Topic: CB750K7 Airbrushed, 70s aftermarket goodies and some engine questions

[onepieceatatime]

I suppose I should record my progress.
I bought 2 non-running, titled '77 CB750Ks and part of a K5 for $500.
I intend to work on both of these at the same time, and hopefully within the next 2 years have them on the road.
All 3 engines were stuck, I got the 2 '77s freed up with ATF/acetone and rocking the bikes in gear. The first one shot the ATF/acetone at least 15 feet across the garage and covered 3 other bikes with it. I figured out that I should put rags over the spark plug holes on the other one.
The one missing the exhaust, unfortunately the valves are not moving either, so that engine is going to have to come apart, and it will likely get an 836 kit in the process. That engine has already been apart at some point so I am curious to see what is in there.







All of this was included in the sale

[onepieceatatime]

The grey bike had the bags and fairing on it when PO bought it, but he took them off. I plan on putting them back on, but may use another fairing that I have.
I have gathered a few other parts, and several rebuild kits, but know I need a few more rebuild kits that I have not bought yet.
The other fairing:

I want to remove the comstar wheel and put these on:


and one of these tanks:


and this Corbin-Gentry seat that I bought for it:

[onepieceatatime]

The air-brushing was done by a place called Vantastics in Carlisle, PA (it is signed). This is the one that will have to have the engine torn apart, and will likely get the 836 kit. It has 6" oversized forks with a fork brace, but  they are bad, so I got some new stock size fork tubes for it. It also has Lester wheels which I plan to keep on it.
I need an exhaust for it still. I either want to find a factory exhaust for the grey bike and use its headers and these NOS Jardines on the black one, or find some other period correct aftermarket exhaust to go on it.

This bike is getting this Corbin-Gentry seat:

some finned engine covers:

and probably these floorboards and highway pegs:

The speedometer is messed up so it will probably get some '75 gauges.


And the other set will go on the grey bike to replace the DOHC gauges that are on it now.
 

[onepieceatatime]

Before I spend any money on new spokes, can anyone confirm that the front wheel from a '76 CB550 and the rear wheel from a CB750K5 will fit the K7? I know I'll have to switch to a 630 sprocket on the rear, but otherwise will it all work?

[seanbarney41]

On the back, you have to keep the k7 sprocket carrier with the k7 engine so that front and rear sprockets align...use the k7 wheel spacers with that too.

[onepieceatatime]

On the back, you have to keep the k7 sprocket carrier with the k7 engine so that front and rear sprockets align...use the k7 wheel spacers with that too.

Thank you. will the one from the comstar fit the spoke wheel?

[seanbarney41]

On the back, you have to keep the k7 sprocket carrier with the k7 engine so that front and rear sprockets align...use the k7 wheel spacers with that too.

Thank you. will the one from the comstar fit the spoke wheel?

yes it should, as long as the comstar is from a 77/78 cb750f..but you do know that the k7 never came with comstars?

[onepieceatatime]

On the back, you have to keep the k7 sprocket carrier with the k7 engine so that front and rear sprockets align...use the k7 wheel spacers with that too.

Thank you. will the one from the comstar fit the spoke wheel?

yes it should, as long as the comstar is from a 77/78 cb750f..but you do know that the k7 never came with comstars?

Yeah, that's why I want to get them off of it. I have no idea what the comstars are originally from.

[seanbarney41]

okay, I went back and looked at your pics of the grey bike...that rear wheel is what as known as a reverse comstar.  It came on certain 79 and later dohc4's, certain models of sohc 650, and some honda twins...so no idea if that sprocket carrier will work or not

[onepieceatatime]

okay, I went back and looked at your pics of the grey bike...that rear wheel is what as known as a reverse comstar.  It came on certain 79 and later dohc4's, certain models of sohc 650, and some honda twins...so no idea if that sprocket carrier will work or not

Thanks for checking. I guess I'll figure it out when I get to that point. The reverse comstars might end up as spares for my 650, if they will work for that. Either way I want to have spokes on the K7.

[onepieceatatime]

I guess it is time to resurrect this thread. I have finally started working on one of the K7s (the black one with the air brushed tank).
A friend wants the grey one, but wants help getting it going, so it may end up in this build thread, or one of his if he joins up here.

