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?

[mark247]

Ok so here is where im up to. I have got the head and bore off and have taken the piston out. Its wierd, i havent measured the rings or anything but the bore looks not bad at all and the piston looks alright too. Im guessing most of the oil is gettng through the valve seals at this point in time, i cant get the springs out here because i dont have a compressor, so im going to get the bike shop to do that. Looking at the valves from the bottom they look quite carboned up, and also the spark plug is shocking, and i cant get the spart plug out because none of our spark plug tools things fit in past the head to get to it, another job for the bike shop i guess..

I have had a few minor mishaps on the way, i missed one of the head bolts and was wondering why when i pulled on the head it wasnt coming off, but i finally found it RIGHT IN BY THE SPARK PLUG, bloody hard to get at but i go it. Oh and when i was unscrewing one of the bolts in the head i knocked a piece off the place where the camshaft bearings fit, but its very minor ( see the left point in the pic ) oh and there has been a bit of rubbing going on with the cam chain, i can see wear on the casting. But for all i know that happened a longggg time ago.

So where am i up to?

Well none of my stuff has come in the mail yet, so i need to wait for that, and i cant get the valve springs out. So next thing to do is get the stuff in the mail, take the head into the shop and get them to put the seals in for me, also get them to get the spark plug out ( i might just worry about that later )

I also need a torque wrench so i can put my new second hand bore back on.

The thing im most worried about is getting the camchain, tensioner and camshaft back on correctly, i really dont wanna stuff that up. But ill worry about that when i come to it.

[mark247]

Here are some more pics ( kinda useless ones )

[mark247]

Well sinse last time i posted about this the guy who was sending me the replacement piston and stuff finally looked at the piston he was gonna send me and found it had dents in the top of it ( probably not going to buy anything from them again ) , so i got my money back off him. I thought all was lost, but i found a really good piston and bore in Tauranga that i got cheap, the shop has just finished "refreshing" that which probably means a quick hone, and some other stuff. But the rings are all good so thats whats important! And i have just got my valve guide seals installed. Tomorrow im going to pick it up and hopefully get it all back togeather in the evening in time to go to the kiwibiker meet up in tauranga :rockon: If i remember i will take photos of putting it back together.

[okeh]

Well sinse last time i posted about this the guy who was sending me the replacement piston and stuff finally looked at the piston he was gonna send me and found it had dents in the top of it ( probably not going to buy anything from them again ) , so i got my money back off him. I thought all was lost, but i found a really good piston and bore in Tauranga that i got cheap, the shop has just finished "refreshing" that which probably means a quick hone, and some other stuff. But the rings are all good so thats whats important! And i have just got my valve guide seals installed. Tomorrow im going to pick it up and hopefully get it all back togeather in the evening in time to go to the kiwibiker meet up in tauranga :rockon: If i remember i will take photos of putting it back together.

Remember the con rod connects to the exhaust valve via the rear wheel.

[xwhatsit]

^Lol

You can get the valve springs out without a compressor. It's best to do with somebody's help, but put the head so it's sitting on something like a small paint pot (so you aren't resting on the studs, if you have those), and stuff a rag underneath so the valves can't push down. Then get an open-ended spanner and press down on the valves; on my 250 it was relatively easy, on your 400 it might require a bit more strength. Then your helper (or you if you have a articulated tail) just needs to pick out the little split collets (don't lose the bastards). Even easier to put it back again.

The valve seals are also very simple. Some long pliers to pull out the existing ones, and then the new ones are just a press fit. Don't need to spend money at a bike shop for that.

Remember; if you need cheap parts, Econohonda is your friend. http://econohonda.co.nz .

[mark247]

Well first off, i forgot to take photos. But i got it working! I cant believe it actually started first time. Sounded like a fish tank with no exhaust but who cares, it still started! I havent put the fuel tank on properly yet, or the seat, or anything like that, oh yeah and the exhaust obviously. But i managed to fix it! Im so surprised it works, i thought i would stuff something up.

By far the hardest part was the camchain tensioner and getting the chain onto the sprocket, i needed another set of hands for that but dad helped me with that which was good.

Oh and my brother had to help me put the piston into the cylinder, that was a bit tricky with the rings and all.

Other than that stuff it seemed to go smoothly.

I cant believe it actually works!!!!

[Kickaha]

^Lol

You can get the valve springs out without a compressor. It's best to do with somebody's help, but put the head so it's sitting on something like a small paint pot (so you aren't resting on the studs, if you have those), and stuff a rag underneath so the valves can't push down. Then get an open-ended spanner and press down on the valves

or get a tube socket same diameter as the valve collar and hit it with a hammer

[xwhatsit]

or get a tube socket same diameter as the valve collar and hit it with a hammer

Yeah I read that somewhere on the net, I tried it, but I was too scared to belt it with any force so it didn't work lol.


Glad to hear it's all back together. Putting the chain/sprocket/camshaft assembly back together is an absolute arse of a job, at least with that design (mine is the same). Done it a few times now but each time still get pinched fingers. Ghey.

[mark247]

I have a question.

To connect the sprocket for the camchain onto the cam shaft you use two bolts. They spin around and around with the camshaft obviously. I didnt use any locking agent on them at all, i just did them up reasonably tight and put it all together, i have ridden 100km without any problems. I was told it is a good idea to put locking agent on them because they could come lose from the spinning and vibrations.

What do you guys reckon i should do?

Keep riding it how it is or take the tappet cover off and lock tight them on?

Has anyone here not lock tighted them on before?

[okeh]

I have a question.

To connect the sprocket for the camchain onto the cam shaft you use two bolts. They spin around and around with the camshaft obviously. I didnt use any locking agent on them at all, i just did them up reasonably tight and put it all together, i have ridden 100km without any problems. I was told it is a good idea to put locking agent on them because they could come lose from the spinning and vibrations.

What do you guys reckon i should do?

Keep riding it how it is or take the tappet cover off and lock tight them on?

Has anyone here not lock tighted them on before?

Go get them welded with the biggest butiest weld ever! So they may never come out again~!

[xwhatsit]

I have a question.

To connect the sprocket for the camchain onto the cam shaft you use two bolts. They spin around and around with the camshaft obviously. I didnt use any locking agent on them at all, i just did them up reasonably tight and put it all together, i have ridden 100km without any problems. I was told it is a good idea to put locking agent on them because they could come lose from the spinning and vibrations.

What do you guys reckon i should do?

Keep riding it how it is or take the tappet cover off and lock tight them on?

Has anyone here not lock tighted them on before?

Put loctite on! That is exactly what caused my top end to seize. Either that or a loose camchain, I can't make up my mind. It's worth it.

Did you replace camchain btw?