Topic: CB750 Motor rebuild... for dummies

[Terry in Australia]

Yay progress.

Some of my parts arrived for the bike from Z1, cam tensioner and chain etc. I should have thought to look at the lights and harness because I will probably have to order some electrical connectors from them as well.
Still need primary chain and tensioner and the gaskets and oil sets, which I'll probably get from Cycle X.

So with only one rounded off bolt the crankcase has been separated.

It says in my Clymers that you can take that clutch base off, but i couldn't figure out how.
I skipped taking of the AC rotor off

and i couldn't get the neutral stopper plate off.

I was having some trouble getting the crankcase separated and I think it was because something was catching on this:



the oil guide?

Yeah mate, no biggie, you'll just need to panel beat the oil guide (for the drive sprocket chain oiler, if you want to keep it) back into shape, if you'd taken the top case off the bottom case, and not vice versa, it wouldn't have hung up like that. As I said, no biggie though, well done on your progress so far!

Hi All
I'm following this thread closely, as I am also in the progress of a rebuild of a K2 motor. Keep up the good work all of you who are helping out us dummies!

Can anyone give me any ideas on how to get the 4 countersunk screws out of the countershaft bearing holder (also described as the layshaft main bearing holder) and the single screw out of the end of the selector drum??? (see picture).

As you can probably see from the picture, I have tried to remove them with an impact screwdriver and it has completely rounded out one of the screws, and started to do the same with all the others. Damn!!

Do I actually need to remove these at all to split the case? My main reason for splitting the case is to replace the primary chains and the chain tensioner roller, which is pretty deteriorated.

Any help gratefully received.

Mal

Yeah mate, that's common, the original screws were "staked" with a chisel to stop 'em from turning, but they aren't actually siezed, there's way too much oil around for that.

Just drill the heads off with a 1/4" or 5/6" drill (you've already "countersunk" them, ha ha!) and pull the plate away, then turn the remains of the crews out with your fingers. Use a little threadlock on the new screws, or re-stake 'em. Cheers, Terry.

[Kevin D]

the drive sprocket chain oiler, if you want to keep it

Terry, are you saying this plate is optional?!?!??  If I take it off there will be no more motor oil leaking out on my chain and all over my number two pipe??!!!!???

[Bob Wessner]

Yup. If you remove it, no oil to be caught and sent down the sprocket shaft = clean pipes and rim.

[SandwichEnthusiast]

But if I take off the oiler off that means less lubrication to the final drive shaft and out to the chain, which means more manual oiling of the chain?
which I'm up for, but would like to know about it.

I know that the screw in the selector drum doesn't have to come out (to split the case I mean), cause I didn't take it out and I was able to split the case. I also don't think you will need to take out the bearing holder (4 screws) to take apart the crankcase, but I don't think that part of the transmission (or whatever that holds down) will be coming out without it.

I know my impact driver was rounding those screws out, and before I screwed it up completely I went out and got a new #2 and #3 Phillips socket from Canadian tire, the bigger bit is much better at holding on.
I also rounded out one of the crankcase bolts, the last one of course, but managed to get it off before tragedy occurred.

Some questions:

1.
Is the "spacer plate" in the middle of the "Neutral stopper" supposed to be straight, as in flat or is it kind of bent? I may have screwed it up when i was unscrewing the nut that holds onto the springs and Neutral stopper arm. Like when i was unscrewing the nut the arms were starting to press on the plate: to the uninformed you should, as soon as you get some slack, move those arms clear of the stopper plate If I did indeed mangle something.

2.
There was some sealing tubing on the oil pressure wire from crankcase to the alternator, which had to be removed to separated the coils etc from the rest of the engine, are those factory seals or something a previous owner did, and good idea/bad idea? comments?