I have been accumulating parts that were missing, and rebuild kits for brakes, carbs, etc.

I rebuilt the forks (actually a different set of lowers too, since I hate chrome) with new stock length uppers, bought a weldless frame kit, cut the frame to be able to pull the top end off in frame.
I have the master cylinder and caliper taken apart and ready for cleanup and rebuild.
I decided to use the K5 gauges on my K2 Ol' Sarge project, so I bought another good set of K7 gauges.

The carbs are seized solid on this bike, but I bought a good rebuilt set a while back, so I can put those on and worry about these ones later.
I did buy a set of factory 4 into 4 exhaust pipes for the grey bike, so the Jardines are going on this one.

I will be replacing the chrome covers as I get this engine ready to run again. I got a black '78 F head cover to put on this engine, and will be putting a set of finned covers in place of the other covers.

I pulled the head cover off. There is plenty of orange silicone for me to clean up before I put it back together. the exhaust valve on #4 is stuck wide open.

The cam and rockers look ok, and I pulled the head off to have a look at the cylinders and pistons.

There is a bunch of gunk in #4.

A closer look reveals what looks to be rolled up leaves.

I also noted a nicely patinaed cam chain tensioner. A new one is on the way, along with a cam chain and gasket kit.


No major dings in the pistons, but I'll still pull the cylinders off and check piston to cylinder tolerance, since there is plenty of orange silicone at the bottom of the cylinders to clean off too.
Here is the head with no cam present. Looks like #3  exhaust is stuck partly open too.


I think I'll end up sending this head out for new valve guides, valve work, etc.

[754]

With the  amount of rust on that tensioner, I would be really wondering what the inside of cases looks like,  not the gears  so much as the bearings ..

[onepieceatatime]

With the  amount of rust on that tensioner, I would be really wondering what the inside of cases looks like,  not the gears  so much as the bearings ..

Yep, I was hoping to get away with just doing top end stuff on this one, but seeing that made me realize that I have to split these cases for a proper inspection.

[PeWe]

Good idea to take all apart for a good clean.
If pistons/rings/bores are worn, cheap alternative available from Cruzinimage.
My K2 has K7/K8/F/F1 (392) pistons. 61.5mm (+0.5).
Max 0.025mm total clearance.

https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

If any of the bores have much rust, go bigger. I had to replace 2   sleeves due to the rust was deeper than expected on an old cylinder.

https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-1-0mm-over-size-pistons-set-62-00mm/

Their clutch work fine too. My K2 has it (+56whp) and my K6 had too when 836cc and 81whp.
https://www.cruzinimage.net/2017/11/16/76-78-honda-cb750k6-cb750k7-cb750f2-clutch-friction-plate/

When apart replace all chains and tensioners. Stock Honda tensioners.

My K2 build got BMW timing chains as primary chains (2 chains needed) which are stronger than stock. Some sell these for 2-3 x price as HD chains.
These chains can be found cheap here at internet car part dealers.

Details
BMW 530 d,  184 HP,  [E39],  Production: 1998/08 - 2000/09,  Engine code: M57 D30 (306D1)
Original partno: 11 31 2 248 729

You can find more about primary chains in this thread
http://forums.sohc4.net/index.php/topic,154183.100.html

[onepieceatatime]

Thanks for the info Pewe. Looks like I have more reading to do.

Yesterday I ordered a set of primary chains, just in case either engine I currently am working on needs them, and 2 primary chain tensioners, one for this engine and one for my K5 engine. It seems as though you are suggesting I just replace the chains as a matter of principle since I am in there anyways.

I have a set of RC 64mm pistons and rings that I might put in this engine, and a set of 1st over NOS pistons and rings, and a nameless 836 kit that I bought 2nd hand but unused which includes that bored cylinders. Unfortunately none of them are the K7/K8 style pistons, so I need to figure out what to do to the head, or replace the head to use them. I am waiting on my ball hone to show up so I can clean up these cylinders as well as the K5 cylinders. Once I have done that I will check the piston to bore clearance and make a solid decision on how to proceed.
I still have the K2 engine that I have to tear down and update as well. Right now, my inclination is to put the RC parts in the K7, the 836 kit in the K2 engine and either stock or first over in the K5 engine. The K5 is going in my K2 frame, but the K2 will eventually be swapped in once I get the oil filter bolt threaded bung welded back onto the lower case.

