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Low compression CB750 engine teardown. Found the problem.

Jun 17, 2021
whats going on guys welcome back to classic octane i'm taylor so in the last video you saw me crank up a classic

cb750

again at the end of that video i was going back and forth on what to do because we

found

out we had a cylinder of That

engine

that had low

compression

so my original plan was to take one of my other CB750s and run it to see if that

engine

was in good condition, swap out the engine and be good to go, but I heard a lot of your feedback. and i decided to do what you guys would do most people in this scenario aren't going to have a spare

cb750

like me to trade in so we decided we're actually going to do a top end rebuild so we have the complete top kit and rebuild including new piston rings um all the gaskets all the seals everything to do basically from this section to the end so that will be the name of the game today this will not be an instructional video Doing this process is going to be more of an overview of me doing it and let's see if we can figure out what's causing our low

compression

on cylinder number four.
low compression cb750 engine teardown found the problem
Let's take this apart before we start. I'll give you a quick rundown of all the parts we have here so this is a complete top and gasket kit I'll link to all of this in the description below but it has all the head gaskets, head gaskets valve, exhaust gaskets, I mean everything we can think of four sets of new piston rings, spark plugs and then I went ahead and got one of those stainless steel allen bolt kits this engine actually has all steel allen bolts stainless since the main ones I like to replace are these on the valve cover just because they know it needs to be a good solid seal even here and it's kind of a little hard to do that with these screws jis so I like to just replace them with allens and then of course the whole engine will match now that it will be allen all the way so that should be all we need.
low compression cb750 engine teardown found the problem

More Interesting Facts About,

low compression cb750 engine teardown found the problem...

I go ahead and start tossing things aside, I'm using my second riser here as a workbench to keep all my parts neat and organized. I removed the entire valve train. It doesn't matter how many times you've done this. It's never a bad idea to just have the manual just to keep your sanity, to keep you informed about the process of each individual step and just to make sure you're not getting ahead of yourself, it's also very important to meticulously design everything like the axles of the rockers and everything to make sure you're not losing any screws you're not putting any screws in the wrong place you know all the rocker shafts and rockers and everything is going back to where it came from so I like to just lay everything out methodically It's very simple for me just reverse the order so now I'm at the point where I'm going to loosen all the head bolts or nuts I should say and it's very very important that I do it slowly and what I'm going to do is just do the reverse order of what the manual calls for to tighten them and it's going to be kind of like a crisscross pattern.
low compression cb750 engine teardown found the problem
I don't remember what's out of hand again as long as I have the manual, but again it's very important to not just loosen this all at once. you just want to loosen them in reverse order and keep loosening until they're nice and loose and what that does is evenly release the pressure off this head if you were to just start from this side and start to unscrew everything that would start the head to lift on this side while it's stuck and that can actually warp the head so I can't stress this part enough to go in reverse order nice and slow of what's in the manual and then we'll get all this out we'll get ready to remove the cylinder head all the bolts are far enough out you should be ready to remove work on all the bolts we need to power our there we go there it definitely has some buildup on the valves that's for sure i want to clean that up a bit overall Doesn't look bad looks like me one or two o-rings missing but luckily we have new ones ok let's put them like here so our number 4 ro is a bit rusty which is what i was thinking was the

problem

. in that last video, if you remember, I was thinking that probably some water got into this cylinder and caused some rusting

