1979 Honda CB750K DOHC Head Repair & Rebuild
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- Опубликовано: 7 апр 2019
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In this episode I’m repairing the head on a 1979 Honda CB750K (DOHC) and reassembling from buckets of parts.
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DISCLAIMER: The Information provided in this video IS NOT professional advice. It is simply a demonstration of how I performed this task and is for informational purposes only. If you are not comfortable performing this type of work, seek professional assistance. Always follow the manufacturer’s instructions when performing maintenance or installing parts and equipment. 
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An Excellent Video - thanks ! You are book marked in my Maintenance list -
Hi Tim..... just to chime in. Watched you're video here and want to mention a couple of things. First of all the CB750 DOHC's are known for valve failure usually associated with tight valve clearance. Owners don't realize that these shim on bucket engines do NOT loosen the valves with wear. They generally tighten themselves and therefor don't have the ticking of a loose valve. Since no noise is apparent the owners don't think a valve adjustment is needed. Then they start having a missing caused by loss of compression through a burnt valve. Can't tell you how many I've seen but it's a lot.
Therefore.... the first thing to do prior to assembly would be check the valves for leakage..... Then repair as needed and reassemble.
Now.... about your torque method..... I always start with hand tools first. Torque in a criss cross pattern like you mentioned to hand tightened. Then repeat with 1/2 torque value and repeat to full torque value.
Hi rayworx, thanks for this comment. Very good information.
@@Tim2Wheels I respect your ability to take criticism! Great video. I got a lot out of this. Subscribed.
Great video. Thanks for uploading it. I have a 79 cb750 (A restoration project) Going to tear down the whole thing and build the entire engine for the very first time. I hope I will get it done. Videos like yours are great help for first timers like me. will need your help. Thanks Again.
Thanks for walking us all through nicely. Great job!
Thank you very much, i was in search for a video of the dohc version for quite a while and your video is perfect for my needs.
Super helpful. I have an 81 cb750 and want to take a look inside the engine and possibly replace old parts. This made the process seem way less intimidating.
Great video Tim,I followed you through all of your KLR videos and I just bought a 79 cb750. It’s like Christmas!
Simply great!!! superb video!!
Hi Tim
I had a CB750F
Lovely bike!
Thanks for the video.
Rgds from S Africa
Clive
A note that Honda Tech told us about when I was at the dealership. When installing the 'tach drive gear' onto the exhaust camshaft.... install gently and slowly turn the tach gear so the gear engages into the camshaft gearing. The tach drive will crack if tightened and the gears are not meshed properly.
Very true. Thanks for adding that.
Thanks for the great video. I'm working on these steps now. Great guide
Very good video showing what I'm going to need to do to reshim my head. I'm used to shim under buckets in WaveRunners and did not realize there was a tool to make this easy without having to remove the cams first
Good video, I’m at this point on my bike and really needed this as I have dumped 4.5k into parts for this build. Can’t make mistakes at this point.
Great video, I have a motor and head at exactly that stage, and was intimidated if I could do it. I have the repair manual and gaskets coming, and with what you showed me my confidence is solid that I can do this. Thank you.
Sounds like a daunting task, best of luck Robert!
Very helpful content. Thank you
I don't work on bikes...and I your videos fascinating! Thank-you (subscribed!)
You Mr engineer it is a help ful and regarding to llustrat the engine assambl thank you
Good stuff!
Excellent.. video thanks for sharing
Thank you sir. Just bought one of these and see how simple the engine design is. Beautiful design. Chain tensioner basics, too.
Nicely done. I have an exhaust valve on mine that won't close properly. Its been sitting for about 10 years and It stays open about 0.2mm and won't seal. Not sure what's happening in there but this video will come very handy. Thanks.
