Low compression CB750 engine teardown. Found the problem. 

Just a few quibbles with the terminology used in your commentary: The "top" ring is properly called the compression ring The "second" ring is properly called the scraper ring Yes, the "third" "ring" is called the oil control ring and consists of an expander in the middle with rails above and below it. As well, after honing the cylinders you MUST use clean white rags, preferably something soft like cotton (old clean white tee shirt material that's 100% cotton without silk screened stuff on it works great) to clean the cylinder bores of all the grit from the hone stones AND the cast iron powder from the liner surface itself. Wipe the bore with a cotton rag dampened with mineral spirits (NO WD40 or similar lubricants) and then toss the dirty rag. Repeat this process until your last rag will not show any black stuff at all. You'll be surprised at how much rag material you can use just to ckean ONE cylinder bore. Your whole engine will thank you for taking this extra step of getting rid of all that abrasive crap. As to installing the block and coaxing the pistons/rings into the bores: you really should thoroughly bathe the rings with engine oil before trying to get them into the bores. You'll find that things go much easier. It would also be a great idea to lube ALL the cam bearing surfaces AND the lobes with engine oil or "startup lube" before installation.

"Im not going to do a 'How to...' video, this is more of an overview" ...Proceeds to do an overview video 10 times better than 99% of 'How to's!' Lol. Watched every bit of it and I dont even have a 750, great vid as ever! Excellent job mate.

Thanks, very nice to watch. Having already disassembled the cylinder head, you could have taken the opportunity to grind the valves too...

I have 3 CB750's and a CB550, all of which I would consider yard bikes with extensive exposure and neglect, but, I am hopeful I can get one or two of them running. You have the most comprehensive classic bike repair channel I have ever seen on RU-vid, thank you for publishing these videos. BTW, where is the vid of this engine running?

always a good idea to remove the valves and lap them in by hand.Particles of rust can sit on the surface of the seats and hold the valves open slightly.Its just peace of mind,and saves having to strip the engine down again.cheers Graeme from New Zealand,enjoy your programmes

I had the pleasure of attending and graduating (youngest to grad age 16) from American Honda Midwest Service school back in 1970 right after the Daytona win. The class was held in Bob Hanson’s Race shop in Racine Wisconsin. My instructor was Bob Jamison who prepped the winning 750. He was a real stickler for detail and this is what I learned from his philosophy. You did a great straight forward job however what you missed was...the troubled cyl probably had rusted (at least) the intake valve. If you’re not intending on inspecting the valves, do a leak down by pouring solvent in the intake and ex to see if it’s getting past the valves. The cam tension rollers get brittle and under all circumstances be changed. The rubber compound on the knockoffs are not near the original Honda part. The valve seals also get hard, and while you’re in there, it’s an evening job to pull the valves, lap them in and put new seals in. Make sure u check and lube the stems.

Just finished the top end rebuild on my 78’ 750 following this video. Thank you Taylor for the information and wisdom. This is the single best video on RU-vid covering this imo

Great call choosing to do the top end rather than just engine swapping. This is much more interesting and helpful.

I really have to complement your skill at methodically explaining each procedure as you moved along during the whole top end rebuilding process. You have a real gift for teaching and making things crystal clear from start to finish! This video is definitely valuable content. Subscribed! Keep them coming’

I wish I had this video to watch 30 years ago! Nicely done.

Mate. I've been watching you for a while now, and I must commend you on your entire approach to your content. I've considered trying it myself, but the whole 'talking to camera, explaining stuff, then editing and publishing these videos, along with actually fixing a real life motorbike, is a bit of a handful!' So you, my friend, are something else. Just thought I'd say something positive (and truthful), cuz we all need that shit now don't we.

Piston Rings are probably set to have the gaps away from the front & back of the cylinder because that would be where the most pressure is applied by the rod/crank as the piston moves through the cylinder.

The reason the head gasket goes on dry is because any sealer used could possibly block the tiny high pressure oil way holes in the cylinder head and that would be very bad news indeed. One thing I noticed is that the head gasket was not laid flat over the oil rings and you just pushed/dropped the head gently into place and kinda hoped that nothing gets pinched. Hope it works out and no oil leaks.

When removing Piston Rings you should place small rags around the 4 Con Rods so any mishap and you dont end up with foreign objects drop into the bottom of the motor..

You do a great job with these videos, very comprehensive without being boring and educational as well.

