1970 cb 4-1 on a 1981 cb?

[tonto]

So im building out a 1981 Honda cb750 custom into a café racer, and ive stumbled upon a great deal for a 4-1 exhaust system from an earlier 1970's model. im under the impression that this engine changed very little until the mid 1980's

is this something that will fit with a bit of elbow grease, or am I just looking at something to swear at?

Armature hour;)

[TOOLS1]

I have seen late exhaust systems on early bikes, so I assume it would work the other way.
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[shinyribs]

Oh, really? Hmm...I just got some options opened up then...

[glara11]

So is this fact or fiction? I also have seen sweet deals on late model exhaust system that id like to take advantage of.

[TOOLS1]

glara11 wrote

So is this fact or fiction? I also have seen sweet deals on late model exhaust system that id like to take advantage of.

Yea, Like I make up everything I post on here!
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[seestheday]

I'm assuming we're talking about a header/exhaust combo.

You would need the collars from the original exhaust for it to work.  DOHC collars are definitely different than the SOHC ones.

[glara11]

Well Ive been doing some research lately and it has been mentioned that 1970-1978 models dont have a oil pan so it wouldnt fit in our 1979-1983 models.

[Murrayf]

the early (1970) cb 750 has a slip on the ends of the pipes and clamps on to a spigot/pipe

that protrudes out of the cylinder head and it squeeze clamped in place by the flange

the later ones have a gasket and 2 bolts that go thru the flange and compress them on

i have no idea how you would ever inter change that??


i have both type of bikes plus exhaust systems off the bike if you need a pic

[LukeM]

With a torch in one hand, and a hammer in the other, just about anything is possible.  

The important part, as mentioned earlier, are the collars that secure the exhaust pipe to the exhaust port. Keep in mind the following: oil pan clearance, access to the oil filter (becuz it would suck to have to remove the whole exhaust just to change the filter), leg clearance (depending on where the muffler is), etc.

If you get it connected without too much drama, I'm sure others on our forum would love to see it.  It'd be worth a write-up.

Luke M

[Murrayf]

Well

maybe i stand corrected lol

i happened to look at another one of my 71s also built on 7-71

its a different model maybe   but it has the regular bolts coming out of the head instead of the clamp

type flange so now i will have to walk away and figure this out as i know both bikes are original

[shinyribs]

On the SOHC engines the exhaust spigots and the crush collar type flanges are interchangeable.  But a spigot style header will always require spigots, obviously.

The exhaust systems will interchange, but only as an entire system.

When I get time ill measure the stud spacing and pipe spacing on my SOHC & my DOHC and see what it looks like.

Oh, and the early bikes do have an oil pan as well. They all have oil pans. Some models (DOHC and SOHC matics) just have deeper pans since they don't have external oil tanks.

[TOOLS1]

The stud spacing is the same on all the CB 750 engines. Both SOHC, and DOHC. I know, because (some of you will not believe this) I have both. The early engines 69-75 had the spigots that bolted to the head, and then the exhaust pipe slipped over it. The 76-83 (both SOHC, and DOHC) just use a crush washer (also used between a spigot, and the head on earlier models) and a finned flange with half moon wedges. The exhausts can be switched by just removing, or installing spigots. Also the port locations are the same on all the engines. However you might have clearance problems under (on 4 into 1 systems) or at the rear where the exhaust mounts.
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[shinyribs]

Way to steal my thunder buddy! I measured them tonight and they match just like you say. The DOHC heads have what look like deeper fins around the exhaust ports. I'm not sure if you used a SOHC spigot style exhaust if you'd be able to get the clamps on or not.

Hopefully,if I ever get the time,I'm gonna try a spare set of pipes on there and see what it ends up looking like.