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  1. #1
    CB750 Member mitch5069's Avatar
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    broken camshaft holder, leaking tachometer oil seal cb750 dohc

    So I was attempting to replace the oil seal for the tachometer gear and I ened up breaking part of the camshaft holder. I was wondering if anyone else has use some type of high temp epoxy to hold engine parts together. I dont believe there is much force on the part that broke off and it is more of a guide for the tachometer gear. I might try to run without it.

    Also has anyone ever replaced one of these seals? It seems to be pretty tough to get out...

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  2. #2
    CB750 Addict motogtp's Avatar
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    Per the Clymer manual I got the seal on my '78 Super Sport out by using a long, small screw (drywall type I think). Screw it into the seal and keep screwing it in. Once the screw lands on the metal on the bottom the threads in the screw will continue to pull it out. I thought that was crazy but worked like a champ.

  3. #3
    CB750 Member mitch5069's Avatar
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    Thanks for the heads up. I actually just picked at it and used a pair of needle nose to get it out. As for the cam hold I'm just going to run it without the broken piece (small part). I think it will work fine without it and I don't want to run the risk of it falling off in the engine.

  4. #4
    CB750 Enthusiast ray_'s Avatar
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    Hi, bringing this thread alive in 2019! Haha... So i am faced with a similar problem. While trying to take the tacho cable off of a 20-year-in-storage bike, the bolt holding the cable twisted on me. When I took it to a machine shop to be removed, while drilling the bolt to easy-out, the camshaft holder for the tacho cable cracked. Although its not broken yet, its definitely cracked. On inspection, I feel that the bearing surface and the bearing part of the holder is intact but just the tacho part is cracked. I am afraid that if i install the holder as is, it would break on me. Can this be fixed? Should i use it as is?

    https://drive.google.com/open?id=1Sr...Tf3_em9M4GmsAU

    https://drive.google.com/open?id=1he...9qzTgGAQ1Nz0bU

    https://drive.google.com/open?id=1Bd...fr17cuFpuUTpOU
    Last edited by ray_; 09-14-2019 at 11:44 PM.

  5. #5
    CB750 Guru amc49's Avatar
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    Yours and do with it as you will. Depends on how long that crack is.

    If the boss breaks off and then assembly gets into the cam you will destroy the engine there. Need a new casting BUT they are part of the head, the holes were all lined bored to match that one head only. If you can find another then you have to shim up or sand the flats down on a piece of glass to get back to the cam clearance there and not for the faint of heart, doing that wrong wrecks the engine too if done just so.

  6. #6
    CB750 Enthusiast ray_'s Avatar
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    Thanks for the information amc49. If you were in my shoes what would you do or rather how would you proceed? I know its a vague question but..... Also, by what you wrote, do you mean to say that if I find a replacement used holder, it would have to be trued to my cam and also clearenced as per my old holder?

  7. #7
    CB750 Guru amc49's Avatar
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    Yes, cam caps are not interchangeable unless reworked to fit, some few get lucky but simply throw one on there and you can burn the engine up in minutes if unlucky. Plastiguage your old one and match to that.

    I would get another part there, of course, it ruins a head taking it and why you don't find many of them.

  8. #8
    CB750 Enthusiast ray_'s Avatar
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    On inspecting thoroughly, I am confident that the bearing surface part of the cap is intact but the tachometer worm drive part is definitely cracked. I am thinking of getting these Argon Welded and using these (might cancel the tachometer drive altogether. Still thinking about that). What do you think?

  9. #9
    CB750 Enthusiast ray_'s Avatar
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    On inspecting thoroughly, I am confident that the bearing surface part of the cap is intact but the tachometer worm drive part is definitely cracked. I am thinking of getting these Argon Welded and using these (might cancel the tachometer drive altogether. Still thinking about that). What do you think?

  10. #10
    CB750 Guru amc49's Avatar
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    Can't say impossible but it would be dependent on the skills of the heliarc guy. Part prepping will come into play too as you may need to grind new trench to support new material laid in, you don't just weld 'on top' of the form there, you grind it out to make the finished part much stronger. My Dad would have been able to do it, I saw him weld cut-in-two 4 speed hardened cluster gears to make one off transmission gear ratios for our drag car. He used to weld the insides of Saturn 1B and 5 first stage rocket fuel tanks and had to pass X-ray inspections all day long. That man could weld anything on this planet using any welding science known on this planet.

  11. #11
    CB750 Enthusiast ray_'s Avatar
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    Update: so talking to a few people and discussing on another forum [needs logging on as a member tho](http://www.cb750c.com/modules.php?na...=383122#383122), I came to know that tachometer drive cam caps were indeed sold separately by Honda themselves. So, by that logic and knowing that no other caps were sold individually, I find it reasonable that the tachometer cap can be swapped among heads without worrying too much about align boring etc. What do you think?

    I am still gathering information so would love to know what you think.

