Honda CB750 DOHC - Engine Rebuild Step by Step - Ep1Mar 28, 2022
guys, what's up? Welcome back to the mailbox workshop. I'm not in the shed in the UK today. We are about to enter phase two of our lockdown, which means for the next four weeks I won't be able to go. to the shed unless I run there and run back to class it's my daily exercise which may be pushing a bit I haven't decided yet thankfully I have a lot of edits to catch up on so I'm sitting here in my hotel room in copenhagen just doing a little bit of video editing in todays video we are going to
engineso this one runs from about 79 on up i think mine is an 82 but they should all be more or less the same so if you have on these bikes and you're going to see a bit more about how the
engineworks or if you're thinking of doing a
rebuildyourself then stay tuned guys let's start by talking about the tools you're going to need to this rebuild so this is going to be done in about five episodes this rebuild s I'm going to focus on the tools you need for the first part so we're talking about removing the head from the bike the valves and all the way to the pistons but not touching the bottom crankcase so first of all you are going to need is a
cb750twin overhead cam engine now i say this because this video is very specific to that engine there may be other offsets where the procedure is generally the same but this video is made for the cb750 uh you're going to need this on a workbench with all the oil drains.
Now getting the engine out of this bike is not a straightforward process. I've made a video on this so if you're just starting out and your bike still has the engine. fitted i'll link to it next and at the end but you need to get your engine on a workbench on a stable surface somehow you need a manual for the bike this video is not meant to replace a shop manual it is supplementary material i I'm just going to share some of the techniques I've learned taking this engine apart and rebuilding it, but in the first instance I recommend the official Honda shop manual it's called that's how I've used it alternatively you can get the Climber and Haynes manuals for this bike they are both excellent and i have plenty of pictures and descriptions on how to do this rebuild if you are looking for the official honda shop manual it is available on all the usual uh honda forums that being said if you subscribe to the channel, soon i will be releasing my own version this document is very clean over the original huh everything is tagged correctly im going to annotate it with tips i found as h I've done this by being helpful so if you're interested in that please subscribe to the channel and I'll be pitching that over the next few months you'll need a selection of wd40 degreasing cloths and latex gloves if your bike is anywhere near the stake you're on mine this will help protect your hands the bikes are gross you have to remember this bike is 40 years old so it will only help you work safe and clean.
More familiar with imperial sizes on your nuts and bolts, it's a Japanese bike so you'll need metric sockets. I used a half inch set but a 3 8 is just as applicable for removing them but just make sure the actual sockets are in millimeters. i will just need assorted light hand tools screwdrivers allen wrenches and pliers always handy one thing you will specifically need is a jis screwdriver. below if you're interested but some of the screws on this bike will look like phillips heads but they're not they're Japanese industrial standards so strap on your screwdriver make sure you get them out cleanly and I'm not going to risk stripping them I took them off head.
I bought a pack of these plastic shims at the local hardware store. They are designed to pack wood when doing carpentry work. They are extremely useful when you are trying to separate motor housings to fit into tight spaces. and supporting the engine, I'll explain more as the video goes on, but if you have to be at the hardware store when you see these, they're normally around five bucks a pack, grab a set as they're extremely useful. You will need a rubber hammer or regular hammer and some wooden blocks. There will be components on this engine that will have stuck in place and will need a little tapping to move them with a rubber mallet or a piece of wood that just protects.
The engine is aluminum so it's very, very smooth, so you have to think about that beforehand. I bought a set of these picks on eBay. I think they cost a dollar or so including shipping from China. I have to say that the quality is surprisingly good. and I would probably use these more than any other tool in my shop they are really very very useful again I'll link them below but when you're trying to get your fingers into very tight corners you'll need these things. Finally, I recommend getting a set of ziplock bags and a marker pen for each lug bolt washer, since it comes out of the engine, you want to put it right in a bag, label it and save it now, applying these best practices from day one on your the shop will make this ten times easier to rebuild this bike don't do what a lot of people do take off a bunch of bonus and think i'll pack it up at the end of the day and then you get carried away building the engine and before you do i know you got a parts box and a nightmare on your hands, so establish good practices from the start.
