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Stockish rebuild? What should I do

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21chayton

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Im currently building a 1978 sohoc and im kinda conflicted on what to do I don’t think I want to go and try to build a powerhouse just yet I just upgraded from a 550 i built and I just wanted something a bit quicker and I think the 750 will be a great place to start because when or if I decide I want more power I can bore up and make a good deal more.

The thing is I really only have about $1,500 for the motor and I want to build a super reliable but fun bike. Now i know I’d like to get a new clutch and electronic ignition but when it comes to the internals of the motor I’m not too sure what I should do

I’ve heard I should get my crank balanced and the crank case honed because off the line there not perfectly straight but I’ve also heard I should send my head to get rebuilt for better flow and a more reliable valve setup and lots of stuff about the rod bolts but if I’m just rebuilding the motor stockish what are best mods I could do for a little more power but the most reliability
 
How many miles on it? Does it currently run? A rebuilt head will cost you $1000 to $1500 (or more) depending on what you have done. I just had a cylinder head "stage two" done for $1500 and that was with friends and family discount. Align honing the cases for the crankshaft is overkill unless you plan to do some serious racing. New pistons and cylinder boring will run you another $1000 to $1500. You can find some basic pricing here (scroll down):

http://www.cyclexchange.net/Machine Work & Services.htm
https://cbrzone.com/sohc.html
I would suggest getting the motor running first and go from there. Ripping it apart and splitting the lower cases is not always the best approach. Most CB750 motors I encounter are still in good working order and the "from the factory" configuration is good for many tens of thousands of miles. Most struggle getting the carbs right, especially the later PD carbs found on the 77/78 models:

http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204
http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
 
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I couldn’t tell you how many miles are on it no more than 30,000 I’m sure, but I do know that the bottom end crank case is leaking a little bit, so maybe it wouldn’t be a bad idea to just tear it all apart. Give it a good refurbish and put it back together.
 
I also was able to drive it just a little bit when I first bought it just to know it was in OK condition before I went ahead and tore it apart and built a new frame for it and I’m not so much concerned about the carbs because on my CB 550 I was able to get the carbs tuned almost perfectly, and I was very happy with the condition that I was in. I’m sure I can do it again with this bikes carb set up I just wanted a bit of a quicker bike I’ve been considering since I building a chopper what I change the ratio so it had a little bit more torque and less top end speed
 
I was just reacting to your $1500 budget for the engine. If you split the cases you can quickly exceed $1500. I don't know if I've ever seen a CB750 leaking between the seams in the upper and lower case, typically the leaks are at the gaskets or seals which can all be replaced without splitting the cases. I race bracket ET class with nothing more than a mild flat top 836 kit and a big rear sprocket:

Valdosta Round 2c.jpg



Never been into the bottom end. The new head is for the next motor that will also include higher compression 836 pistons.
 
Oh hell yeah that’s bad ass is shit so definitely upgrading the sprocket size would give me a much quicker bike and I’m almost considering an 836 kit but I think I want to just rebuild the motor play around with it and if I’m not very happy with how it is then I’m gonna do an 836 and possibly some head work, but for some reason it just looks like the leak is coming from where the case splits I could be wrong and it could just be running down from the cylinder head but I’m just gonna tear everything down and see how it looks. Build a really really sturdy bike, so I almost think it’d be a good idea to tear down the case anyway and put some bearings in just to make it that much more sturdy but why is it that when a split the case it becomes so expensive
 
That last picture looks like it could very well be leaking between the upper and lower case halves. Splitting the cases gets expensive when you fall down the rabbit hole mentality of "while it's apart" might as well do the following:

>New primary chains and tensioner
>New primary drive cush rubbers
>New bearings throughout, including crank, rods, and transmission bearings. This is where you touched on align honing the cases for the crank.
>Undercut transmission for reliable shifts and to avoid false neutrals
>Overbore with new pistons. New connecting rods will add another $800 to $1800 depending on how much OCD kicks in.
>New cam chain and tensioner

For now, you could pull the motor, flip it over, remove the lower case half, reseal the upper to lower case halves and put it back together. If you can do that while resisting the temptation to do any or all the other things I listed above then you have a lot more will power than I do! :cool:
 
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It looks like your bike has been in an accident. One frame tubes is bent and the other side looks like a welding repair had been done. With the engine being solid mounted to the frame, the engine can show signs of damage after the fact from an accident like the case halves leaking. You sound like a bit of a dreamer, and that's okay but what you are talking about doing will cost you a lot of time and money. It would be much cheaper for you to buy a crotch rocket with the money you are talking about spending and leaving the 750 as is.
 