problem

s on the valves or, you know, caused some rust on our piston rings, something like that, I think I had it. on time.
low compression cb750 engine teardown found the problem
Lapse mode, so we may have missed removing the cylinders, but there's no more bolts or anything, it literally comes off right after you remove the head, so looking down here, this is our number four cylinder, definitely has a bit of corrosion on it. It's a little dirty on it, but I don't feel like scoring much or anything, so I honestly think that'll come back with a sharp. I don't think we're going to need it. doing any kind of machining or you know boring it a bit is nothing, it's that kind of damage we'll know for sure when we perfect it but it looks pretty good.
I'm going to go ahead and go through the house through all of them just to get a nice cross hatch and everything back. the rest of these but one thing i noticed so okay cylinder number one here the piston and piston rings you can see the piston rings are loose uh on your ring lands they're like if they were able to float. they're a little bit springy so they actually jump when in that cylinder they push against the cylinder wall and give you a good ceiling aka good compression and they're nice and free on all but these number one pistons four, which is our problem. cylinder you can see the rings are completely stuck in their lands like they're not even pushing anywhere which is exactly what you'd expect to find for an engine that only had low compression not no compression so we'd expect to see no compression . a hole in a piston a bent or broken valve some like serious damage but since it was low we're like ok probably not catastrophic and this is exactly what we're finding here it looks like water probably just got into the cylinder it ran down the side of the piston a little bit it made these rings seize really close together and we're not getting a good enough seal honestly if you're just a little shade tree mechanic you'll probably free them smack smack this again and it will probably work perfect. since I'm here I'm not going to do that I'm going to go ahead and replace them with our new rings since I have them anyway I'll probably go ahead and replace them all so we have new rings. throughout all i want to see now is the best way for me to start cleaning all this stuff right down to the pistons themselves i can take them out and honestly i'll probably steam them out some old timers are going to get really mad if i do that but steam blasters aren't very aggressive looking and won't remove metal which means we should be able to remove this carbon and leave a nice clean finish.
The steam jet is used on the internal parts of the engine all day, every day, no problem. It blows my mind because they are so easy for me to clean afterwards. I can clean all glass beads. Blow them out with compressed air. You know, wash them. Make sure there are no glass bead debris or anything left on the piston that won't be going to be easy for me to do on the cylinder head just because there are too many cracks and crevices for the glass velocity to get stuck so I'll try to find a different way to clean this combustion chamber, to clean all this carbon construction. up and all but i'll bring you back when i figure that out you can see i've cleaned one side and all i'm doing is using a little bit of solvent spraying it here and letting it sit for a while so i'm using a little bit of scotch brite pad you can use a bunch of different things for this just don't use anything that's stronger than aluminum you know you don't want to walk in here with a razor blade and start scraping too crazy or a heavy duty steel brush or something like you just don't want to be able to scratch or gouge any part of this because it can create kind of a hot spot and some issues this is a very finely machined surface and you just don't want to introduce any kind of nicks or anything so I'm going to pass over here again.
I let it sit for a while while I scrub it. I am using a small brass brush. If you're nice to it, that's fine. Scotch brite works well and cleans everything. I am also doing the same on the pistons. I was curious about the depth of the carbon buildup, but I actually sprayed solvent with a scotch brite and it worked fine, so I don't think we need to. Vapor blast anything so I'm going to do that and then I'm also going to be working on the mating surfaces for the head gasket, the cylinder base gasket, all of that stuff, we want to make sure everything is super clean , we also want to make sure that all these little o-rings and everything are accounted for so we can replace them with the new ones. the ones where everything you know is coming back together so i'm gonna keep scrubbing let's go over to this side and clean the pistons and bring them back when i get everything clean and we're ready to put our new piston rings on to make good progress.
Here I cleaned all my surfaces again with just a little bit of solvent, a little bit of scotch brite and everything went pretty well. and also took out one of our pistons, this is the one that had the rings stuck in there. I was able to pull them out very carefully and they did break and were definitely corroded in there, but there doesn't appear to be any damage. to the ring lands or the piston itself so I have no reason to think these need to be replaced everything seems nice clean nice there is a little bit of discoloration but this is very smooth which is what you are looking for , you just don't want this to be choppy or any other. kind of big gouges something like that is going to become a hot spot and could potentially become a problem so what I'm doing now is going ahead and double checking the ring gap on all of our new rings I've perfected these too two cylinders.
