New to the CB750 and have header question.

[breakbeatz69]

Hey everyone! So I’m new to the forum but not new to bike forums or metric bikes in the least. I’ve been building choppers and bobbers out of sportsters a little but my main bag of tricks comes from building choppers and bobbers out of the Yamaha XS650. I’ve done numerous xs’s with everything being done in-house by myself, I’ve had a couple bikes featured in magazines, and blah blah blah.

But now I’m tackling the one that always seemed to allude me, and that’s….you guessed it…a 1981 CB750c. I’m super stoked to finally do one of these bikes but I’m LOST when it comes to the DOHC style engine. I can tear down Harley Evos and Yamaha parallel twins like nobody’s business but this CB750 is a different monster entirely. I did order a Clymer service manual because, for one, I’m not an idiot that thinks I’m just gonna fly through this bike because I know a thing or two about motorcycles, and two, I know I’ll need it to understand this beast of an engine….so at least I got that going for me.

So to get into my first of many questions to come, I got my donor bike home today and got it in my shop and wanted to put it on my motorcycle jack right away but realized the 4 into 1 exhaust was in the way of the jack being able to sit on the lower frame rails. So I went to take the exhaust and headers off and when I did only 5 of these half moon shim looking things came falling out as I removed the headers. I checked the exhaust ports and there weren’t any stuck in the engine or on the headers themselves. So I have 5.

Now, I’m no mathematician, but with 4 exhaust ports having 5 of these things doesn’t make sense. So I looked online to find new ones to buy and couldn’t find shit in the way of any new ones. I did however find a set of 8 used ones on eBay and promptly ordered them since they were pretty cheap. Now, 8 makes sense to me, 2 for each header, but what in the fuck is the purpose of these things (whatever they’re actually called)??? I know there’s also gaskets that go into the 4 exhaust manifold ports, but what is the purpose of these half moon looking pieces? Are they some sort of weight support due to all the steel from 4 headers hanging off of the engine? Or do they serve another purpose? And are they necessary to have, or are they a part that can be deleted?

If you made it this far thanks a lot for riding with me until the end. My posts from now on won’t be as long winded now that I got my introduction out of the way and everyone has an idea of my experience level. Any and all comments/advice are welcome and thank you all in advance for your help. FTW!

 

[brettp]

When installed with the stock exhaust, the tightening of the "crowns" at the header use those to pull the exhaust towards the head.

 

[breakbeatz69]

When installed with the stock exhaust, the tightening of the "crowns" at the header use those to pull the exhaust towards the head.

Thanks for your reply. And what would be the result of not using them and just using the crowns and gaskets?

 

[brettp]

Well, those are used for a 4 into 4 exhaust. My 4 into 1 came with different crowns specifically for that exhaust. Not sure if they're usefulor in the way with aftermarket exhaust.

 

[breakbeatz69]

Yeah I’m just trying to understand if these pieces have any real effect on the 4 into 1 exhaust system that I have on the donor bike I bought. Like do they have any sort of sealing effect that the gasket wouldn’t do alone by their selves? I guess my real question is do I have to use them. Like, they pull the headers towards the gasket is what I’m getting from what brettp said. So if you don’t use them, the headers won’t seal tightly to the gaskets? If that’s the case that’s a piss poor design by Honda. Why not just leave a little more meat on the bone on the header ends and just have the crowns smash the header end directly into the gaskets and into the engine? Maybe I’m not understanding something here.

 

[mrtwowheel]

It might help if you post pictures of the engine side of your headers and the hardware that you have. Aftermarket headers use different hardware. Myself, I like those pieces on stock headers, good design, easy to assemble. You probably know that the gaskets are buried in there and need to be dug out, hard to see. Welcome, we like pictures.

 

[breakbeatz69]

It might help if you post pictures of the engine side of your headers and the hardware that you have. Aftermarket headers use different hardware. Myself, I like those pieces on stock headers, good design, easy to assemble. You probably know that the gaskets are buried in there and need to be dug out, hard to see. Welcome, we like pictures.

Will do, I get some pictures when I get home this evening. I’m pretty sure they’re stock headers though.

