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HOW2: Top End Rebuild Honda CB125 (Benley Motor)

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PostPosted: 14:41 - 22 Oct 2011    Post subject: HOW2: Top End Rebuild Honda CB125 (Benley Motor) Reply with quote

Keep getting asked about top end rebuilds; torque values, timing the cam and 'stuff' for these engines. Its all here, BUT lost in the depths of 'Show & Tell'; so; lifting post from thread "Corporal Punishment! Foto-Resto ANOTHER '86 Honda CB125T"
OK, well, a month in, Christmas 'out of the way', and numerouse 'bugs' addressed, the bike was getting slower, not faster.
The quantity of blue smoke emminating from its tail pipe, was increasing almost in direct proportion to the decrase in speed, and down to 45mph, it was fairly clear it needed major motor attension.
The first 'Rebuild' was undrtaken.
With the amount of smoke pouring out the tail, a rebor was almost inevitable. Priced up, a pair of pistons and rings are roughly 60, a rebore about 30, and then you need gaskets, so its a 100 job. Meanwhile, logistically, you strip motor down to the bottom end, thn hoik heads and barels to M/C shop and wait a week for them to be cut, while you twiddle thumbs, so an e-bay special barrel kit for 85, not only saves a couple of quid, but waiting around too!
And I had hoped to have the whole job 'done' from lifting spanner to take engine out, to putting spanners away, engine back in, in a day... in the end it took two, but still not bad going!

Engine out:
Rocker Cover 'off'
Breaker bar to 'crack off' each of the cylinder head studs, in sequence, then ratchet to wind them off.
Then the rocker assemblies can be removed to expose the camshaft, and the cam chain sprocket loosened off.
Removing the cam chain is err... fiddly! But then the cam can be withdrawn from the sprocket and head.
Then the Cylinder Head can be lifted off.... then the barrel. (Makes it all sound SO! easy!)
Cylinder Head studs loosened off with clever 'cam' action stud wrench, then removed.
Gudgeon pin removed from piston, and the pistons removed
and the base gasket face cleaned up.

