Honda GB400: Replacing Cylinder, Piston, and Valve Seals

[mark247]

Hey guys,

My replacement second hand ( but in way better condition than mine ) piston, rings, and cylinder are in the mail to me at the moment, along with a new set of valve seals. Hopefully this will stop my bike using 250ml of oil per tank of gas... I'm going to replace all the stuff myself and was wondering if anyone would like me to take photos etc to explain it all? I'm sure someone, somewhere, out their is interested... then again, maybe not?
It's going to involve me removing the camshalf, taking the head off, and removing the piston from the conrod etc etc. So yeah, just wondering if anyone would be interested?

[xwhatsit]

I would be interested Want to see what my next bike looks like on the inside.

[mark247]

I would be interested Want to see what my next bike looks like on the inside.

CB250 would probably look very simular to the gb400, i think?

[xwhatsit]

CB250 would probably look very simular to the gb400, i think?

Yeah, it's basically a more modern RFVC headed version of my engine, I think. In addition to being twice the capacity.

[mark247]

Yeah, it's basically a more modern RFVC headed version of my engine, I think. In addition to being twice the capacity.

Mind my ignorance, i know RFVC stands for Radial Four Valve Chamber. But whats so special about it if anything? lol

[Jacko2]

HiYa,
I'd be interested in seeing process.
Cheers

[mark247]

HiYa,
I'd be interested in seeing process.
Cheers

Sure thing, i will find some batteries for the camera and take photos and stuff while doing it all. Glad to see someone else is interested!

[Jacko2]

The girl has a 500 as a run-about. I grab it whenever I can till she kicks me off. So much fun. I'm gonna get one.

[mark247]

The girl has a 500 as a run-about. I grab it whenever I can till she kicks me off. So much fun. I'm gonna get one.

I wish i could afford a GB500, that little more power would be cool. But in saying that, i need to kept restricted when riding, i find i try to go as fast as possible everywhere.....

[xwhatsit]

Mind my ignorance, i know RFVC stands for Radial Four Valve Chamber. But whats so special about it if anything? lol

Well you'll see when you pull the valve cover off. The four valves splay out so they're pointing off at all different angles (radial). Compared to my head, which is more conventional, and they are much more upright, and each valve pair (exhaust and intake) are square.

The reason for doing this is so the combustion chamber is more hemispherical. My combustion chamber is more flat. The reason for this is a bit technical; not really sure, but it's more efficient or they can get more compression for less detonation or something like that.

Those `Hemi' V8s -- similar thing. Hemi == hemispherical combustion chamber.

[mark247]

Ok well, i havent got the piston and stuff yet in the mail but tonight i decided to rip into my bike. I have gotten the fuel tank off and everything and i have just finished taking the cylinder head cover, cam chain tensioner, and cam shaft out. I havent taken any photos of it with the head off yet because my camera has dead batteries so i have had to use my phone, which takes shit as photos, but i did take two of the bike before i started, and when i had ripped the tank and seat off, which really doesnt look that impressive. When i got the cylinder head off i noticed my cam chain tensioner set up is different to the one in the manual :S In the manual is says there is a "cam chain tensioner shaft bolt" in my bike there isnt one, i think the shaft which holds the tensioner in place on my bike is just held in by the cylinder head cover when its on... if there was a bolt to go in top of the shaft in my bike, there is no thread for it to bolt into, so i dont htink its missing.... im 99% sure my bike is just a slightly different design.. Other than that all is going swell. Oh i remember one think i stuffed up on, when i was pushing on the "camchain lever" with my big ass screw driver i think i kinda ripped it a bit... its made of hard rubber stuff, but nothing broke off so i think its all sweet...

Anyway, all is going well. I will update later as soon as i get around to doing more..

[xwhatsit]

Wow that is a seriously similar design, even the frame, to my dear little RS. It's like they just redesigned my bike and put a different style tank and seat on it. Cool. Gives me hope.

That camchain lever, if it's like mine, then you were pressing down on a little wedge, in order to let the bigger wedge (what does the actual tensioning) release. Correct? If so, then don't worry if you munged it a bit, just so long as it's still able to wedge the other slide in place. What that's there for is to stop the tensioner untensioning if the camchain presses on it too hard.

It seems odd that there is a bolt in the manual and not in your engine. Not like they would've changed design for something like that. Did you get the tensioner out? You sure there isn't a hole in it somewhere for the bolt? If so, it's not a huge matter I think, as there will be backwards pressure on it anyway from the camchain.

Are you replacing the camchain? You should do it, it will only cost $80 or so. Very important, might as well do it now while you've got everything apart. That nice new piston could have a couple of dents in it if you neglect that too long (Honda singles are known for their camchain issues apparently).

[mark247]

Wow that is a seriously similar design, even the frame, to my dear little RS. It's like they just redesigned my bike and put a different style tank and seat on it. Cool. Gives me hope.

That camchain lever, if it's like mine, then you were pressing down on a little wedge, in order to let the bigger wedge (what does the actual tensioning) release. Correct? If so, then don't worry if you munged it a bit, just so long as it's still able to wedge the other slide in place. What that's there for is to stop the tensioner untensioning if the camchain presses on it too hard.

It seems odd that there is a bolt in the manual and not in your engine. Not like they would've changed design for something like that. Did you get the tensioner out? You sure there isn't a hole in it somewhere for the bolt? If so, it's not a huge matter I think, as there will be backwards pressure on it anyway from the camchain.

Are you replacing the camchain? You should do it, it will only cost $80 or so. Very important, might as well do it now while you've got everything apart. That nice new piston could have a couple of dents in it if you neglect that too long (Honda singles are known for their camchain issues apparently).

How do you go about replacing the cam chain? I cant find it in the manual...

[xwhatsit]

I have heard of some cam chains having joining links in them, but mine certainly wasn't and I doubt yours will be. So you need to take off the right side cover (where the clutch is). You may have to remove another couple of gears to get access, but without too much trouble you can get at the bottom camchain sprocket and just pop it off along with the chain. Then putting the new one in is just a simple matter of feeding the chain down the barrel and putting the sprocket back on.

Not really a hard job once you've got the top end apart.

What sort of manual have you got? It ought to say somewhere, it's a job that needs doing more often than replacing the piston.

[mark247]

I have the Honda service manual for the GB500, aaaaaaaaaah it says 500... ( its a pdf so i never looked at the cover ) that will be why its a little different... I wonder if the torque settings will be the same for the gb400?