
    Using the XR/XL head won't be a problem. It's probably been fitted because the RS one cracked.
    Sadly, they all do eventually.
     
    • Like Like x 1
  4. TechHeadFred

    TechHeadFred Well-Known Member

    Messages:
    1,180
    Likes Received:
    732
    Trophy Points:
    498
    Joined:
    May 11, 2014
    Gender:
    Male
    Occupation:
    IT Systems Engineer
    Location:
    Canberra, ACT
    Home Page:
    My Bike:
    2007 CBF250F, 2004 KLR650, 1992 FZR400, 2 x CB250RS (1980 and 1982),
    From memory the XL250S had a 2-in-1-out exhaust system, which would explain the slightly different head.

    I have two CB250RS, one A and one C, have had both running and one even registered and on the road a few years ago.
    I also bought a third one that is now just parts. Among those would be a spare head that had a seized piston in it that might be ok after a good clean. It might need a re-bore though, couldn't honestly say for sure!
     
    • Like Like x 1
  5. matthewb77

    matthewb77 Member

    Messages:
    7
    Likes Received:
    2
    Trophy Points:
    13
    Joined:
    Aug 19, 2021
    Gender:
    Male
    Location:
    Auckland NZ
    My Bike:
    1981 Honda CB250RS
    Thanks for all the help - I've ground the existing ring off of one of my pipes and welded another small ring further up (for the diamond-shaped bracket to push against), which works well in holding the pipe.
    [​IMG]

    I've also noticed something moderately concerning: the right-hand, sparkplug side head bolt has been enormously overtorqued, resulting in it breaking through the fin on which it is meant to rest. So the bolt head is now recessed a bit. Does this seem like a big problem? I've only seen it running for five minutes so I don't really know if it was leaking gases (also the exhaust wasn't on properly so that was leaking obviously). The head gasket appears to be supplemented by silicone.

    The left-hand bolt is correct, right hand wrong I think:
    [​IMG]

    Looking between the fins at the bolt head and washer:
    [​IMG]

    Fred - my understanding is that the my current head will lose me a couple of horsepower (because of rockers and porting) which I don't really care about, and I've already modified my exhaust, so I'll stick with it. Just hope it doesnt crack too soon - any tips for mitigating that? Gregt's old post suggested porting the head but that'll probably cost more than I paid for the thing. I guess just run it a tad on the rich side and be a bit gentle?

    For the record here's gregt's post about the XL/XR/RS heads: https://www.2fiftycc.com/index.php?threads/honda-cb250rs.2304/#post-140504


    Thanks again
     
  6. gregt

    gregt Well-Known Member Premium Member

    Messages:
    276
    Likes Received:
    233
    Trophy Points:
    248
    Joined:
    Mar 26, 2016
    Gender:
    Male
    Location:
    Hororata NZ
    My Bike:
    many and varied
    From memory the head bolts sit low so why is the front one raised ? Did the PO use a long bolt there and put a spacer under it ?
    I'd be seriously surprised if the head has collapsed around the bolt hole. I might be wrong, haven't got one here atm to reference.

    If I was you, I'd use the thing as is while looking around for another good head. Once you find one - doesn't matter if it's RS, XR or XL - make sure it's got good valves and seals and the threads are in good nick too. Then send it down here for a port cleanup. It's only about 15min work with the right gear. Call it $20 plus postage.
     
  7. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

    Messages:
    5,975
    Likes Received:
    4,476
    Trophy Points:
    1,148
    Joined:
    May 4, 2013
    Gender:
    Male
    Occupation:
    Retired
    Location:
    Tamworth, NSW
    My Bike:
    1937 Royal Enfield 250, CF Moto 250 V5, Honda's XL250, CBR250, VT250F2, FT500 plus a few others.
    That is how the head bolts sit on the XL head. Nothing is broken. The rear bolt is shorter.
     