[PeWe]

Better to overdo it instead of redo it shortly after.
Pull engine and take apart cost some in gaskets and seals only. Plus oil. Do it again new gaskets and seals.
MLS and RCS gaskets can be reused which is one reason to use them.

If using old pistons. Check if ring groves are worn. Rings should not have play up-down.

[onepieceatatime]

Better to overdo it instead of redo it shortly after.
Pull engine and take apart cost some in gaskets and seals only. Plus oil. Do it again new gaskets and seals.
MLS and RCS gaskets can be reused which is one reason to use them.

If using old pistons. Check if ring groves are worn. Rings should not have play up-down.

Thanks.
I haven't been this far into an engine before, so I don't have the experience with pistons, cylinders, etc. The most I have done is heads, valves, timing chains, etc.

[onepieceatatime]

I didn't do anything too exciting last night. I took apart, cleaned, and rebuilt the front caliper and master cylinder for both this bike and the K2. Then I searched through my bolt bins for some fender and caliper mounting bolts that were missing. I messed around with a couple other little things in the garage, then bent the mount on the kickstand back straight and replaced the missing spring on it. Then I pulled the cylinders. The rods look as lovely as the cam chain tensioner. I guess next time I am in the garage (after I finish the Subaru, since got the other parts I was waiting on) I'll have to clear some bench space for this engine to come out of the frame and be split apart.

[onepieceatatime]

I started to mess with this project again, since I'm once again stalled on Ol' Sarge.
Whoever the ham-fisted buffoon was that opened this engine before doesn't deserve to own tools or bikes.





I managed to get them out, and have found out that they are a 12 x 18 knock pin (or dowel depending on where you look) and are available from Partzilla for $2 each, plus $7 shipping.
Does anyone know a better (cheaper) source?


I ordered some 2nd over pistons for this engine, because I am guessing it will need them, once I start to clean it up some more.

[onepieceatatime]

I pulled the engine out of the frame, and got it on the bench tonight.


Then I began taking it apart to see what kind of rust or other damage was lurking inside.

Rusty bolt and repaired clutch lifter plate.


And a crusty clutch basket. I have a complete spare clutch assembly, so even if this one does not clean up no big deal.


A bit of rust here too, but it looks like it should clean up ok.


The connectors on the 3 yellow wires are somehow fused together. Nothing I tried would get them to pull apart, so I ended up cutting them. I can put new connectors on during re-assembly.


Thick nastiness in the oil pan, looks like a ogre sneezed in there. When I cleaned that out, I found a lot of metal shavings in the pan too. Not what I was hoping for.


Cases split, gears in lower case look ok.


This looks like the worst of the rust inside the engine, but where did all the metal in the oil pan come from?


The #3 connecting rod doesn't want to rotate nicely like the others, and rocks on the crank. I don't think I like where this is going.






If any experts are reading along, do I need to replace that transmission gear, or is it likely to clean up with a little bit of effort?
I guess the decision I need to make now is a choice of 3 options.
Do I look for a crankshaft and connecting rods, do I look for a K7 engine to replace this one with, or should I put this one off to the side, and finish my K5 engine with the K7 head that Hondaman is fixing up for me, and put the K5 engine and sprocket carrier in the K7 bike.
This would effectively convert me to 530 chain from the 630, but is there anything else that would need to be done?

Will the K7 head work on the K5 engine without problems? I just had the K5 cylinders bored to 2nd over and installed the new pistons. Would I need to run the K7 pistons if I use the K7 head?

If I am effectively scrapping this engine, I guess I need to get to work on the K2 engine so it is ready for the Ol' Sarge project.

[PeWe]

K7 head is marked 392 between 2-3 on inlet side, right?
No problem on earlier engines.

I have a 392 head to be restored which will go on my K6.
It get the earlier valves, springs etc. but upgraded versions.

My K2 got cheap Cruzinimage +0.5mm K7 pistons which is an upgrade. Gives higher compression.
Check www.cruzinimage.net for a new set of K7 pistons. Total clearance max 0.025mm when bore. 0.020 must be fine. $120USD + shipping + bore job.