I'll show you the polishing process for these two here in just a minute, but while I was doing this, I thought I'd turn the camera on so I wouldn't go too far. in front of me so there's three we'll call them ring sets the top two are just individual rings the bottom one is what's called an oil control ring that has this little corrugated looking springy piece in the middle and then two skinny type of retainers on each side so it makes a little sandwich and that's really what PS the oil on the bottom of the piston scrapes it off the cylinder walls so you have these two which are very similar but if you look the profile one of them has slightly rounded corners and that will be your top ring so consider that number one now this is kind of a two tone color it has like silver on the edge where it's a little bit closer to black on the surfaces this is a solid black this will be our number two rings that are almost perfectly square on the edges and then also have little markings these have little r's going up so it just makes you Knowing that it will be in this configuration when it is on the piston itself we will also want to make sure to record these gaps so that two gaps don't overlap because that point becomes a potential place for compression to creep in so we want them spaced a few apart. 120 degrees every time we go to install. them, but what I'm doing right now is Double check the gap of the ring and if you're not familiar with how to do that process, I'm going to carefully put it on the cylinder here.
I'm going to turn our piston upside down and I'm going to push. down maybe about a third of the way I like to use the ring it lands on the piston to make sure I have this perfectly square in the bore like this so I'm basically just pushing it down until the cylinder surface match. all the way around the bottom ring ground so what i can do is take my feeler gauge and go over here and measure that little gap and what i'm personally looking for is a gap of about 12 000 inches the spec in the manual it's just it just tells you the max spec it says if it's larger than 27 mil replace the rings there's a lot of speculation on the internet as to what they should be set to when you go in fresh I've seen it as tight as eight up to 18 I figure I'll find myself in the middle and I'm going to aim for 12 thousand which surprisingly and fortunately is exactly what they're measuring so I'm just sticking that in there 12 000 of an inch tells me we don't have to do any filing on this if that was very tight , let's say there was no gap or there was two thousandths of an inch or something that they have you can file these rings and you just want to make sure you remove very small amounts manually uniform ra and simply go up to whatever specification yoube looking for I'm sure someone in the comments will tell me that 12,000 is the wrong thing to do, but we'll do it anyway, so I'll just go ahead and just specify each piston ring for each cylinder to turn into this. cylinder rings and so on, just in case there are small variations in bore size we don't deviate but this is how piston ring gap is checked. going to get some lube and then this is the sharp one I have it's just a three stone spring piece that you put in a drill so you can stick this into the barrel it will press evenly and then you can turn the drill and run this it doesn't take much we're not trying to remove a ton of material here all we're trying to do is clean up any little corrosion spots and then introduce what they call a cross hatch pattern which is basically fine scratches on X shape that only help with compression.
They help with those rings so they have something to bite down on to seal well. I'm just going to sprinkle a little bit in there. now let's erase it we'll see what it looks like not sure if you'll be able to see it and it probably looks a little more aggressive than it is in person we don't want to be able to feel any of those marks or anything. we just want kind of light x-shaped scratches for lack of a better word on the cylinder The ders make sure there isn't a big build up of any of that and that's it so you're literally talking about maybe 10 15 seconds max into the sharpening process again less is more, I'd rather go back and do it three or four times rather than do it. it's too much and you end up, you know, you bored the cylinder by accident, you knocked out the last one, ready to go, got number four with the new rings in place, figure I'll show you here at number one, just go through it once For you guys, Guys, I have my number one rings here.
We will remove the old ones again. Keep in mind that piston rings are very fragile, so if you bend them too much one way or another, I'll show you this one. bend it too far and it'll explode into a million pieces and you probably should have worn eye protection before you did it but luckily we're good to go so keep that in mind don't go crazy bending this stuff off. just get one edge work slowly and steadily and make sure ain you're not ripping anything you're not scratching it and you can just pull them out they'll all be junk these little oil checks can be a little annoying but you just find the area where the space is. is and you can usually put a little nail in there and separate it like this we'll find the gap in this corrugated piece that's right here and it's outside you can take the pistons out to do this if you want to I'm just going to do it with them in the connecting rod area still just to help hold it up so when we get back we're going to do the oil control ring first slide it around and lay it out that way. bottom slot and then we have our two little middle rings that go on the top and bottom of that boom that one is on so just rotate it to make sure everything moves nice and free everything is where it should be and we can move on to the next one you remember from a second ago is the one with the perfectly square edges in this particular pair is the one that's a solid color we want to make sure our little mark is facing up work it without bending it too much number two rings in place we're going to set that gap right here at the four o'clock position or so and we're going to go to our top ring, we're going to want to set that gap to around 120 degrees so I'm going to shoot that somewhere around this area of seven o'clock seven eight o'clock something like that only you know a decent amount of distance you don't want the gaps right at the front or right at the back of the pisto n or not entirely sure why but the manual says that and there we are so all our rings are in place the staggered gaps we're ready to go flush and repeat um now that all the rings on the piston they are in place we are ready to put them back together i dont think i mentioned it before but my timing chain tensioner is in good condition this would be a good time to replace those wheels if they are very worn or you know you have a and kind of problems that are like 15 bucks not a bad idea to do them huh mine like i said is in good condition honestly i think this motor was a part not too long ago because almost all the gaskets beat the gasket just came out if it is like a 40+ year old stock engine that has never been opened before i have to scrape a lot and clean a lot more to get the gaskets off this one the gaskets came off so im thinking i probably dont know the decade p roast 15 years maybe this motor has probably broken apart before so here are all the o-rings and gaskets and everything that comes in your high end gasket kit all pretty self explanatory um they just go by so fast these are our four really skinny o-rings that actually go on the bottom of the cylinders over here you can see the old o-rings in those little grooves so that's where those four o-rings go we'll move on to these these are for our tappet c overs so they replace the o-rings here.