I'm in the midst of a top end rebuild on my bike. This vid gives me hope. lol.

Someone already said it, but it bears repeating,....you are an excellent teacher. Very complete presentation in understandable terms. Well done!

The beauty about SOHC Hondas (and Z1 Z900/1000 Kawasaki's also H1 H2 triples) is that there is so much after market stuff for decent prices. Their continuing popularity and sheer numbers sold, especially in the U.S means its commercially viable to mass produce parts. I'm into 80s Japanese stuff, but the 2 year product cycle, often with complete redesign, means finding parts for some bikes a nightmare.

My ADHD has now got me binge watching all your videos. And now you have a huge fan up here in Everett, WA. And this all started with your video of the most awesome explanation of rebuilding the carbs on the CB550. Absolutely great videos. Very good editing. There's no unnecessary waste of words or time. Thank you sir.

A - Im a strong believer in cleaning up the engine prior to removal and conducting a leakdown test on it at that time as well . In addition if it wasnt a runner I'd be tearing down the bottom end and checking bearing condition and clearances . Also the bore & pistons should have all been measured . Minimum piston - wall 0.001 - 0.002 clearance . Ring end gap top 2 = 0.007 - 0.016 . Oil ring = 0.012 - 0.035. little tip ... you can increase the taper prior to boring / honing ... makes installing the pistons a snap. Cam timing . I always check again AFTER cam chain tension has been set . Use assy lube cam bearing surfaces .Your videos are well done.

I'm glad you bought the pliers, spiraling on rings is not a preferred method.

Great job man, but, I would add i always clean out the ring grooves on the pistons with a piece of old piston ring in case there is slight carbon build up under the rings. Here in the UK we call the top ring with the chamfer a ridge dodger , It's there in case you have a slight lip on a bore to stop the ring hitting it (Which you probably know already )

I had planned on doing this work on my K2 during the downtime this winter. This will be an invaluable help along with the various sacred texts I've collected.

Hi Taylor. 20 years ago I had a CB750. Still a hole in my heart. One of the things that was poor design from Honda was the timing chain. To this day I can hear a CB750 from blocks away because of the rattling sound. Has anybody designed a replacement tensioner/guide/chain/whatever to improve the design so it doesn't detract from that beautiful design? Thanks for keeping these world-changing bikes rubber side down. Paul N.

Love your vids. This is like ASMR for motorcycle nerds.

You mentioned location of ring end gap not being front or back of the cylinder and not knowing why. Think about the rotation of the rod and the directional force of it against the piston. If the fairly sharp ring gaps are pushing against the cylinder at the point of highest pressure (front and back) there is a high likelihood of cylinder and ring damage. Left and right of the cylinder experiences a negligible fraction of this force. Great informative video as always, showing DIY isn't as daunting as many may think. Btw would be a treat if that cam got a shot of assembly line or oil on its way in.

Just a perfect explanation which we owners of cb's thank you a lot. Thankyou master

Was wondering why no assembly lube on install?

Very good instruction video ! Thank you !!! Greets from Belgium (Europe) 👍

love those large , air-cooled fours

Good job. Outstanding. Installing the pistons is the part I hate. I set no. 2 and 3 in the up position and install those in the cylinder first then rotate to even (also allowing the cylinder to drop with the center pistons} and then install the outside pistons no. 1 and 4. I don't have your finesse.

I've used Easy-Off Oven cleaner (it is carbon after all) on piston faces and valve bowls, then a parts washer.

Got rid of a 750 4 years ago. Wish I still had that thing.

Definitely made me get the confidence to rebuild my top end! Awesome content!

FYI - I have an OEM Honda FSM (factory service manual) for the SOHC CB750, 69-78. The FSM piston ring gap spec for top and second ring gap is 0.2-0.4 mm (0.0079-0.016"). The oil ring rail gap spec is 0.1-0.3 mm (0.004-0.012"). An old rule of automotive thumb I was taught says a safe top piston ring gap for a normal engine are OK at min. 0.004" x bore diameter (in inches), so for a 3" bore (76.2 mm), the top ring gap should be min. 3 x 0.004" = 0.012". That is just a general rule, though. Getting the actual ring manufacturer's specifications is the best way - by far. If you buy aftermarket rings - follow the instructions that come with them. If they don't come with instructions - follow the FSM specs. The Honda OEM FSM have that in the specifications chart in the chapter on the engine. The general rule of thumb for the 2nd compression ring is 0.0035" x bore. The oil control ring rails should be generally the same as the 2nd as a minimum, but the top limit of the gap dimension on those can be much larger - not as critical.