Be disciplined. This doesn't sound very exciting, but believe me, it will make your life so much easier. we're going to need to get on with the rebuild, so first we're going to I'm going to remove the cylinder head cover. This is held in place with eight cylinder head cover bolts. It's always good practice to work these in a pattern that releases tension evenly with all eight bolts removed. The stock cover should lift off easily. put in the practice bag label the parts and you may notice at this point that the center four bolts are a bit longer than the outer ones right in the center of the engine we have a timing chain guide and oil line which can be removed by removing all three retainers bolt on so next we're going to remove the camshaft mounts these need to be removed for bravo charlie hotel juliet i'll show you these now the letters are etched into the camshaft mounts but keep in mind that bravo and charlie are the letters covered by a small town, also note that two bolts have already been removed with the oil line so if you find it is missing don't worry as there are two small oil caps from plastic that should be able to be lifted free now a single bolt in the center of the engine holds the cam chain guide retaining blade this can be removed at this point the camshaft mounts The pins are lined up for a small metal pin two on each mount the easiest way to remove these is a pair of needle nose pliers some of the pins will stay in the mounts and some will stay with the motor just make sure you pick them all up and toss them in the bag along with the brackets if any of these pins are really stuck in place the easiest way I found to get them out was to just use a vise to grip them tight then twist and hammer them from back at the same time I was freeing one or two that had seized my engine at the front of the engine is a timing chain tensioner lug nut bolt that can be immediately loosened with the lug nut loosened you should be able to push it back down on the bottom side of the cam shape keeping pressure on this tighten the cam chain lock nut again and it should keep the cam chain in a state loosened with some play at the rear of the engine you will find another chain tensioner loosen this then using a vise loosen the rear cam chain and tighten the snug nut as before unfortunately my camera was unable to record the video of this section.
We need to remove the cover of the Pulsar generator. This should be the only one on the bike with the Honda logo on it. Behind this you will find a large nut that will allow you to spin the motor, you need to turn it until a small 1.4 character appears in the window at the top, this tells you that ash 1 and 4 are in the highest position. If you did it right, the charge should be positioned the same way as in the video, as long as the lobes of cylinder one are pointing at each other, that's fine. Next we can remove the golf kilo foxtrot and lime camshaft mounts in that order, although I just released the tension on these gradually as before. you click down the pins as you go and bag them along with the camshaft with all four camshaft mounts removed the intake camshaft can be lifted via the cam pan the exhaust cam sprocket it's a large gear at the front of the engine it's held in place by two small ones The bolts can now be removed, just be aware that you may have to rotate the engine 180 degrees to access the second bolt once it's removed both bolts, we're going to remove the remaining camshaft mounts. tach gear alpha then eco i have labeled the tach near gr on these diagrams and it is visibly different than the other camshaft holes at this point i decided it was a good time to remove the gasket although you can actually remove this anytime.
Next, we're going to remove the exhaust camshaft. Now keep in mind that when you do this, the timing chain may fall into the engine case i. This short 12 inch length of steel or wood was put in just to keep it from falling over. works perfectly it is recommended that these chains be reinstalled in the same orientation they were removed from the engine if you want them to work in tension in the same direction so as not to reverse them now they are quite oily so quite difficult to actually check how they go around this is how the chain went on this motor they want to prevent it from tipping over when i reinstall it so with this particular chain i installed a set of cable ties on the top edge so i know where the cable ties are on top of the engine and put a pair on the front and a single cable tie on the rear which means in a few months when I reinstall this I know it's in the same order.
I'm not in danger of smearing Sharpie ink on these parts and I can 100 guarantee you'll go the right way, so just think about how you're going to do it, but here's how I did it. at the rear of the engine you have a short s shaped oil line uh this co might be unbolted now its very very very easy to damage this as it undoes the two bolts you can twist the actual oil line and twist it , which will then restrict the flow of oil to the head which you definitely don't want so what I did was just use a small wrench on the two nuts at the end of the oil line and then the bolts can be secure to eliminate any risk of torque while on this side of the engine the two timing chain tensioner lug nuts can be completely removed on the front of an engine you should have a small plate to show you the size of the engine there are two small bolts below these now they can be removed next we are going to remove the 12 dome or acorn nuts on top of the engine it is important that you do this in a crisscross pattern similar to the sequence I am using here and just do a quarter turn or so on each nut as you take them out they may now be quite tight and you may need to use a small hammer on your wrench to free these i highl recommend you don't use a torque wrench so it's very easy to remove the bolts that go on these and it's a lot of work to repair so work slowly so smooth you should get there after a few passes you should have most of the stress removed of these and you should be able to remove them entirely.