3.
Somebody already mentioned that I should have flipped my crankcase upside down and taken the bottom off, instead of the other way around. I saw that in the manuals, but the pictures of the case just laying on the studs kinda put me off, am I just being a wuss? will they bear the wait of the whole crankcase on them? I was thinking of putting the studs through either the gap in one of those clamping work horses, or my workbench (old kitchen table which can split open for an extendo-leaf)

Thanks again for all the help guys, I'm still hopeful about getting this thing on the road when it gets nice.

[Kevin D]

Sandwich
 That spacer plate is flat and it needs to be, otherwise and it will be jamming with the shift shaft tangs. I think this is the most intricate part of the engine and I took a few photos to help me reassemble it.
 You gotta get those screws out and remove the bearing holder - the upper piece fits inside the upper shaft about 3/8".
 

[Kevin D]

Kind of a milestone here. I lifted the crank today and removed the chains.
Halfway point? The easy half I think. I have two months left if I'm going to make my April date. I don't yet have any new parts and I'm not sure I can get this stuff clean enough for a decent paint job. 

Sandwich - chain lube is thick and stays in place - mostly. Motor oil is thin and goes all over the place. And... I used a chunk of wood beneath the engine when I flipped it over, and a short brace in front to keep the weight off the studs. The 2x4s I added are spaced just wide enough to match the frame rails so I can prop the front end of the bike up.

[754]

A long time ago I left out the oil tray for the chain oiler,

then shat my pants and worried endlessly about trans bearing wear..  but about a month after I fired it I sorted out a few little things on it, then put 5K on it in a month with no noise, but I still worried a bit.. I think the bearing relies on splash lube..

 Re propping it on the studs.. depends on if you are in a hurry or not.. if the 20lbs of weight per stud would hurt them..they would never hold your top end on.. but petting something under it is not too hard..

[Terry in Australia]

Yeah, as I said, don't worry about the chain oiler, it was removed on the later engines with no other mods to improve oiling to the final drive shaft, with no problems. Cheers, Terry.

[ceruzziracing]

I'll jump in as well, also doing a rebuild on what I believed to be an untouched 18k motor. Well I thought it was untouched until I pulled my pistons. Stupid question probably but here goes... would the pistons have been numbered on the inside during initial build at the factory? Reason for asking, mine have had numbers scratched into them on the inside (1-4) like someone might do during a rebuild. Problem I see is that they are numbered exactly opposite to their relative position in the engine (1 is in the 4 spot, 2 is in the 3, etc). Plus, I could've sworn that this motor had never been opened (from the history I'd gotten from previous owner).
Only questionable wear I've found so far is on the #3 rod end (wrist pin end), plus all wrist pins starting to exhibit some grooving, however, plating remains intact. Haven't split the cases yet.
First mistake so far---trying to get rotor off with three jawed puller. Think I've succeeded in buggering up the inner threads on the crank end (can't get the bolt to start to thread easily). Plus, I never did get the rotor off  . Where did you guys buy your puller?

[paulages]

First mistake so far---trying to get rotor off with three jawed puller. Think I've succeeded in buggering up the inner threads on the crank end (can't get the bolt to start to thread easily). Plus, I never did get the rotor off  . Where did you guys buy your puller?

off of another bike--a 550 rear axle does the trick. that said, i usually don't bother removing the stator.

[ceruzziracing]

Nice, I'll have to grab one on eBay. Good tip, thanks!

[rachet]

I fit the criteria for this thread perfectly. . . I have a CB750 ('78) and I'm a dummy. . .   I am excited to use all this knowledge next winter to rebuild my engine.  Currently I am working on a non-CB Honda but that is for another thread.  Keep up the great work and I look forward to more Info and pictures.