The exhaust joints are the copper ones. We have these a little thicker, more like a cylinder type o-ring. Those go here. There are four of them right at the top of the reels and they come together. with a stock and these four holes four actual o-rings which are not that thick two of them go here and here you can see the little gap there and then on the other side oh yeah in this on the block itself is where the another two go here and here so you can see where those two bolts have kind of a little recessed area that's where those four o-rings go the smaller four o-rings go on the top side of the cylinder head and replace these little ones o-rings here so there is one there, one there and one there for the oil lines. should say that's where those go and t So of course you have the main head gasket and then this lower greenish one which is the cylinder base gasket that goes between the block and the cylinders on this side.
The head gasket of course goes between the cylinder and the head, that's pretty obvious. the thinnest is the valve cover gasket and then you have things like the starter and timing chain tensioner that sort of thing so all of that is probably self explanatory but if anyone ever you've done this before, it might seem a little intimidating, but it's really not that many gaskets, just find where the same size is the same number of o-rings and swap them out, so I'm going to go ahead and start throwing all those gaskets away. I'll bring them back when we're reattaching the cylinder head here because it can be a little tricky to get all the pistons in the cylinders at the same time, making sure we don't catch and break any of our piston rings.
There are a lot of different theories. It is the best way to do this. I have done it in many different ways. I'll just make sure all my ring spaces are set up again. Okay, I've seen people put pieces of wood under here to hold the pistons. Piston ring compressors you can put here. I find them more painful than that they helped on these engines if you think about building a small block chevy or something. These cylinders are built into the block so when you put the piston in you just compress it slide the piston in and then the piston ring compressor tool is like on top and it just pops out of here the tool will stay stuck in the middle so he has to use it as a hose clamp or something.
Along with that explanation that I'm not going to use one I'm just going to apologize manually putting them in manually there's a little cone on the bottom of each of these cylinders that can help you get the piston rings to slide in there it takes a little bit of finesse g and patience but it's actually not that bad pull our cam chain over here make sure our tensioner and everything is in the bore and we'll start aligning these pistons in each cylinder so we're just going to go cylinder per cylinder and then we make sure all our o-rings are in place we have our cylinder type of o-rings and then our standard o-rings make sure you have your little pins on the outside and the chain tensioner all of that in the right place , we'll go through with our new head gasket making sure we don't bend this too much in one direction or the other and this goes on without any copper spray or anything.
Copper spray is for copper joints. these are graphite i think i just have to put it over this little one down so it should sit nice and flat and we're ready for our cylinder head so it will continue in the same way we just need to make sure we feed this cam chain all the way to the final. up through clean surfaces no o-rings or anything I left it in place. Hold it while I run this chain through the tongue here. We are not pinching anything. We have a lot of tacks to line them up. here ok now we're ready for the four gis i'll call them uh bolts that go through these two holes these two holes and also get those little hockey pucks there's actually no bolts through these central ones but we'll also put the little blinking plugs in there so we keep everything nice and sealed so we'll throw them away and they'll just tighten.
I don't actually know if there is a torque spec for this or not. I'll have to look it up. A really important torque spec is for the 16 head nuts that go through here and that's where we follow that sequence and we'll hand-tighten them all and then slightly tighten them. and then we'll probably work through one or two sequences of torque and i g have to look up what the setting is, it's not a ton, it's somewhere like 15 18 foot-pounds, something like that, but more importantly we follow that pattern to get a good even clamping load across the head. fit everything nice and tight so now i want to double check our cam is one the motor spins ok also our cam chain is still on the sprocket so yes it is moving it seems to move nice and smooth , it's not very easy to move so we know it seems to be making good contact with the sides of the cylinders as much as we can tell I'm also going to take this opportunity to go ahead and get the engine in the right place to install the cam that it's the t mark on the timing plate here for one and two cylinder so that's TDC for one and four I mean put it right at the t mark right there which means the number one and number four are in absolute top dead center which is where our cam n needs to be installed we are ready to go for our cam retainers whatever their official name is just slide down on their little guys bolts you can do it ah i see our problem our little old seals are stuck in there we need those there we go now we slide on it runs so much