The only other comment I would add is that it appeared that you installed the cams dry. Maybe you oiled the cam bearings, tappers and cam lobes off-camera, but I didn’t see any lube apparent as the cams were installed. Even one turn of a dry plain bearing can cause significant damage. It is best to use assembly lube rather than motor oil for such an installation. One reason is that assembly lube tends to be stickier and more viscous and will stay in place even if the engine sits a few weeks before first run. Second is that the lube tends to be high in zinc and or moly to provide good wear protection during that first start when all of the oil passages are dry and pressurized oil is not yet getting to the cams.
21:30
You must be a teacher?
Very Very good post!!!
Do or have you thought of rebuilding the whole motor?
I have that motor and if you go have one I would personally buy one from you!!! I don’t have anything wrong with mine but I’m 54 years old not going to have another bike and would like to put HP cams and done other fun stuff. Lol
Thanks
Dan
I just came on to watch this video about the shape of the headgasket. You seem to got the same one I did does it matter that it’s not round and it hangs over the cylinder a little bit.
The Honda manual is correct. Bolts are specified by the shank diameter, not the size of the wrench used.
As to torquing, I can’t speak from a mechanics perspective, but from an engineer’s perspective you only want to torque in stages if the manufacturer recommends that. The reason being that once the motion of the nut or bolt stops, you go from the dynamic friction regime to the static regime. Static friction is usually at least 20% higher than dynamic and can be higher than that depending on the materials, thread pitch, dry or lubricated, etc. The danger in partial torquing is that you can get too close to the final torque such that the final torque setting may not be high enough to overcome the static friction and thus you may not get the bolt to the specified value even if the wrench clicks. That is why many manufacturers now use torque to angle and torque to yield style fasteners. A torque to angle approach ensures you get to the final torque as you aren’t torquing to a torque value, but torquing to a rotation angle and that rotation angle is completely indecent of static vs. dynamic friction. However, the downside of these type of fasteners is that you generally yield the metal and thus the fasteners can’t be reused and must be purchased new each time.
You are generally safe if you stop at 50% or less of the final torque value, but if you get to 70% or more, then the odds of getting the correct final torque diminish dramatically. Ideally, torquing a fastener should be a smooth and continuous motion. If the process is ever interrupted prior to achieving final torque, you should completely loosen the fastener and start again, not back up and continue on as folks with ratchet style torque wrenches often do. Looks like a fun project.
From a technicians perspective (35 years Honda Certified/Registered tech-retired now) your torquing in stages is exactly correct. I like to torque gently snug in the inside-to-outside pattern. Then I return to the center bolts and torque to 1/2 of the torque value. I repeat to 5 ft/lbs less than specified final torque. Final torque to specified torque repeating the inside to outside pattern.
Nice and helpful video!
How did you manage to retract chain tensioner so you could have some slack to install the camshaft gear?! This is the hardest part!
great video! quick question, how did you clean the gasket surface ? blade or sent it off to a machine shop?
The normal way to describe a bolt is the nominal diameter over the threads not by the wrench size. The crazy BSW standard did that years ago.
BUENAS NOCHES SEÑOR MUY BUEN TRABAJO ME VOY A APOYAR PARA UN PROYECTO DE RESTAURACION DE UNA HONDA CB 1980 GRACIAS Y SALUDOS DESDE PACHUCA DE SOTO HIDALGO
Hey, Tim! AWESOME video. Thank you so much! I’m currently restoring and modifying an ‘81 cb750c that has the same DOHC. I am wondering if you could please point me in the right direction to find one of those great printouts with the instructions like you have in the video? I’m only a few weeks in and already I’ve messed up knowing which bolts go where to reartach the starter clutch cover on the lefthand side (some bolts are longer than others). I have it only loosely secured at the moment so I don’t strip anything. I have Haynes and Clymer manuals, but I need something like you have here. Know where I can find one specific to the ‘81 Honda Cb750c? Thanks so much!
ifihadaboat85 there a a PDF version on live if you google 81 CB750 service manual. I down loaded one last month and they are free
How do you align the tach holder to the can ?
Nice video by the way !!!@
Good video I'm slowly taking my head iff fir vrojen bheader bolts x5 I'm surprised u didn't use copper slip or similar ..... I know you be using nuts to fasten headers but if they ever need to come out you could have my dilemma
These CB750s, specifically the 82 Nighthawk.. Can the cylinder head be removed with the engine still in the frame?
