1979 Honda CB750K DOHC Head Repair & Rebuild

[Music] all right hi and welcome back to another episode of Tim two wheels so in the garage today I have a project bike that belongs to a friend of my son's it is a 1979 Honda cb750 k the those are very popular bikes to be turned into cafe racers and that's exactly what was done to to this bike I'm not real clear on the history of this bike he bought it used and it was already kind of stripped down and converted into a cafe racer but the bike had some issues that the head had been removed from the engine and in the process there had been several broken bolts in the head and so the project today is going to be essentially going in removing a lot of these broken bolts getting the head back in good condition cleaning up and getting the head reassembled right now it is literally a basket case it is in the head is in several buckets and you'll see that later on so for today's project were going to be reassembling correcting those issues and reassembling the head on a 1979 Honda cb750 K now the 79 was the first year that at Honda transitioned over from the single overhead cam to a dual overhead cam there's quite a few videos out there on the single overhead cam the older engines there's not that many on the dual overhead cam which is one of the reasons I wanted to make this video so for anybody who's looking to reassemble a head hopefully this will be helpful to you alright so let's go ahead and get started with this project okay so this is the original cb750 engine that came out of the bike it's got a good bottom end the problem was there was some damage to the heads on the top and some stripped bolts of broken bolts and things of that nature which can be repaired and we made do that but the owner of this bike has a donor engine that they want to swap out just the top half and put it on this bike and see if we can get it running as cheaply and as easily as possible so that's the project so on this bench this is our donor motor which you can tell because it's it's all black but this engine that the owner of the bike had managed to to get to use has some issues in the bottom and the bottom half but has a good top half or top end so that's what we're gonna basically just swap the top end off of this motor put it on the original motor which has a good bottom end and see if we can get this thing up and running all right so I'm gonna stop right here and say that the original plan was to remove the head complete off of the donor motor and put over onto the original motor that's what the owner wanted to do that was that was his plan after getting in there I was able to remove the broken studs that were on the original head get it cleaned up so I advised him that if it was my bike I would just reassemble the original head and that way you're keeping the original motor all original now the plan has changed from swapping the head to just reassembling the original head and using any parts that were needed that had been broken off of the original motor take those from the donor and put onto the original so that's what we're gonna do so in addition to the two donor engines or I'm sorry the the original engine and the donor engine he does have all the parts that came off of the original engine so we have literally a basket case here we've got the parts that the two cams timing chain those parts here we got some covers and bolts and breather tubes air filters for the carbs down below I have a bucket and down below here I have a bucket with the carp Bank in there and a lot of the other miscellaneous parts the coals all the other parts that we need to get this guy back up and running so this is the head off of the original bike Cotter key here but this is the original valve cover and since this is a 79 model year it is a dual overhead cam again everything is in good shape here there were a couple of bolts apparently broken off or stripped [Music] okay so here on the cylinders as you can see now I've got the all the pistons cleaned up a lot of the carbon off of those I've got the surface cleaned up nice and smooth everything is good and clean we now have a new head gasket that I can apply to this and we can go ahead and begin assembly [Music] okay so one trick on the reassembly you want to get your chain tensioner your cam chain tensioners retracted if you will so the there's really two that you have to worry about one is this one here in the rear that is more of a vertical and you what you need to do is loosen around back you know there's two two nuts cap nuts back here let me spin around just a little bit more seeing a view of that but there's these two cap nuts one here and one here which is in between the intakes so you want to loosen those and then press in on that bow that's on the cam tensioner and then tighten these back up and that'll hold it in and retract it okay so and the other one is this one that runs horizontally here and this is for the this is for the chain that runs between the two camshafts that times them so they're in the front of the head there is a bolt here now on this bolt you want to be careful you want to loosen this nut here first that is your lock nut what had happened with the previous owner is try to take this out without loosening that lock nut and just by removing the bolt head you will twist this off and that's exactly what happened to the two the old one and here you can see this is what was broken off inside and I had to drill that out and use a an easy-out type of a bolt extractor to get that out of there and it was a it was a bit of a pain but finally I got it so when you loosen this which I already have it you'll see how that springs up here there's a piston or a pin that goes in slides in this this bolt tightens against that so you what you want to do to put the chains back on is to get in here and press down you can use your finger or use something got