Valve guides

[scotto75]

Hi everyone,

I've got oil getting in my cylinder barrel of my CG125 causing lots of blue smoke on start up, so I have assumed it is coming from above. The compression is in the high 190's, just slightly over 200psi with a bit of oil in, so I feel the piston rings are okay.

I've replaced the exhaust valve stem seal, but it's still burning oil. It stops after the bike has been ridden for a period, but leave the bike overnight, and it's smokey again on startup.

I understand that replacing valve guides involves removing the cylinder head and getting an engine shop to do a pro job.

My questions are....

1 - is it worth getting the valve seats done and replacing the valves and seats, piston rings, etc at the same time?

2 - if so, is it not quicker and cheaper just to buy a replacement cylinder head and piston kit?

Any advice will be greatly received!

[Rogerborg]

I'd be costing a complete copy-engine from a Chinese bike.

Try searching for 156FMI or 157FMI, although you will need to check that it's a CG copy as that code can also apply to Suzuki copy-engines.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[stinkwheel]

I wonder if that's it.

I normally associate leaking valve stems with smoke on the overrun (so when you wind off the throttle after a period of being on the power). Probably getting worse with use rather than better as the oil thins.

Is it actually using oil?

An oil control ring can be cracked with no significat loss in compression.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[scotto75]

It's hard to say exactly, without getting someone to follow me watching when it happens, but its's usually worse when I change gear, so releasing the throttle, and pulling away from idle. The second half of my morning journey is on a dual carriageway, an it does feel like it eases a little after this.

I filled it up a week ago and had to put almost 300ml in this morning, so it's using a fair amount as the CG only holds a litre anyway.

[stinkwheel]

Well. If you take the head off, I'd have a good look at the rings too.

I'd imagine a new set of valve guides would be pretty cheap.

Valves themselves are upwards of £20 each.

Valve guides are £17 each.

You can fit and remove them at home by heating the head in the oven and drifting the old ones out. The new ones can be fitted by freezing the guides and heating the head then drifting them in. You need to make a drift out of something soft (aluminium, hardwood) that's slightly smaller than the overall OD of the guide with a hole u the middle that the body of the guide can fit into.

Many people would take it to a machine shop and get them to press it in.

All the above said. You can get a pattern head on ebay for as little as £40 delivered. You'd need to swap your valve gear across and lap in the valves.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[scotto75]

Thanks stink wheel. As you say, a pair of valves and guides will be £40, but for £60 I can get a new head off eBay with the valves and springs already fitted. A set of rings will be another £20, but for £40 I can get a new cylinder and piston kit.

Don't really want to spend £100 on parts and then have the bike out of action for a few days, but I'm guessing it would cost more to have a mechanic do the work.

It needs to get done as I'm not happy cruising about in a cloud of blue smoke or running with only two thirds oil.

[scotto75]

I guess I could give fitting a set of guides a go myself, but always thought it safer to let an experienced professional do any head work. Do you think it's an easy enough job to be able to diy at home or just easier to pay the extra £20 for a head with valves already fitted?

[stinkwheel]

I would personally fit the valve guides on the basis that they are honda parts.

I'm a bit contrary that way though and there's definately more than £20 of work in the job.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Teflon-Mike]

[davebike]

Honda don't list them as a separate part and I not surprised most modern engines have them cast into the head most have flanges inside the head to keep them in place replacing would be a major machining job

I say if you replaced the guide seals you have a worn out or damaged oil control ring or rings

There are poor quality valve guide seals out there and poor quality rings but not so common
____________________
Dave
NC750Xdct + others at work !
davebike1@gmail.com

[stinkwheel]

[Nobby the Bastard]

I thought that they can generally be pressed in and out.

When my (original) exup engine was being stripped, my mechanic checked the valve guides for wear, having said it was't a problem to replacement if they were. As it happes they were fine.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[scotto75]

Thanks for all the replies. If changing the exhaust stem seal hasn't done the job, I too am inclined to fit a new cylinder and head to have peace of mind.