  8. matthewb77

    matthewb77 Member

    Messages:
    7
    Likes Received:
    2
    Trophy Points:
    13
    Joined:
    Aug 19, 2021
    Gender:
    Male
    Location:
    Auckland NZ
    My Bike:
    1981 Honda CB250RS
    Thanks. I've had it running and it doesn't seem to leak gas around the head. Interesting about the bolts - the seat of the right hand one looks a bit funny (in a damaged kind of way) but I'll definitely trust your wisdom and not touch it.
    What does cleaning the ports up involve? sounds like a good idea (and price) regardless so I'll definitely take you up on that when I find one, thanks.
     
  9. grcamna5

    grcamna5 Member

    Messages:
    31
    Likes Received:
    12
    Trophy Points:
    18
    Joined:
    Feb 1, 2021
    Gender:
    Male
    Location:
    Davis,CA.
    My Bike:
    Honda CB125S
    Subscribed
     
  10. gregt

    gregt Well-Known Member Premium Member

    Messages:
    276
    Likes Received:
    233
    Trophy Points:
    248
    Joined:
    Mar 26, 2016
    Gender:
    Male
    Location:
    Hororata NZ
    My Bike:
    many and varied
    I've done a few over the years. It's basically cleaning up the cast in obstacles in the exhaust ports - and trying to straighten the worst curvature a little. I also make sure there's an unobstructed air passage through the outside of the head. I"ve seen heads where the casting flash left in there has blocked airflow totally - which doesn't help at all. Inadvertent italics sorry.
     
  11. grcamna5

    grcamna5 Member

    Messages:
    31
    Likes Received:
    12
    Trophy Points:
    18
    Joined:
    Feb 1, 2021
    Gender:
    Male
    Location:
    Davis,CA.
    My Bike:
    Honda CB125S
    Has anyone seen what the difference is between the early CB250RS head(kickstart only model)where the engine is supposedly rated at 26 hp and the later RS-Z electric start 33 hp model ??
     
  12. matthewb77

    matthewb77 Member

    Messages:
    7
    Likes Received:
    2
    Trophy Points:
    13
    Joined:
    Aug 19, 2021
    Gender:
    Male
    Location:
    Auckland NZ
    My Bike:
    1981 Honda CB250RS
    This website claims that they have higher compression - reading a manual or spec sheet might confirm this. Someone else will probably know more.
    "Cylinder heads were usually OK for about 40,000 miles on the kickstart model, then they would start to crack between the valves with consequent compression loss and hard starting. The electric start RS-D versions had higher compression heads and these could fail very quickly. I’d definitely advise buying the kickstart RS-A version if you intend to use the performance."
     
    • Like Like x 2
  13. grcamna5

    grcamna5 Member

    Messages:
    31
    Likes Received:
    12
    Trophy Points:
    18
    Joined:
    Feb 1, 2021
    Gender:
    Male
    Location:
    Davis,CA.
    My Bike:
    Honda CB125S
    I would like to see if a cracked head could be repaired properly so that it wouldn't crack again.
     
  14. gregt

    gregt Well-Known Member Premium Member

    Messages:
    276
    Likes Received:
    233
    Trophy Points:
    248
    Joined:
    Mar 26, 2016
    Gender:
    Male
    Location:
    Hororata NZ
    My Bike:
    many and varied
    A lot of people have spent a lot of money over the years trying...
    Generally, air-cooled 2 valve heads can be repaired satisfactorily. But air-cooled 4 valve heads not so.
    2V heads have more room and after you've pulled the inserts and welded everything up, there's still some untouched chamber roof left. Which will help it stay reasonably dimensionally stable.
    4V heads once the inserts are pulled have very little untouched roof. Weld it up and it all moves,,,,After machining and replacing inserts and plug thread, heat it up again - running - and it all moves again.

    Basically you can't fix this kind of design fault by putting it back to how it was, They crack because there is not enough material in the casting to offer a heat path which will keep that area at a safe temperature.
     
    • Like Like x 3
  15. grcamna5

    grcamna5 Member

    Messages:
    31
    Likes Received:
    12
    Trophy Points:
    18
    Joined:
    Feb 1, 2021
    Gender:
    Male
    Location:
    Davis,CA.
    My Bike:
    Honda CB125S
    I understand. thanks,Bill
     

Share This Page