Taylor you do great work and then present it well. Couple things I’ve learned to do. Permatex or something under the pucks or they leak. Torque cam towers and caps or risk screwing up threads. Red loctite on cam gear sprocket. Big problem if they loosen up. Torque head bolts and do it again in 24 hours and again in 24 hours until it holds a constant torque. You can put a weldless frame kit in allowing you to do a top end job w/o removing the engine. You can always weld the frame back together. Keep up the good work.

Great video, One thing I'm sure you do anyway is I spend at least half an hour on each cylinder scrubbing after the honing process as that fine stone and metal paste sits in the honed cross-hatch and takes alot of work to get out, I use paper towel and lacquer thinner until after a wet scrub the towel comes out white with no signs of grey, without doing this engine life is severely shortened, I have race motors going around that I built in the 90s smoke free with great compression.

The reason you don’t put your gaps towards the front or back is because that is where the cylinder will wear into an “oval” shape. They are trying to reduce the chance of compression leakage overtime. Should the gaps be on the front or back when the cylinder wears out there would be massive leakage in the area.

Good video. Thanks for making it. CanI suggest next time add a comment on how to test now well the valves are sealing. Such as using a flash light to see a gap. Or gasoline/kerosine in the valve to look for drips. Not that hard to lap the valves since you are in there.

Well done video! You have emphasized loosening the head bolts a little at a time in two or three steps. That is very good advice. I would add to that to FIRST loosen the six 6 mm bolts (four under the rubber plugs and two at the cam chain area). When you loosen the nuts on the main cylinder head studs there can be a lot of stress on the 6 mm bolts. On later engines this is more critical where the cylinders are part of the crankcase. There are torque specs for all of the screws and bolts on the engine. If there is no specific torque spec for a bolt, use the general torque spec for that size of fastener. You can find that information at the front of the Torque Specifications section in chapter one of the Honda Service Manual. The silver gray color on the top piston rings is chrome plating to extend the life of those rings. You didn't mention cleaning the cylinder bores after honing, or I missed it. It's really good to get all of that honing grit out before reassembling the engine. I recall that the cam chain tensioners would get corroded and stick in the tensioner. I found on the early years of CB750's that there was a small threaded hole in the back of the tensioner rod that I could thread a screw into that would act as an indicator that you could watch to confirm that the tensioner was working. This was useful during a routine service.

Just allow the camchain to drop into the crankcases, fit the cylinders and cyl. head , then retrieve thy cam chain useing a telescopic magnet or a piece of hooked wire, ... makes life so much easier.

I hope no bits from your exploding piston ring ended up in the crankcase! I would probably have stuffed a rag in each of the crankcase holes for the pistons as I have butterfingers anyway ... great video by the way - thanks!

Your correct in ascertaining the trepidation most folks place on a refresher with engine teardowns, associating it with marked levels of terror that things will be thrown out of kilter from small missteps taken along the way. A hundred diversions probably accounts for most unsuccessful repair procedures more than misdirections that take place. Great synopsis of one's route to travel when putting these plants back together. Best of luck with your frame and suspension refit and upgrades. Warmest regards.

Hope you washed out the cylinders with hot water and soap. You should be able to run a clean white cloth through the cleaned bore without a mark. Lightly oil afterwards.

Very methodical, low key presentation. Well done.

Awesome vid man! Wish we could have seen the vid of it getting pulled and what you did for that. Good job on stressing the head bolt removal and installation process. Could have used a little lube for the cams and pistons, but when I saw you adding oil at the end, definitely shows you know what's up. Keep up the good work!

Great video....very informative, I like the way you work. Just one niggle for me......I think whilst the head was off I might have been inclined to check the valve seats and re-grind them to know the that whole head was good.

thaks a lot for all the info. i think the ring gap not being at the forward or rear of the pistion is because there's some force fore and aft as the pistion mooves up and down which makes the bore oval/oblong over time.

I appreciate your videos. Enjoyable and informative. This one, like so many of your others, gives me confidence to take on new challenges. Thank you.

Sweet project dude! Just my two cents as a small engines mechanic, you should try soaking your combustion chambers and piston tops in wd-40 and a brass brush, been doing this the last four years and it works wonders. Again awesome content and great work!