I'm afraid I could do with a bit of clarification regarding this bit though...

[Teflon-Mike]

[scotto75]

[Teflon-Mike]

That's past its best!

Gunked up oil control ring, would be prime-suspect for the blue-gugg effect... BUT, as so many things, it's rarely a singular protagonist but a conspiracy of'em.

Bore will tend to only get scored when the cylinder is so worn out that the rings can pop out the piston & break up & snag, or the motor has siezed and the rings have snagged after being smeared in the piston grooves.

The 'wear' pattern tends to be that from new; metal rubbing on metal, will tend to make the gaps between bits grow, and grow fairly evenly. After initial bedding in, the motor will 'loosen up' and the rate of wear will be fairly constant and apparently negligible; as it adds up though, it will start to accelerate as the piston rings, designed specifically for this, made of springie iron, 'expand' in the grooves to fill the gap as it grows.

Towards the end of service life; a lot of ring is poking out of the piston grooves and less is filling the groove; so they start to loose thier sealing efficiency, and you start to get increased oil-blow by.. but even then, it can be fairly insignificant, for a fair while.

But, it marks the onset of a period of accelerated wear; where with larger clerances, and less sealing efficiency, blow by reducing the amount of lube between cylinder wall and rings, the bore wears out that much faster, and the cycle adds to the causes making it wear faster still.

By the looks sounds that is probably about where you is at!

Bore is beyond service limits and wearing at a quicker rate; piston rings are struggling hence blow-by & gumming of the scraper ring.

Guides? Heh-mu-fug.

New seal on the top, you probably aren't sucking much if any oil down the pipe.... BUT... motor had been run hard enough and long enough to see bore ready for a rebore... how far behind the cylinder wall will the valve guide, and valve seats be?

As mentioned; put to a paid mechanic, 'cost' in the job would be taking bits to pieces and putting them back together again, at £30-60 an hour... in that, £30 piece parts become almost insignificant in the 'cost' of the fix; so even if they aren't completely knackered, replacing as course, so you dont have to spend the big money of pulling the bugger apart again to replace them when they become the weak-link, makes sense.

DIY? Its only saving you money 'cos you don't cost your time.. or like me, figure its a more pleasurable way to waste time than watching a bunch of fellas kick a pigs bladder round a field, or dangling a bit of string in a puddle hoping to out wit a fish! In which vein, buying spanners and gaskets and stuff, CAN actually also be a cheaper way to waste money, than buying season tickets, SKY subscriptions, this years supporters scarf, bait, tackle and Lemsip!

But brings us to a conclusion, do you really NEED to know what the 'cause' is, absolutely, and nail it down to that one main. singular, major protagonist, or even hang all the conspirators in the plot? Or do you just want to fix the problem?

And if its hobby-mechanics, that don't have to earn their keep... well... yeah, you can live with that much more 'risk of failure'... as every failure is an opportunity to play with spanners and more fun to be had... BUT it don't get you to work on Monday Morning!

Personally? have it down to the base gasket; I'd be wobbling the con-rod on big-end, and the crank in the mains, to assess how much life is left in the bottom end; and then probably chucking new cylinder & piston kit on the top, as cheaper, usually than a rebore on these little motors; Head, if cheap as suggested like-wise; and once all back together chucking the left overs into the Little Honda Engine Bits breeding pile in the corner, and forgetting about them, until I need something to chock a Land-Rover wheel or a bit of alli to rebuild a cooling fin or something.... but nothing stopping you leaving them on the bench and cracking out the DTI gauge and mag-stands to measure them up for size and roundness etc and discover just how far gone each protagonist was, IF that's of such interest to you.... But that's me, and once fixed, I have more than enough other teasers to keep me and my spanners entertained!

Like trying to find where halfl my effing volts are escaping from my Range-Rover battery! Or, reconditioning a pair of carburetors for one of the 125 Super-Dreams, OR repainting my Montesa's frame, ORR ooh! that one don't need anything!!! Maybe I should try a new handle-bar & head-lampo arrangement on the 'Seven-fifty'.... it's been avoiding the spanners for TOOO long! Lol!