[Terry in Australia]

I'll jump in as well, also doing a rebuild on what I believed to be an untouched 18k motor. Well I thought it was untouched until I pulled my pistons. Stupid question probably but here goes... would the pistons have been numbered on the inside during initial build at the factory? Reason for asking, mine have had numbers scratched into them on the inside (1-4) like someone might do during a rebuild. Problem I see is that they are numbered exactly opposite to their relative position in the engine (1 is in the 4 spot, 2 is in the 3, etc). Plus, I could've sworn that this motor had never been opened (from the history I'd gotten from previous owner).
Only questionable wear I've found so far is on the #3 rod end (wrist pin end), plus all wrist pins starting to exhibit some grooving, however, plating remains intact. Haven't split the cases yet.
First mistake so far---trying to get rotor off with three jawed puller. Think I've succeeded in buggering up the inner threads on the crank end (can't get the bolt to start to thread easily). Plus, I never did get the rotor off  . Where did you guys buy your puller?

Well it's definately been apart mate, but it might not have had a rebuild per se. My mate has a K2 he's owned since new, and it'd never been apart until his brother fell off it and smashed all the fins on the head and cylinder block, and as it was repaired under his insurance, the dealer replaced those items with new, along with the tank, pipes (alternator side) etc. Cheers, Terry.

[ceruzziracing]

I figured as much, must have been rebuilt south of the equator, all the pistons are in exact opposite order!

[bryanj]

Doesnt mean its backwards, just that the person who numbered em counted from the other end!!

[Terry in Australia]

Doesnt mean its backwards, just that the person who numbered em counted from the other end!!

Probably more likely Bryan, but I think "Ceruzzi" was taking the pisss outta me. (they're all so cruel to us Aussies..............)

[troppo]

[/quote]

Probably more likely Bryan, but I think "Ceruzzi" was taking the pisss outta me. (they're all so cruel to us Aussies..............)
[/quote]
so true, just cause we live on the underside of the globe, they look down on us and keep all the good bits to themselves

[754]

I herd you dont have to look under the motor ledge to get the bearing codes. they are on top down there !!    

[Terry in Australia]

I herd you dont have to look under the motor ledge to get the bearing codes. they are on top down there !!    

Only when I fall off........ 

[Roscoe]

Hi Guys, I'm a total newbie to this. I've been given a 1978 CB750F2 that is in an absolute mess. It aparently has dropped a big end bearing and if I can figure out how to post a picture of it you'll see what condition it is in. I have so far completely stripped it down, had the frame sand blasted and  powder coated black so it looks like new. But I need to pull the engine apart to get at the big ends but I'm a bit nervous about doing so. If anyone could help in this area with advise I would sure appreciate it. I'll currently from New Zealand but about to move over to Australia to live so most of the bike has been packed away waiting for the house to sell.
Cheers
Roscoe

[Roscoe]

Hi me again, After reading the thread I notice that it seems that you can split the engine crankcase without taking the generator rotor off? Is this correct?

[Jerry Rxman Griffin aka MuthaF'er]

Yes you can but you might find it easier to remove the alternator rotor (if you need to) with the crank still in the case. 

[Terry in Australia]

Hi Guys, I'm a total newbie to this. I've been given a 1978 CB750F2 that is in an absolute mess. It aparently has dropped a big end bearing and if I can figure out how to post a picture of it you'll see what condition it is in. I have so far completely stripped it down, had the frame sand blasted and  powder coated black so it looks like new. But I need to pull the engine apart to get at the big ends but I'm a bit nervous about doing so. If anyone could help in this area with advise I would sure appreciate it. I'll currently from New Zealand but about to move over to Australia to live so most of the bike has been packed away waiting for the house to sell.
Cheers
Roscoe

Bring it around to my place when you arrive in Oz Roscoe (I'm in Melbourne) I've got a nice F2 crank and rods just sitting on the shelf waiting for an engine................. Cheers, Terry.

[Roscoe]

Hi Terry, Thanks for that. We're moving to the Sunshine Coast, Mooloolaba as soon as our house sells. I'll keep in touch with you. Cheers, Roscoe

[Roscoe]

Yes you can but you might find it easier to remove the alternator rotor if you need to with the crank still in the case. 

Thanks for that. Can I use a 20mm bolt as the puller to get it off or do I need to buy the proper puller? Cheers, Roscoe