I'm actually looking at doing my own cb750 project, also in Austin! I had to watch this video just to mentally prepare myself. Great explanations and video!

Nice one Ty, you make me think that I could rebuild a Honda CB750 engine, you make it look soooo easy. Great video man. 😎👍👍👍🇬🇧

Yeah, I still have my piston ring pliers from 20 years ago for the one time I needed them and do not regret buying. And the ring compressors.

Created one of these engine rebuild video series myself... it took me a while to setup cams, etc ... so I really appreciate the fresh content Taylor. Hope all is well. The sonic cleaner (that I purchased after seeing yours) did wonders for my pistons... 50% simple green, 50% water. I noticed you didn't add Hondabond on the pucks?

I just might be buying an old CB 750, your info is wonderful. Keep up the good work!

I see you start all four pistons in the cylinders at once... I start the middle two first then the outer ones and that seems to work well for me. good job on the video and it looks like you do good work. great to watch ...you are a natural teacher.

Very good in explaining everything step by step ! That is very clever of fasting the engine to the engine stand

Why wouldn't you rotate the crank until just two pistons are up - and get those in and then rotate it 180 deg. and deal with the other two? I may be wrong, but it seems to me that working with only two at a time would be easier than wrestling with all four at once.

Great information. Could you offer an explanation on why you didn't take the opportunity to lap the valves on at least cylinder 4 please?

Curious as to why you would go all that way and not bother with a new cam chain or even look at the crank. Great video, love this stuff be thankful it's a single cam.

you should have inspected and lapped the valves

Well done Taylor, you've saved another engine and it was good to see what was wrong with it, the video made good content .....👍🏴󠁧󠁢󠁷󠁬󠁳󠁿🇬🇧

Before opening any engine I perform a leak down test. I want to know if the problem is with valves or cylinder.

When you shatterd the ring to show us some of it went down the piston hole~You might have wanted to put safty glasses on and a rag in the piston hole~Always keep a rag in the piston holes when rebuilding an engine

Awesome video! Helps a lot to visualize what I need to do when it's my first rebuild. Keep up the videos on the cb750. Can't wait to see the next ones!

Absolutely love your content on the Honda cb’s. Trying to find a 70’s one right now to build one, absolutely love the sound and how much you can customize the look of this bike 🙌🏼😌

I had this excact scenario with a 750 Honda, i managed to free up the rings by carefully heatcycling the piston,and managed to loosen all of the stuck piston rings

Congrats Taylor once again for another well done job & explanation. I got 2 toys both cbs to restore, a cb750c & a cb1100f sports. I will get there. Thanks for sharing ur clear videos helping everyone. Keep it up!

Had a 1978 Honda 750 Four F2 got rid of it when the tappet noise went bad that was back in 1983 and have regretted it ever since wish I had your knowledge back in the day.

Thanks very much Taylor. Very good informative video. I learned a lot. It won't be as intimidating as it would have been. As you say, slow and methodical is the key. Cheers, 👍🇨🇦

Nice video once again. I have always tought that you hone the cylinders to have something in the cylinder walls where the oil would stick better

Great video as always but, damn, I get huge workshop envy :-)

Thank you, always enjoy your videos.

Hi friend, thanks for the video, I need to do this job to a cb550f, now I'm not in a panic,

dry ice blasting is the go, tho in a moulding shop we used powdered cork blasting

The best man! Go Classic Octane!

Thank you so much for this video. It makes me feel like I can take on a similar project. Keep up the great work you are doing. Big fan!

Darn, wish i watched this a year ago. I did my upper gasket because my 550f was leaking oil. I discovered massive carbon buildup on the cylinder heads and the valves. I sprayed for two days sea foam spray, brake cleaner, carb cleaner and some came off but it was a centimeter thick solid caked on carbon buildup. Then i lost my job and could afford to order the full kit or many of the tools i wish i could have had. It took me a lot of time and a lot of brass brushes i had laying around to break down and clean the carb. The rings seemed ok so i left them because i was losing money. I got it all back and running but wish I could have done a better more thorough job. Now im selling mine because I need the money. But i put a lot of time into restoring it from the previous owners lack of care.

Rings are constantly spinning in the cylinder bore. Clocking them is nice for first start but definitely not a requirement.

I know it takes energy to explain wuch is highly appreciated because of my inexperience thnks

got to do a top end on my honda vf750 soon looking forward to it!