  12. #12
    CB750 Guru amc49's Avatar
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    You do what you want to. You've been told what to watch for.

    https://www.partzilla.com/catalog/ho.../cylinder-head

    That is an official Honda parts breakdown and although the part is #3 and has a separate name and number it is also boxed in with the others as part of a major sub-assembly............the head itself. The caps get the flats cut and then they bolt onto head at torque and then the entire head gets two linebores done to finish all cam journal bores. The tolerances can vary up or down a bit to make the caps slightly over or under a true half circle and why you match them to the head. If you simply cut one 'big enough' to fit all heads with no problem then you have possibly a massive oil leak and the valves cannot be set so easily around that location because of the excessive cam looseness, they are already too loose as it is.

    One other thing............just because you can find a Honda part number does not mean the part was ever available, many specialty DOHC race parts had part numbers that when ordered even when the parts were available would say not available. That cap may well have a number assigned just to ID it in case of manufacturing issues, in other words for internal Honda use only. To zero in on production issues they have to have a part number to relate the issue to. I can also think of some engine mount shims that are like that on DOHC, they were fitted on the line if needed but you could not get them over the counter even with the number for them.

    Again, no matter, you know what to do.
    Last edited by amc49; 09-22-2019 at 05:14 AM.

  13. #13
    CB750 Enthusiast ray_'s Avatar
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    Hi again! So I did what you suggested and bought a replacement camshaft cap, had it checked for clearance and alignment at a reputable machine shop. The technicians have said that the cap will do and now I am in the process of putting together the head on the engine. Also, I got a valve job done too with new seals (valves were rusty and leaky). My question now is:

    1. Given the work done, will it require a specific break in procedure? If yes then what would be the right way to do it?

    2. I have done my best at cleaning the cylinder block surface using a scrub pad and WD40. The end result is not shiny clean but the surface is smooth and clean enough to not feel any undulations on my fingers. Any thing else I can do? Was that the right way to do it? Plan on doing the same for the head surface.

    3. The pistons have carbon on the tops. I plan on reassembling the engine with the carbon as is since I do not have a scraper that's gentle enough for the Aluminum. That ok?

    4. Anything else I need to pay attention to? Does then oil pump gasket need sealant?

    Thanks in advance!

  14. #14
    CB750 Enthusiast ray_'s Avatar
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    https://drive.google.com/open?id=1Kr...dLJe3URHpubM3V

    https://drive.google.com/open?id=1Kq...SykwbP4Tc56Saw

    Here are some pics of the cylinder block and the piston carbon build-up

  15. #15
    CB750 Guru amc49's Avatar
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    Gasket needs no sealer if the flats are OK.

  16. #16
    CB750 Enthusiast ray_'s Avatar
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    Okay. Thanks for the reply. I just took the head apart after coming from the machine shop and noticed that they managed to scratch a few of my bearings and even ding up one . Now I am concerned if the damage is too severe or will it be okay? Here are some pics.... the one highlighted in red is deep and can be felt with a fingernail while the other one cannot be felt.

    Wold really appreciate your expert opinion.

    https://drive.google.com/open?id=1Kt...nW8dmafwNVjpvc
    https://drive.google.com/open?id=1L6...rOw9z-NGSl8qAD

  17. #17
    CB750 Guru amc49's Avatar
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    Low spots don't hurt anything, it is the highspots that get you. A say Xacto razor knife pulled around the edge to remove the high if any is there, it looks close to OK as is. The second pic can have a piece of 600 sandpaper run across it to remove the highs, of course you have to clean the head good as you are right by an oil passage. The head is filthy anyway and needs cleaning badly.

  18. #18
    CB750 Enthusiast ray_'s Avatar
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    Quote Originally Posted by amc49 View Post
    The second pic can have a piece of 600 sandpaper run across it to remove the highs, of course you have to clean the head good as you are right by an oil passage. The head is filthy anyway and needs cleaning badly.
    What's a good way to clean the head? I was planning on using some water+detergent and some elbow grease for the external cleaning but was really unsure about what to use for cleaning the insides (cause I have just got the valves and stem seals installed). Will cleaning the oil channels with compressed air and draining them with gasoline or maybe oil do? How do you generally do it?

  19. #19
    CB750 Guru amc49's Avatar
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    Do NOT use any water-based ANYTHING to clean an assembled head! You cannot possibly get out all of the water from all crevices and hiding spots and if you let it dry then the valves may well rust stuck in the guides.

    I myself use fuel as it is cheap but you must apply common sense doing it, I ALWAYS do it out in the open and no smoking of course. I for one have the head apart to clean it, it allows getting all solvent out of the oil passages, not doing that can seize cams up pretty quick. You can blow out with air to open them up but you must pour oil over the cams before installing valve cover or the dry oil passages will let cams score in the few seconds of dry running.

    Proper for a head about to install is valvestems all lubed and cams at the journals too.

  20. #20
    CB750 Enthusiast ray_'s Avatar
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    Quote Originally Posted by amc49 View Post
    Do NOT use any water-based ANYTHING to clean an assembled head! You cannot possibly get out all of the water from all crevices and hiding spots and if you let it dry then the valves may well rust stuck in the guides.
    I understand. But isn't that so if the water gets inside the head? I am thinking of cleaning the spark plug region using this technique (with the cams, holders and buckets out) with the valve cover tied on using a rope. The cover would help keep anything from falling inside and thus allow me to clean my spark plug area. The only issue I see is the water dripping out of the plug holes and sticking to the bottom of the valves which I guess I could dry off and then spray with WD40 to prevent rust. Could that work?

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