My 392 head did not have that big chambers as I thought. ~22.3cc -22.8cc.
Better if all chambers are equal. Grind them to same volume and shave head a little.

[BenelliSEI]

If any experts are reading along, do I need to replace that transmission gear, or is it likely to clean up with a little bit of effort?
I guess the decision I need to make now is a choice of 3 options.
Do I look for a crankshaft and connecting rods, do I look for a K7 engine to replace this one with, or should I put this one off to the side, and finish my K5 engine with the K7 head that Hondaman is fixing up for me, and put the K5 engine and sprocket carrier in the K7 bike.
This would effectively convert me to 530 chain from the 630, but is there anything else that would need to be done?

Good question? You’ve done lots to get to this state. I’ll be interested to see what others suggest, but my experience with engines in that state is you can spend a lot of money. Considering your initial investment, why not look for a motor that hasn’t suffered so much??

[PeWe]

+3
Better engine to spend money on.

[onepieceatatime]

K7 head is marked 392 between 2-3 on inlet side, right?
No problem on earlier engines.

My K2 got cheap Cruzinimage +0.5mm K7 pistons which is an upgrade. Gives higher compression.
Check www.cruzinimage.net for a new set of K7 pistons. Total clearance max 0.025mm when bore. 0.020 must be fine. $120USD + shipping + bore job.

The head that came off my K7 engine is with Hondaman being rebuilt. This is the only picture I have of the markings on it:

Someone had this engine apart before, so I cannot guarantee that it is the original head though.
Good to know I can use it on the K5 engine and at least save the expense of that head rebuild. Thank you

I have a set of those pistons on the shelf that I bought in anticipation for the need in the K7 engine. I also have an 836 kit (pistons, rings, head gasket, and bored cylinders) on the shelf; this kit will likely end up in my K2 engine. In addition to those, I have a set of the earlier +0.25mm pistons on the shelf, and a set of the earlier +0.5mm pistons in freshly bored cylinders on the K5 engine.

I have a head marked 410, which Hondaman recommended that I do not use, as well as a set of RC Engineering 64mm pistons and rings that I will not be using.

I have new cam chain and primary chains and tensioners as well as the primary replacement damper set I purchased from Cycle-X for the K7 and K2 engine.

I have 3 CB750 engines in the garage: K2, K5, and K7. I have 2 CB750 frames: K2 and K7.
Because I already have a spare engine that I have spent money on rebuilding, the K5, I was considering that for the K7 bike, even though output shaft, chain size, etc are different than the K7 had. The K5 head needs at a minimum valve guides replaced, the K7 head just got new guides and replacement valves and springs.

I have done a bit of looking in my area for a K7/K8 engine and not found anything reasonably priced yet.

[754]

You can use almost any year lower end contents to get the parts you need, so you may get a cheap motor that has fin or case damage...usually cheaper.
 I would be leery about the ball bearings in that transmission  unless still covered in oil.
 Are the metal shavings steel or aluminum  ?

[onepieceatatime]

You can use almost any year lower end contents to get the parts you need, so you may get a cheap motor that has fin or case damage...usually cheaper.
 I would be leery about the ball bearings in that transmission  unless still covered in oil.
 Are the metal shavings steel or aluminum  ?

It was aluminum in the oil pan.
I wonder if getting an earlier engine just for the crank and rods would be worth the cost, that would require new crank bearings too, correct?
I was also concerned about the condition of the transmission bearings. They look ok, but if I do try to save this engine, they will be replaced as well.

Sent from my Pixel 4 XL using Tapatalk

[Stev-o]

You can use almost any year lower end contents to get the parts you need, so you may get a cheap motor that has fin or case damage...usually cheaper.
 I would be leery about the ball bearings in that transmission  unless still covered in oil.
 Are the metal shavings steel or aluminum  ?

I wonder if getting an earlier engine just for the crank and rods...

When I blew up the engine on my K4  [Big Bang Bike] I bought an entire lower end for $100 and used that to build my motor back up.

Bought 836 pistons and jug [from Bill B] and was able to reuse my head. Runs like a dream...