like a small hammer handle here that I can use to get a little better leverage and see I'm pushing it down to where that piston slides all the way forward and then I'm gonna go ahead and tighten that nut to where it will keep it retracted like that now I'm not tightening the lock nut at this point I'm leaving that backed off because what I'm gonna do is once I get the TAM chains in there we'll be backing that off and then locking that in place okay so now the head is sitting back on the next step is going to be to tighten down the actual head bolts so I'm getting the washers in place make sure things cleaned up and on here and then we'll go through and tighten them in a specific sequence snug them down then we'll torque them just spec so there is the washer that goes on and then the cap head nut I'm just getting each one started by hand right now and then I'll go through snug them up with a wrench just snug and then come back and actually start to torque in the right sequence in a crisscross pattern and I'll show you that detail here in just a moment the specifications say to torque this to between 26 and 29 foot-pounds if so what I've done and and there is a specific order that you should go through as with any torquing of anything you want to go through in a crisscross pattern and I've gone through and I originally have my torque wrench set to roughly 16 foot-pounds and I've gone through and torqued them all to 16 foot-pounds in the proper sequence and so now what I'm going to do is go back to the final torque specs set that and I'll go back and retorque them I don't like to go and this is debatable I guess amongst mechanics can can probably chime in here and tell me whether there's anything to this or not but I'd like to go through and basically torque at half value or somewhere around half value maybe a little more initially especially when you're doing something as big as a head this size on a on a bike or on any engine then I go back and instead of torquing from zero all the way up to max torque each one I like to torque them all down kind of have torque then go back and torque to my final torque specs so for this I'm using a Tecton 24 335 torque wrench which is available through tool vector comm I'll put a link below for you or so I'm going to go ahead now and back this nut off and go ahead and bring it on up to my final torque specs so there's 20 foot-pounds there and it says 26 to 29 so what I'm going to do is is come on up to 28 that's not quite to the max but it's going to be very close and then I tighten down my lock nut here and this these are 14 millimeter nuts that are on here by the way the it's funny that the actual Honda service manual says that it's a ten millimeter cap nut which is is not right perhaps the bolt that size itself is ten millimeter that that makes sense but I just wanted to point out that it is a 14 millimeter socket to fit this also in this diagram which is really hard to see but essentially according to this this is one two three four and then you come out here to five six and then we'll go seven eight and then jump back over here to go nine ten eleven twelve so that's the the sequence in which we're going to be tightening these all right so let's go ahead and get started here we're gonna go number one up here which is actually the H capstan and this is the front of the motor the exhaust side this is the intake side just for reference in case you want some orientation there so again I'm just going to come in here and gently apply pressure until I get my torque right there rope so then we come over here and do number two there and I'm just holding the engine to keep it from sliding around here and then this was number three back here then we come over here for number four in that crisscross pattern so then we go over here for number five and then we're gonna come over here for number six and then seven was back up here there we go eight is gonna be over here and then lastly we have nine ten then 11 and 12 alright so that's all the head bolts head cap bolts are torqued down to spec alright so next we can reattach the bolts here in the front that go up under here and and then we'll begin reattaching the oil line in the back and step through the rest of the reassembly sequence as specified in the service manual ok one of the things I'm going to show you here as you whether you can see down in here or not but the timing chain I have just laying time machine I have laying down in there gently so that I can retrieve it and what I've made is just this little piece of wire just made a slight hook and with this wires for is just so that I can reach down in here and grab that time and chain and pull it up out here where it's supposed to be that I've got this wooden dowel that I just lay in between there and that's going to hold hold the chain up as I work with it now this is the exhaust cam chain or cam chef sprocket this is a dual cog because the larger the larger gear is driven by the cam chain which is through my or my thumb that goes down to the the actual Kent crankshaft and then the smaller sprocket drives the chain that goes over to the back to the intake cam so on this side of it with a larger sprocket you'll notice these four flat areas and two of them have marks on it to here and here those two dots so those are alignment marks that you want to get basically parallel with the top of the case at the top of the head as you put this on so just keep that in mind now initially you can't put the cam chain or the crank the shed the crankshaft timing chain right on the larger gear won't give you enough room to work I usually just drop it down in between or put it on this smaller gear so let's go ahead get this in here and get it aligned see I'm keeping the the dots aligned in this manner I'll basically parallel with the top surface of the head now also as I showed you earlier we've got