Nice video. I was wondering, I saw the squirter can but noticed you didn't highlight or apply assembly oil on the camshaft ends or other area's. While I am certainly NOT the expert, every engine assembly I have ever seen that was done with high precision always use assembly oil/grease in these places to prevent premature wear before the engine comes up to full oil pressure. Seeing as how everything is cleaned and dried, just wondering how you feel about that?
Oil also over time drains back and leaves almost zero oil if not started periodically. After the winter hibernation, I always crank the engine to full operating pressure with the kill switch off just to get oil to places it need to be before placing a load on the engine. Just my thoughts out loud. Thanks.
You stated that the when installing the cams that exhaust cam lobes point towards the spark plugs do the intake valves also point towards the plugs?
Yes, both intake and exhaust should be pointing in towards the plug.
You still have a spare electric
Stator cover bolt?
Shame your battery died that was the part I was looking for good video anyway thanks
Sorry about that. I get into the work and forget to check the camera. Thanks for watching.
When you were installing the camshaft gear and the camshaft, it seemed like you missed a tooth while aligning them. If you look at the alignment marks of the camshaft, they are not parallel to the head. Why is that?
Hello Tim, I had a question for you. I have herd that when replacing the head gasket on this engine one should also replace the cylinder sleeve gasket down below as it can tear when loosening the head bolts. Is that true?
Yash,
In general, it's a good practice to replace gaskets when pulling the head and cylinders. I did not remove the cylinders from the case, so I did not replace the cylinder sleeve gasket. I personally don't think it will be damaged if you don't remove the cylinders from the case. However, I am not a professional mechanic. That's just my opinion.
Thanks for the question and for watching.
@@Tim2Wheels hi Tim.... I wish I could say I've been as lucky. Here's the best I can suggest. Some bikes have cylinders that have a screw/bolt that holds the cylinders onto the engine case. Those are usually ok to leave on when replacing a head gasket. Just be sure to never loosen or remove the hold-down bolt. Some engines don't have that bolt or bolts. If you relieve pressure on the head gasket you should replace the base gasket as general practice. Odds are not in your favor if you don't replace the base gasket.
Fastener sizes are the diameter never the broach
Is this motor still in service?
Hello brother great video can you tell me what is the mm piston size of the CB750K
The OEM (stock) piston is a 61.00 mm bore. There are aftermarket pistons available in 0.25 mm increments.
Thanks for the question and for watching.
Tim2Wheels can you please give me a link where I can buy engine rebuild parts.
Hi Tim hubby is restoring CB 750K 1979 limited edition. The bike was stripped by previous owner, all back together but can't work out where the four oil deflectors go in the head? Can you help
Rose, his best bet is a good service manual. If you don't have one, email me at tim2wheels@gmail.com and I'll see if I can help.
Buenas noches hermano, donde puedo conseguir el tornillo del tensor de la cadena secundaria. el del minuto 8:58
te agradezco de antemano por el video.! saludos...
Hello Edgardo,
I got this one from a spare motor used for parts. You may check with a shop in you area that works on this type of motorcycles. Otherwise, you may find one on eBay?
Thanks for watching.
Hi Tim, I'm about to purchase one of these monsters mounted in a CBR929RR frame, and sincerely I think that is the only one here in Cuba, so i wanna make a question, are these units reliable?? I mean in mechanical terms.. greetings from Cuba !!
Hi Carlos, The Honda CB750 engines are good engines and very reliable. with a little knowledge and good care they can last a very long time.
You have a Problem Keeping your Nuts & Bolts straight ........
You tightened the cam Adjuster BOLT to keep the cam chain tensioner down ..... Not the Nut, the nut is the lock . 9:58
مكانة ٧٥٠ سي سى كامل بكم
My dad is on meh account XD
I'm 5 minutes in and still talking reiterating, shame.
Bye.