Post-Script:

Old shedology rule of thumb for valve guides is, without the stem seal or springs in the way, to pop the valve in, and with oily thumb push the valve stem down into the guide until the ball of your thumb is sealing the top of the guide... then rapidly pull the valve about half way down the hole, and let go... see if it springs back under the vacuum!

If it pops back all the way and 'rings' nicely on the seat.. its pretty damn good!

If it pops back about half way or more, its pretty good.

If it sort of sluggishly worms its way back in....take it out, wipe the oil off it, use a cotton bud down the guide to clear gunk out of that, and try again with a light smear of thin oil, like vegie oil for the deep-fat fryer!

If it just drops out... yeah.... its probably 'shot'!

shedologist 'leak test' is to leave the head combustion chamber side up on the bench over night, with the valves in, and the combustion chamber filled with light, veggie oil or two-stroke, see if the level drops.

Petrol or White spirit, anything thinner than diesel, will almost certainly 'leak' by capilliary action past the seats, so not tell you anything.

But will only tell you if the valve seat is 'OK' ish; wont tell you whether anything gets past the guide; hence the 'suck test' above for them.

Rings, likewise, in the bore; pouring oil or petrol into the pot and watching the level wont really tell you much, unless it literally pisses out; its a pretty crude 'laberyth seal' technically; the rings dont form a seal all the way around the bore, they have gaps between the ends, and between themselves and the piston grooves; principle is that the 'path' pressure has to take to get past them is like a maze, rather long and difficult, but it can and will get past. Efficiency of such a seal is reliant on the fact that the pressure in the cylinder isn't constant, and under the effof high pressure of combustion, the tiny % that might get past will be insignificant, and fleeting.. most will stay in the pot and push piston down.

Hence you assess the sealing before strip down with a compression tester 'dynamically', or after strip-down by measurement of clearances.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[MCN]

The only way to determine where the most leakage is coming from is to pressurise the cylinder with compressed air through the plug hole.
Listen for air leaks from the rocker cover, exhaust pipe, inlet manifold.
It is not an exact science but it can help.

To be more precise you need access to the top cover and the crank case which you won't get on the CG125.

The valve guide can be fooked but the seal should keep oil out if good. You can tel if the guides are worn by pushing them side to side with the seals off and look for movement. There should be no movement.
You need a rather specialised tool to measure guide bores too.
Pressing out and pressing in guides is quick. Heating and cooling is cheaper is you don't have access to a press.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[scotto75]

Thanks MCN. A leak down tester is on my birthday wish list, but Mrs Scotto never gets me anything I ask for.

Teflon-Mike - thanks for all the information. That last post is full of genuinely useful information that I'm sure will be of use now and with all the other potential top end fiascos future me has yet to encounter (I'm currently trying to make on CB250 Superdream from two basket cases).

Also, there was a lot of amusing home truths... Little Honda Engine Bits breeding pile made me chuckle.

I don't think I need to know what the cause was, and as you rightly say, it is more likely a combination of factors, but this is a rare opportunity to poke around and feel like a proper experienced motorcycle mechanic and have some first hand experience in those things I read in my Roy Bacon or Mark Zimmerman books or BCF workshop threads!

Regardless of what happens next, you can guarantee I will be trying out the shedology valve and combustion chamber test just for kicks.

[Teflon-Mike]

[Teflon-Mike]

[scotto75]

Wow! You have to hand it to the Superdreams, they're like marmite and hate 'em or love 'em people always seem to have a strong opinion on them.

I'd much prefer to have a CB250 G5, or an AJS 350, than my two bikes. Well, I say bikes, but what I have is two log books and a lockup with a growing Honda Superdream Bits breeding pile.

I've only just got the engine on my workshop table and the task at hand is beginning to dawn on me. Still, I would rather be doing this than anything else, and am naively seeing this like benign the property ladder, and eventually I'll be able to transcend the bedsits for a period semi in the suburbs.

[stinkwheel]

[Teflon-Mike]