better look it's just for show don't get ahead of yourself if something doesn't fit right just take it off and check again, okay, i don't know if i mentioned this when we took it apart or not. but our cam bearings seem to be in very good shape no scoring or any kind of excessive heat we will discolor them ok see my manual here slide them down turn cylinder 104 on the timing mark ok now we are ready. to put the cam back on so put it back on we need to slide it through the cam chain being careful not to twist any of the lobes here just work it nice and slow go ahead and remove my welding wire ummm ok so now let me show you on this side the little marks that you have to make sure ar We line up on the side of the can here you have this little groove and then you have these two lines we want the groove to be in the position 12 o'clock, so up and then I'll make our two little lines match up. with that cam retainer right there so that's exactly where we want the cam to stay so you have to be very careful that it stays in that position and that you can install this cam sprocket without moving that cam too much so we'll put the cam chain on the sprocket we'll hold it in place look I think if I were to bolt on the cam I would have to move it like this and that's a little off I think let's try moving this sprocket one a boom of tooth right there so we were missing a tooth and that can throw off your timing on your bike so I hope it all makes sense what I just did from playing with that cam sprocket as you install it making sure the cam no it doesn't budge and that you can put the chain put the sprocket on the cam w here we took our two bolts out earlier and everything is good and lined up no issues to report we are ready tothrowing those two 10-millimeter bolts in here and actually bolting that sprocket to the cam for good.
I'm knocking out the rocker shafts now they're just the reverse of how they came out just slide them in from the side the outside is slotted so you can use a flat head screwdriver to easily orient and orient them so you can put these little bolts in there and I'll have to look and see if these have a torque spec. I know the top of the cam bearings, camber caps have a torque spec, but I don't know if these do or not. Now it's time to get back to our bearing. caps and these tighten to what the book says from six and a half pounds to nine and a half foot pounds so I'm going to run and pull all this stuff to tighten them and I think the next step is probably our cam chain tensioner I'll have to see when let's get ready for the timing chain tensioner now so just have our three bolts on this thing is super simple it basically just has one spring so you undo the locknut compress the spring down lock it in place for the installation. lined up and it just slides in and we'll tighten our bolts once the bolts are tight we can undo that locknut and it'll snap into place adjusting our chain tension and exactly what it needs to go to valve clearance now, so to get the engine in the correct position you basically turn the engine clockwise until you see this number one intake valve go down then back up then align your t mark with a little notch on the block now that it's in the right position for us to do intake and exhaust on number one as well as exhaust on number two and intake on number three and we're looking at two thousand on intakes three thousand on the exhaust then we're going to rotate the engine 360 ​​degrees in line which dials again that will allow us to do both on number four as well as intake on exhaust on number three hope that makes sense it's all in the manual so I'm going to remove all the valve clearance and then we should be ready for our valve cover and our breather now we're ready for the valve cover I like to just hit everything here with a little oil one because this engine is going to sit for a couple weeks before it gets going and then two everything is really dry now and on initial start up i wouldn't mind it being a little more lubed that's good nothing crazy got a new gasket in there , it's time for our deck and I've got all my new allen screws in here too just like that we'll just start running them.
I will run them in a crisscross pattern. It's not as critical as bolting the head on but it just keeps everything from snapping or warping or unbalanced so I'm going to run this all through and we're ready to t go ahead I'll put the breather in anyway we'll have to remove it to get back to put this motor in the frame but so nothing falls off here we're going to give this motor a nice clean light paint job just to freshen it up and i want everything to be nice and sealed and there's a new o-ring in the kit to the breather cover also don't forget and the final step is our eight on our tappet covers just use a small flat blade to pop them out slide the new one in to tighten them good that's it for the 750 top end rebuild as you can see it's actually not terribly complicated if you just work methodically put all your parts in the order you already know how to get out of the engine follow the manual follow this video like i said i really didn't want to make an instructional video o but i wanted to show you the important parts it's not terribly complicated i really hope this demystifies the high level rebuild process i know a lot of people are scared to do it that's another reason that led me to want to make this video on instead of just taking out another motor, testing it, and putting it on the bike. of parts upgrade all the top end know all your oil seals are ok your piston rings are ok your head gasket is ok everything has passed tune it up correctly this is a very good starting point guy now this engine you know in In theory it shouldn't have to be serviced for a long time and that's what I like to send my customers out with a bike that I know is in good shape and I won't get a call a month later saying you know it's starting to vomit oil from the head gasket or something crazy so all is well and taken care of it will be a couple weeks before i put it back in the frame just because i need to finish the rest of the mods for the bike so it will be a while before that we'll actually hear it, but I'll be sure to walk you along.
I appreciate you seeing it. I'm sure this one was long. If you have any questions or anything in the comments below. See you next time

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