madmtnmotors said:
That last picture looks like it could very well be leaking between the upper and lower case halves. Splitting the cases gets expensive when you fall down the rabbit hole mentality of "while it's apart" might as well do the following:

>Undercut transmission
>New primary chains and tensioner
>New primary drive cush rubbers
>New bearings throughout, including crank, rods, and transmission bearings. This is where you touched on align honing the cases for the crank.
>Undercut transmission for reliable shifts and to avoid false neutrals
>Overbore with new pistons. New connecting rods will add another $800 to $1800 depending on how much OCD kicks in.
>New cam chain and tensioner

For now, you could pull the motor, flip it over, remove the lower case half, reseal the upper to lower case halves and put it back together. If you can do that while resisting the temptation to do any or all the other things I listed above then you have a lot more will power than I do! :cool:
Click to expand...
I like all those ideas now I’ve never looked into undercutting the transmission what’s that all about and I’m having a hard time finding all the bearings for the bottom end for the 78 where might a good place to look be
 
Loose Chain said:
It looks like your bike has been in an accident. One frame tubes is bent and the other side looks like a welding repair had been done. With the engine being solid mounted to the frame, the engine can show signs of damage after the fact from an accident like the case halves leaking. You sound like a bit of a dreamer, and that's okay but what you are talking about doing will cost you a lot of time and money. It would be much cheaper for you to buy a crotch rocket with the money you are talking about spending and leaving the 750 as is.
Click to expand...
The picture is pretty bad the bent tube is just from the handle of the table I had the bike on and I’m welded up a hardtail kit around the motor so that’s what all the welds are for
 
madmtnmotors said:
That last picture looks like it could very well be leaking between the upper and lower case halves. Splitting the cases gets expensive when you fall down the rabbit hole mentality of "while it's apart" might as well do the following:

>Undercut transmission
>New primary chains and tensioner
>New primary drive cush rubbers
>New bearings throughout, including crank, rods, and transmission bearings. This is where you touched on align honing the cases for the crank.
>Undercut transmission for reliable shifts and to avoid false neutrals
>Overbore with new pistons. New connecting rods will add another $800 to $1800 depending on how much OCD kicks in.
>New cam chain and tensioner

For now, you could pull the motor, flip it over, remove the lower case half, reseal the upper to lower case halves and put it back together. If you can do that while resisting the temptation to do any or all the other things I listed above then you have a lot more will power than I do! :cool:
Click to expand...
And I haven’t seen any sort of gasket for the cace halves, is that just one of those things I need to use gaskets maker on or what
 
21chayton said:
The picture is pretty bad the bent tube is just from the handle of the table I had the bike on and I’m welded up a hardtail kit around the motor so that’s what all the welds are for
Click to expand...
Why would you do that to the frame?
 
21chayton said:
I like all those ideas now I’ve never looked into undercutting the transmission what’s that all about and I’m having a hard time finding all the bearings for the bottom end for the 78 where might a good place to look be
Click to expand...

Sounds like you are well on your way to spending in excess of $5,000 on just the engine...


21chayton said:
And I haven’t seen any sort of gasket for the case halves, is that just one of those things I need to use gaskets maker on or what
Click to expand...

Any sort gasket between the case halves would affect crankshaft main bearing clearances. Hondabond, used sparingly, and absolutely NO SILICONE.
 
madmtnmotors said:
Any sort gasket between the case halves would affect crankshaft main bearing clearances. Hondabond, used sparingly, and absolutely NO SILICONE.
Click to expand...
a
Just out of curiosity, has anyone tried an anaerobic sealer on the case halves? I was a GM mechanic for over 20 years and GM had an anaerobic sealer that was used on stuff like transfer cases, front differentials and manual transmissions. It was similar to this and is designed to cure in the absence of air and the presence of metal. It was also less messy to use than Hondabond.
 
Old_Crow said:
a
Just out of curiosity, has anyone tried an anaerobic sealer on the case halves? I was a GM mechanic for over 20 years and GM had an anaerobic sealer that was used on stuff like transfer cases, front differentials and manual transmissions. It was similar to this and is designed to cure in the absence of air and the presence of metal. It was also less messy to use than Hondabond.
Click to expand...

Agreed. I've used the anaerobic sealer that was specified by the manufacturer to seal the case halves on two stroke outboard engines.
 

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