To bore, and how much?
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Thread: To bore, and how much?

  1. #1
    Junior Member woodland_hoodlum's Avatar
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    To bore, and how much?

    Doing a top end rebuild on my '73 350G. First-timer getting this deep into the engine, feeling pretty good about it so far but am hitting the point where I need some advice-
    To preface my questions, I started this job because the left side was not firing and I came to find that there was bad compression- about 30 if I remember right. Compression on the right side was around 150psi. I added some oil to the left side and tested again and it came up to about 90 psi, so I figured it was rings.
    So i've taken it down, and took measurements throughout both cylinders, and found that BOTH of them were out of standard specs, for the most part.
    Both sides had a total average measurement of 2.5210" with the largest measurement in a couple places in each cylinder being 2.5217" (keeping in mind that this was my first time using snap gauges and micrometers, so error was likely, however I took over 35 measurements in each cylinder to double check, then averaged measurements in each position)
    So, do I need to bore? How much? I've seen that Common Motor has oversized pistons up to 2mm over, but the factory manual says to replace the sleeve if boring over 1 mm is needed.. maybe im misunderstanding the language in the manual? Anyway, any advice is appreciated!
    Thanks.

  2. #2
    Senior Member ancientdad's Avatar
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    If the FSM says that (and I don't know that for sure) it's just Honda being typically overprotective in the interest of reliability, as always... my 450 (and a previous one as well) is bored 4mm oversize on the stock sleeves and I ride it on the weekends like any other street bike I've owned in 50 years of riding - and it survived the heat of Barber last October as well, riding it around the grounds every day for the entire week (I was there with a vendor so longer than the 3 day weekend the public sees) including all the traffic of Friday through Sunday. Going 1 to 3mm over on stock sleeves is not that big a deal
    Tom

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  3. #3
    Senior Member mike in idaho's Avatar
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    Honda only offered oversize pistons up to 1.00 mm oversize, for anything bigger than that they expected to sell you a new liner or cylinder. I would only go as big as is necessary to clean up the bore, the more meat you leave in the cylinder the more room for future rebuilds. Parts in the .25 to 1.00 oversize range will likely be easier to find in the future, using genuine Honda parts means quality will be consistent also.
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  5. #4
    Senior Member tbpmusic's Avatar
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    Let your machinist tell you how much you need to bore to clean things up. The cost will depend on how many "passes" he thinks he has to make.
    Then order what he says - parts in hand, he'll be able to do a proper job.
    Bill Lane
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    Senior Member chaindrivecharlie's Avatar
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    Sent from my SM-N920R4 using Tapatalk
    Charlie

    1972 CL 175 K6
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    Joshua 24:15

  7. #6
    Senior Member TOOLS1's Avatar
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    Here are the pistons you need.
    https://www.ebay.com/itm/68-73-HONDA...8AAOSwAuZX5gbw
    I have used several of these piston sets and they are very good quality. I have bored several cylinders for members here for these pistons.
    TOOLS
    Bill H likes this.

  8. #7
    Junior Member woodland_hoodlum's Avatar
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    Quote Originally Posted by chaindrivecharlie View Post


    Sent from my SM-N920R4 using Tapatalk
    Charlie, why so quiet??

    Thanks everyone for the info! That all gives me a good starting point.
    Best,
    Spencer

  9. #8
    Senior Member chaindrivecharlie's Avatar
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    It had all been said, and i heard a lil voice in my head that said zip it!!
    Thats my story, and i'm sticking to it!

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    Charlie

    1972 CL 175 K6
    1972 CL 350 K4
    1981 CM 400 C

    Joshua 24:15

  10. #9
    Senior Member Erwin's Avatar
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    If it is just a road-going bike: Go as large as you need to, not more.

    You can go up to 67,5mm, if you want. That will work in the original liners. But you'd have to but aftermarket pistons like from Todd Henning Racing. Honda sells piston up to +1mm in increments of .25mm. If you buy Honda pistons, be sure to get the piston rings that have the three part oil ring.
    1972 CB350K3 General Export | 1976 CB200T K0

  11. #10
    Senior Member tbpmusic's Avatar
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    If you bore fore displacement, that limits your future - if you ever need another bore you're screwed. Largest I ever saw was 12'th over (3mm) - if your 12'th over fails you'll need new sleeves.
    Otherwise, just bore enough to clean the cylinders up, leave yourself some headroom in the future.
    Bill Lane
    Honda Twins Founding Member

    No advice, just info

    '75 CB200T, '71 CB450 K4 Half-Breed, '72 CL350 (Sold), '81 CM200T, '72 C70M
    and a little red Chineeze scooter

    http://www.bikeexif.com/honda-cb450-restoration

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