the tensioners backed all the way out to give us as much slack as we possibly can well flip this gear just a little bit to try to keep it these parallel to the top like that doesn't be exact what you do wanted to get as close as you can all right so now we have the the exhaust cam I'm going to begin working in from this side through here and you have to twist so that the lobes fit through the actual up through the cam to the gear then finally we get over here and then before that's done I'm going to take the other timing chain and work it to get it in place and I'm just going to lay that over here give us some slack no the lobes here we want they're pointed up right now we want those to angle in toward the spark plug for cylinder 1 turn this there's also these alignment marks here on the end and then we'll work to align the bolts back here trying to keep this other time machine go ahead and bring that chain up okay so I've got the chain on the larger sprocket with the alignment where I think it needs to be now the chain is on the smaller sprocket get it up onto the gear where the holes are aligned with the camshaft all right so now we have these two small with the two bolts that go in there and the specs do call for putting some thread locking material I was struggling with getting the cap off of my blue thread Locker so I'm going to put that on here [Music] and doing some of this work my camera quit recording and I didn't realize that the battery had died in it I was just working away and but basically I've got both camshafts in the timing chains connected I got the oil crossover tube installed see it dripping a little oil here is because I have the all the covers on and the oil pockets are filled now with that clean motor oil they're all pockets being these sections here where the the valves are you want those valve shims and the cups submerged and in good clean oil so I'm getting ready to put the valve cover back on in fact I started putting the valve cover on only to realize my camera wasn't recording so at any rate apologize for skipping over some of that I have all the cap bolts torqued to spec it was at 12 foot pounds is what these are all torqued down to and everything is in place so at this point I do have the new gasket zoom out of it I do have the new valve cover gasket in place on the valve cover and I'm gonna flip that over and get that installed here should I do that my valve cover coming dislodged so this is still 10 millimeter now when you go to reinstall the valve cover bolts you'll notice there is a shorter version like this and so there's a shorter version and a slightly longer version so the longer ones will go here in the center and as with everything you always want to get those started by hand you know just just snugging these down by hand gently let go check the manual to see if there is a torque spec for these all right so the longer head cover does have a torque spec and it's 6 to 9 foot pounds is the specification here my smaller torque wrench only has inch pounds so I need to convert and which makes sense there are 12 inch pounds to every foot pound just like a measurement on a tape measure so 9 times 12 I want to come back to a 108 that should be 108 right there all right okay the last thing is there were a few broken a few broken exhaust valve or exhaust ah I've been too much on valves there were a few broken studs exhaust manifold studs on this particular head so I need to pull some off of our donor motor okay so at this point I'm gonna show you how to remove a exhaust stud if you don't know already but here's one that I took off the other motor and basically what you do is put two nuts back to back and lock them in tight so you got it it's called a jam nut so you put one nut on jam it these happen to be flange nuts so I'll put the flanges together so it have more bite but when you put them together and wedge them tight it essentially makes a head for the bolt and then you can just screw that in to wherever you want to put this as you can see it's turning the whole shaft and just making this whole thing tight get that good and tight now I can put the wrench on the back nut to hold it these are just 12 millimeter and then break it apart and just remove the heads are the nuts and boom you have an exhaust manifold stud installed okay so I hope this was somewhat helpful for you I do apologize for missing some of the assembly footage you know that I was into it didn't realize my camera had died so putting that rear cam shaft in you know it's it's like putting the front one in you go through a very similar process of getting to line it wasn't worth tearing the motor down again just to get the video footage so if you found that it's a critical piece I apologize however if you just get yourself a good service manual you can it has all the steps in there that that you need to do or if you're not comfortable doing this kind of work just always take it to a mechanic who is comfortable in doing that so there's a few other pieces to this bike that I'm going to be working on the we're going to be cleaning the carbs in the next video and then also have a small like the the fuel Inlet tube on the carb had a crack in it this happened to be bronze or copper tube and so we're gonna read braise that solder that to fix that leak and and then so I'll make another follow-up fitting over to that piece so as always thank you so much for watching hope this was helpful this is Tim T wills and that's how I did it [Music] [Applause] [Music] you [Applause]

Channel: Tim2Wheels

Views: 35,169

Rating: 4.865922 out of 5

Keywords: 1979 CB750K, Honda CB750, CB750 DOHC rebuild, Café Racer, Motorcycle repair, Tim2Wheels, T2W, T2W Garage, Motorcycle, Motorcycle Upgrade, DIY, mods, cb750 cafe racer, cb750 engine rebuild, cb750 engine teardown

Id: mVqDiFMRrt8

Length: 28min 32sec (1712 seconds)

Published: Mon Apr 08 2019