 

[breakbeatz69]

Here’s the photos….

 

[mrtwowheel]

I don't know if those are stock pipes, never had a set of 4 into 1 pipes. Those do look like stock type ends on your headers. So you have 4 collars and some of the "half moon" things when you really should have eight. Can you show a picture of the collars and the other pieces?

They're easy to assemble. Slide the collars over the ends of the header then position two of the half moon pieces between the collar and the swage on the end of at least one pipe, place the pipes in place to the engine, secure at least that one pipe with nuts to the engine. Then the rest of the collars and half moons can be positioned and loosely secured with nuts. The thick part of the half moons go towards the engine. There is no torque specified for the nuts.

 

[mrtwowheel]

If you want to blast and paint your crowns/collars and half moon shaped retainers that design works great. I painted mine with high temp ceramic paint.

 

[breakbeatz69]

Here’s some pictures of the half moon pieces. All of them are beat up and bent or cracked in some area of the piece.

 

[brettp]

They might still work. But if you have to replace half, just replace them all.

 

[mrtwowheel]

Yeah you've got original parts there. They weren't placed into the collars right, slightly out of whack, that's why they're bent up. I've found that before and straightened them good enough to use. Don't know about yours.

Edit....on second look, those are usable, just straighten them so they fit into the collars.

 

[mrtwowheel]

Yeah you've got original parts there. They weren't placed into the collars right, slightly out of whack, that's why they're bent up. I've found that before and straightened them good enough to use. Don't know about yours.

Edit....on second look, those are usable, just straighten them so they fit into the collars.

But you gotta have eight.

 

[breakbeatz69]

Yeah I ordered 8 used ones on eBay because I couldn’t find new ones anywhere. These are the condition of the 8 used ones. They’re rusty but they don’t look all bent and beat up…..

 

[Loose Chain]

Judging by the amount of oil buildup on those parts I would say that engine is a bit of an oil burner. What are you plans for the engine?

 

[breakbeatz69]

Judging by the amount of oil buildup on those parts I would say that engine is a bit of an oil burner. What are you plans for the engine?

Well, the engine ran when I bought the bike. It sounded alright but I’m obviously going to go through the engine once I have a better understanding of it and replace components that should be replaced. If it is an oil burner, obviously new piston rings would be something to look at if oil is getting past the rings and into the exhaust manifolds.

I know on the xs650, there are certain engine upgrades such as a few different types of seals, clutch push rod, overdrive rear sprocket…etc that are kind of standard things to replace right off the bat. Are there certain things the same on these dohc cb750’s that are must have upgrades or replacements just to have a better running engine?

Also, I’ve already opted and ordered the following upgrades for this bike because they’re just great upgrades to add to other metric bikes I’ve done. So on the way I have an electronic ignition upgrade, a PMA system upgrade, a mikuni double carb setup into a 4 way intake manifold, and some other shit I know I’m forgetting.

 

[Loose Chain]

Well, the engine ran when I bought the bike. It sounded alright but I’m obviously going to go through the engine once I have a better understanding of it and replace components that should be replaced. If it is an oil burner, obviously new piston rings would be something to look at if oil is getting past the rings and into the exhaust manifolds.

I know on the xs650, there are certain engine upgrades such as a few different types of seals, clutch push rod, overdrive rear sprocket…etc that are kind of standard things to replace right off the bat. Are there certain things the same on these dohc cb750’s that are must have upgrades or replacements just to have a better running engine?

Also, I’ve already opted and ordered the following upgrades for this bike because they’re just great upgrades to add to other metric bikes I’ve done. So on the way I have an electronic ignition upgrade, a PMA system upgrade, a mikuni double carb setup into a 4 way intake manifold, and some other shit I know I’m forgetting.

You might be putting the cart before the horse.

 

[Motogrady]

I wanna see the 2 into 4 carb setup.

 

[mrtwowheel]

A lot of that black or maybe all of it could be carbon. That bike was ran without the exhaust retainers, maybe for a long time, like no seal at the exhaust for a long time. Do a dry compression test before you spend more money.