1982 CB750F...Better Devil

Snout

Active Member
Interested to see what you find. My 1981 CB900 is a similar animal and has always had some rolling noise in the bottom end, worse when cold. Heard starter drives are a common noise in these too. At 49000 miles it is likely due for some love, bit it still rips so I rode on.
 

Jimbonaut

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There's a lesson in here somewhere, probably two...

Found the problem, or at least one of them. I bored this engine out to 823cc, and the pistons I bought also came with an oversize head gasket. Which, it turns out, I didn't install. I installed the regular 750cc head gasket that came with the Athena gasket kit I bought. The thing is I woke up in the middle of the night about a month ago asking myself which gasket I installed. The right one or the fucking wrong one. I internally debated with myself for a week and told myself I installed the right one.

Fucked that up. Should have listened. Lesson learned.



You can see the deformation on the ring, I'm thinking that has to have been what was causing the noise, at least some of it. There was also some oil on top of the pistons -



which I'm thinking should not be there. Any ideas why it is? Is it also part of the wrong head gasket cock-up?

I'll pull the block tomorrow and get to the base gasket. Would the incorrect head gasket cause or influence a leak at the base?

Any other damage that can be caused by my fuck up?
 
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irk miller

You've been mostly-dead all day.
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DId it smoke when you ran it? It's possible the oil is from the head gasket being effed up, but usually oil on the pistons is from oil dripping through the valves while it sits. Your situation isn't exactly normal, so looking at normal causes and solutions, well...
 

Maritime

Well-Known Member
I would assume the wrong gasket is the culprit but Ass U Me so keep looking. It may also have prevented proper seating and torquing on the head, which would cock up the base. But I would also double check the locating dowels are seating 100% in the holes for the base as well.
 

Jimbonaut

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It didn't smoke, no - I didn't see any sign of burning oil. All the valves and seats were lapped and new viton valve seals installed. I'll pull the block and investigate further...
 

Snout

Active Member
Good to see nothing catastrophic in there. You will get it sorted and be back on the road in no time.
 

Jimbonaut

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Been a while since the last update. Got a bit sidelined what with one thing and another, and the engine rebuild stalled big time as it took forever to source an OEM base gasket. That's now on order and will hopefully be here end of this week.

Started work on the seat pan, I'll be using 16 gauge steel for this one. This won't be a brat seat but styled on the old Kawasaki seats and in particular the seat on @doc_rot 's beautiful KZ750. The pan therefore will need a lot more fabrication to make it work. I got an idea here (I don't remember the thread) where someone used an ear protector cup to shape the peak at the front of the pan, and I'm all out of ear protector cups. So that'll be made from steel too, and welded on. Then I'll foam (that'll be a first too) and then pass it over to my man Rod Alves for upholstering. Needs a bit more massaging but it's getting there -





I scored the steel with a dremel to help with the bends, and rigged up this ghetto break to finagle them -



The 2x4's are bench-vise'd one end and screwed together the other. The screwing (and unscrewing - lots of both) was made 1000 times easier by the Ridgid impact driver I bought on the weekend - $120CAD together with a drill. Register the things online and you get free warranty for life. Never pay for another battery or part again. Amazing.
 
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Nice!! Been waiting for this.

How did you get the smooth bend in it? I had to use a round pipe and mallet over that, but you're just using square 2x4, but looks like you're getting a smooth upper curve nonetheless.

Lookin good.
 
Also, is that cardboard the actual width of the pan? Or will the steel pan be narrower than that?

I'm thinking i cut mine a touch too narrow. Good for looks, not for comfort. Won't find out until its done.
 

Jimbonaut

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In order to get the rounded curve I dremel'd three fold lines where I wanted the curve. I then sanded a curved profile onto the edge of my wooden workbench, clamped the pan onto the bench, used a map pro torch to heat the area I wanted bending, and bent the pan around the curve. Takes a while to get the curve right, and a bit of massaging to make everything fit.

The cardboard is the actual width at the pillion, but flares towards the tank. The pan follows the same lines. The shape of course is entirely a personal choice (the pan on my other CB750 followed the frame rails exactly), but this seat will have a completely different shape and therefore I need a wider pan where it meets the tank. I wouldn't be too concerned that yours is too narrow - once it's upholstered I'm sure it'll be fine.
 

Jimbonaut

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This is what I mean by scoring the fold lines with the dremel (using a cut off wheel) -



and here's my ghetto round-edge-bending instrument -



Everything's pretty lo-fi in my garage.
 

Snout

Active Member
Points should be awarded for doing things low tech and still getting things done. I think that is the true spirit of home building. An old man I know told me of a cousin of his that merged a model T and a truck rear end to make a home job tractor with only a hacksaw and a hand drill. Looking good there.
 

doc_rot

Oh the usual... I bowl, I drive around...
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its very easy to get a radius bend like that using a wooden "slapper" and a T-bar. you can make both quite easily in a home shop, altough welder is required for T-bar. How are you attacheing the seat? it may be advisable to add some sturcture to give it rigidity against the forces of the seat cover.

go to 8:00 to see it in action
 
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that metal plate looks like aluminum. He's bending it like a paperclip.

Not to speak for Jimbo, but last seat pan he did was also 16 gauge steel and said he had no issues with lack of structure or rigidity. This one will probably be the same.
I did mine with 18 gauge and i'm adding a "frame" as it is definitely a bit flimsy.
 

doc_rot

Oh the usual... I bowl, I drive around...
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that metal plate looks like aluminum. He's bending it like a paperclip.

Not to speak for Jimbo, but last seat pan he did was also 16 gauge steel and said he had no issues with lack of structure or rigidity. This one will probably be the same.
I did mine with 18 gauge and i'm adding a "frame" as it is definitely a bit flimsy.
I have successfully bent 16ga steel in this fashion. remember you are not completing the bend in one strike. 16 GA is clearly strong. My question was on the mounting system was in relation to what tolerances for fit are acceptable. everyone's opinion will be different in that regard.
 
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Jimbonaut

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Yeah, 16 gauge is becoming my go-to for this kind of thing. It definitely feels a lot more substantial than 18, and both axis really don't flex much at all. I'll be securing the rear with rivnuts (which I'll bolt into through the brace I welded between the frame rails) and the front will bolt into a cross brace on the frame just in front of the battery box. It'll be secure.

Speaking of which, the pan is shaping up pretty well, and after this eve's encouragement/massaging/muscling it's pretty much in shape -





Trim a bit off the back of the pan, build up and weld on the hump at the front and then foam.
 
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irk miller

You've been mostly-dead all day.
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These are what I use to mount a lot of my seats. Can be bought cheap on Ebay. This is a GSXR pillion seat, but lots of sport bikes utilize the same system.

1592395918103.jpeg
 

Jimbonaut

Well-Known Member
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But, but what are they Irk? What should I google? If I can work that mechanism into both the pan and frame then I’d be levelling this thing up for sure.


Sent from my iPhone using DO THE TON
 

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