Hey, with these nuts removed, now we can start removing the cylinders. This is where you want to use your rubber mallet or a piece of wood to protect the engine lock if I just have a steel hammer the important thing here is to take it real slow just get a millimeter or two on each side once I got a small gap i inserted one of my plastic wedges into it the head may just want to rock back and forth as you remove it the wedges give it something to push against and but its happening this should do it alot easier so you can see much better here how I have used these little plastic packaging you will find. which when you apply upward pressure on one end to what will happen is it will rotate through the center motor and the other end will come down so to prevent this from happening and just rock back and forth through the center of the motor , you should put some plastic gaskets on each side as you go, this means the motor has something to push againstsupport so it doesn't fall off when it hits the other end and you just gradually insert a millimeter or two of plugs at a time just to avoid that really to-and-fro movement and then you know every little tap of the hammer that you put into the engine, actually it will move the cylinder head up a bit as you remove the cylinder head, just make sure you have a way to secure the timing chain to prevent it from falling through the insulation with the cylinder head removed, I decided to remove the shims and lifters, they are clamped only from the suction of the oil so I found that a small metal pipe was enough to go under these and remove them from the head when backing these up Just make sure you note what position they were in so you can make sure they come back to the same spot in this point.
I also decided to remove all four spark plugs. At this point, you can also remove the front timing chain tensioner bolt. There is another bolt on the timing chain tensioner on the inside. at this point you'll probably be able to see the tops of the pistons for the first time so you can see here I've got a lot of flaky carbon bits on mine there's a lot of debris in here I really don't know where it all comes from but I certainly glad i got it off so i cleaned it up good. The lower front edge of the ashes.
There is a single bolt that can now be removed. This releases the cylinders again. I have used it. a block of wood and some plastic wrap to remove this so when you lift the cylinders it's important not to let the pistons in the upright position fall out so here I have pistons one and four in the raised position just make sure they don't hit too hard and damage themselves at this point you can remove the cylinder gasket as you can see on my engine I have a lot of debris that has settled on this to keep it from falling unnecessarily into the bottom case, just vacuum it or wipe it down finally.
In this video we are going to remove the pistons on each side. There are two small piston pin clips. I removed them with a small pick and a pair of needle nose pliers. They are under surprising stress, so I recommend using a pair. of safety glasses when you do this they will need to be replaced so if they shoot across the room don't worry too much you will have to buy new ones anyway, with the piston pin clips removed you can simply remove the piston pins piston. I can even do this using a small pair of needle nose pliers or I've also found that a sharpener pushed out the back of these removes them very effectively once you've removed two of the pistons you'll have to rotate the engine 180 degrees so you can access the piston n pins on the remaining two pistons and just because i wanted to see what kind of condition the pistons were in when they came out i gave them a quick clean and then lit them here you can see how fast the carbon comes out and She finished off with a beautiful sparkly top so guys that's probably a good place to end this video before we do let's take a moment to outline what we're going to do before we get this engine running again in the first place , the top end, huh. looks pretty manky to be honest it sounds like you got a good clean including lapping all the valves checking tolerances and also going to replace all the valve seals with slightly higher specs I'll probably cover this in part three or series four rebuild i wasn't planning on rebuilding the bottom end but looking at the large amount of debris that settled on the cylinder gasket i think a lot of it probably ended up in the crankcase no small task breaking this up so i make up my mind Pretty uh a lot but I feel like if I don't open that up and clean out the debris in there it's going to give me a lot more long term problems with the bike so I'll do that in episode 2. splitting the crankcase And giving it a good external clean on the engine would definitely give it a thorough clean.
Some of the covers but I'm going to start the rebuild in earnest so first you're going to get new new gasket seals all over the engine. It looks like these haven't been touched at the factory or certainly not in living memory so I'm definitely going to replace those I'm going to check and check the bearings as I go I make sure if there are any that need replacing , I do it as I go along and I'm going to check the brand of the clutch as well. It sure has enough life left to give me many years of idle motoring.
Finally, the carbs are stripped and removed, ready for the explosion of steam. I'll make a video showing those splitting and then a second rebuild video coming up on the channel soon so if you're interested. Watching me make carbs be sure to hit subscribe guys that's it for this video. I hope you enjoyed it. or any feedback on these dual overhead cam engine rebuilds or the channel in general, please drop me a comment below. I would really appreciate it. I will get the next video as soon as possible. As it comes out be sure to hit that subscribe button until next time, take care and stay safe, cheers.
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