I too love your work and videos. Can I just ask one question though? Would/should you not give the valves a ‘lap’ while you are in there? And then fit the new valve stem seals that are probably in the top end kit? Not being critical just wondering your reasoning as we can all see you have the skills to to do this job too. Wayne.

such a great vid, thanks for taking the time to share!

I'd use two rectangular magnets that cover both the cam and the cylinder head on either side of the camshaft to keep the cam from spinning while you're trying to rotate the cam sprocket.

Did you forget to replace the valve seals? They came with the gasket kit. Also then you could hand lap the valves if needed.

This is very helpful since I'm planning to eventually do a top end rebuild on my 78 CB750K!👌🏽

You can score the bore if you have gaps front and rear, not over the gudgeon pin either or you could theoretically snap the end off.

what beautiful work. outstanding job.

Just for your information the cross hatch is there to hold oil , so you have an oil film on the wall.

This brought back very old memories. The only thing I'd have done differently is pack old socks/underwear/t-shirts into the barrel holes under the pistons before trying to take rings off; I'd put them there anyway just to stop things getting into the motor. As noted, rings are brittle; the last thing you want is to have to tear down the motor because a piece of piston ring disappeared into the belly of the beast. Many, many years ago, a pal with a 69 CB750 bought a new Yoshi cam complete with new cam supports and a full gasket kit. Missing from the kit were O rings for the oil lines to the head so he went to a local parts store and bought O rings. They were too big and squished when the head was torqued down, significantly reducing the oil ways. About a mile down the road on the rebuilt motor, the new camshaft seized. Very sad.

Cam Rocker Box brother, oh and by the way brother I wanted to say that the Piston Ring compressor tools are pretty badass if you're installing the Pistons into the cylinders ahead of time and then installing the cylinders on top of the rods and connecting them to the rods afterwards, because I always wondered about the piston ring positioning when they are installing Pistons connected to the rods already, the Piston Ring positioning is definitely going to change that way so with the Piston Ring compression tool installing them ahead of time into the cylinders is really really helpful, and will definitely help to keep those piston rings seated in the positions they need to be

For a video that is not how-to, there is a lot of how-to. This is a great look at the process, and thank you for sharing. Hey, I bet it would be a REAL bummer to get the cylinder block and head on, look at the assembly bench and notice the cylinder base O-rings you forgot to put in. :P

When I still believed that a compression test was the most useful test to figure out the status of the top end of an engine, followed by a bore scope, someone suggested a pressure test. I thought he meant compression test and just misspoke. But he patiently explained how pressurising any cylinders that are down on compression, will quickly reveal WHY it’s down on compression. If the air escapes through the exhaust, it’s the exhaust valves that are at fault. If the air escapes through the carburettor or inlet manifold, it’s the inlet valves that are at fault. If the air escapes through the oil filler, it’s the piston rings that are at fault. I have literally NEVER seen this advice on any problem finding vid, engine rebuild vid, bike or car vid, or forum anywhere. This is obviously great advice, but how come no one ever seems to be aware of it???

Great job on the video and the rebuild .. I have experience with that myself.. The only part I didn't understand is why you separated the cam sprocket from the shaft. But I watched the whole video and it is very good....not easy to do that. You are helping a lot of people to do it right.

Nice vid. Just a bit of advice... It's a good idea to glue those rubber disks (under the cam holders) in place. I've had one leak on me. PITA because (as you know) you have to pull the motor again to fix this.

Did you use anti-seize on the SS fasteners? That is a must do, since SS fasteners in aluminum will cause galling and/or seizing. Is there a reason why you didn't lap the valves? I would have replaced the valve stem seals as well.

Excellent! I'm glad it was just stuck rings. It would have been nice to crank the engine to see the new compression value on cylinder 4. I guess we will have to wait a couple more weeks for the "finale". Thanks Mate!

How do you? Know you have the right rings without gapping them first without physically checking the gap which means you've got to clean the cylinders hone if necessary. And then insert the piston without the rings on it so that you can get the ring flat level and check the gap at the end of the rings. Each bike has its own specifications for that gap. Finally rebuild the engine..

fantastic vid. I put a big bore/big cam kit on my Grom clone a few months ago and it was nice seeing it's pretty much the same process for this give or take 3 cylinders 😂

Excellent, loads of respect and support Rodge David Kidderminster UK