Time to strip the head.
Smaller of my two Valve Spring Comprssors, was just about the right size, to remove valve collets and drop th valves out.
However, see later; during 'rebuild 2', I adapted it with an old socket and some pvc insulation tape to better gt on the tiny little valves!
breaking with convention, that says strip the head completely, and push valves through a bit of cardboard and number them as to whers they came from, and similarly try and organise collts and springs.... I thought 'bollox', lets just do ONE valve at a time!
Shown here, is the valve stem seal, which crumbled when I tried to remove it, and was possibly as contributory to the excessive oil consumption as worn rings and bore. The kit included new ones, so they were changed.
Next task was to 'lap' the valves in on thier seats, which is a fiff-faff time consuming 'chore' using grinding paste btween valv and seat, and a 'sucker stick' to twist and press valve against stick and rub the past in to grind a nice smooth sealing face, and get a good seal.
This removes any 'deposits' and ought to get rid of any light 'pitting'. Its NOT a 'valve & seat grind', that will take out bad pitting on the seats and put nice sharp sealing 'edge' on the valves.... but my local M/C shop wre not keen to do that job... they didn't have a set of grinding stones small enough!
Mind you, them diddy valves were not easy to get the sucker stick, intended for inch plus car valves onto, either! But I did it!
One valve 'done' and the valve refitted, three more to go!
When all THAT is done, can start putting it all back together.
Pistons, New & Old
As verything during rebuild, it has to be lubricated so that there is 'something' ther to protct baring faces when you first wind the engine over, before starting it and gtting oil pressure.
I use LM or lith grease on most barings and things like the cam-chain and rocker assemblies, as this gives a good 'coating' of lub that dosn't drip away lik oil would, and melts into the oil when the engine first gets warm. Increass oil viscosity a tad, but you ought to do an earluy oil change post rebuild run in anyway.
HOWVER, her I am lubing the pistons with Two Stroke oil. This is lightr than grease or engin oil, BUT has th major bnefit that it is designed to burn. So, lubing the piston and rings, on start up, it will burnb off more easily and not leave th deposits engine oil or grease would. Rings ar 'soft' cast iron, and often slightly pourous, and some people will soak new rings in Two Stroke oil before fitting. I did... wll, long enough toi make a cup of coffee, anyway!
Start with the bottom ring, and work up, BUT in th case of many four strok engins, they hav thre rings, the bottom one an oil control ring, or scraper ring, which is often actually a'compound' ring, as shown.
This has two very THIN and unshapd rings, and a 'wobble' ring or 'mesh' ring between them. Thse can b inordinatly fiddly to get on and in to the piston groove without braking or sticking.
Rings have to be GENTLY walkd onto the piston and then down the skirt into thir respective groove.
Then, built up, ONE gudgon pin fitted. Note the orintation of th piston; its often marked with an arrow and or 'Front' or in this case 'IN' in th valv pocket machining to align with the inlet valve... so that goes at the back of the engin then!
When you fit the pistons to the con rod, you will slid th gudgon pin in from the outsid, as THAT is th asier side to do it from, and you DONT want th pin coming straight out the othr side of the piston, or struggling to fit circlip to the inside, between all the barrel studs, so fit the INSIDE circlips before fitting to rods.
New Pistons fitted to the con rods
Base gasket and Cylinder head studs replaced, and the barrel fitted on.
Head Gasket goes on, and then the cylinder head.
Recycled photo or 'removing' the cam chain, because I forgot to take one putting it back on! Likewise refitting the cam chain sprocket! ALL after the idgery nadgery job of timing the cam in!
And more recycled photos, reversing the sequence, fitting the rocker assemblies, and the cylinder head bolts, after MUCH muggering about getting the cam chain tensioner aligned and bolted up! Cylinder head bolts given 'final' tightening with torque wrench, to book setting, of I think 20ft lb. Not shown because my capable camera woman had gone to bed!
JOB DONE.... for now!
All that remained was to slot motor back into bike.... A-gain... and set it up!

Take note; this shows the major sequences and procedures of the job. It does not show nut and bolt detail, or proviode specific cam timing instructions or bolt by bolt torque settings. Such detailed info is in the Work-Shop Manual, which is the definative guide, for which this is NO substitute, and to which I STILL have to reffer for such numbers and details when I do yet another one of these bludy engines, so if I need it to do one of these motors, habing done umpety of them before, chances are you'll be pretty lost, pretty quick without one too. Just think; manual is 15 and this would be a 300+ job if you had to pay some-one to do it for you. Its NOT a high price to pay for the specific detail information it contains!
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?'

Last edited by Teflon-Mike on 10:39 - 11 Nov 2017; edited 2 times in total
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World Chat Champion

Joined: 15 Jun 2005
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PostPosted: 18:50 - 22 Oct 2011    Post subject: Reply with quote

Good write up.

I especially enjoyed it for two things you don't often see in Haynes manuals:

1) Dirty tools and fingernails

2) Fag smoking assistant (with clean fingernails)

Thumbs Up
Space Monkey #7
Don Eladio is dead. His capos are dead. You have no one left to fight for. Fill your pockets and leave in peace. Or fight me and die!
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World Chat Champion

Joined: 17 Jan 2010
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PostPosted: 12:30 - 23 Oct 2011    Post subject: Reply with quote

Very good Mr Teff and your spangly assistant Thumbs Up

I learn much from you Thumbs Up
I am the sum total of my own existence, what went before makes me who I am now!
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There is a gap of 6 years, 34 days between these two posts...

L Plate Warrior

Joined: 24 Nov 2017
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PostPosted: 09:08 - 25 Nov 2017    Post subject: Reply with quote

Does the aftermarket cylinder & piston kit have provision for the seals that go over the stud dowels? Or does it just rely on the gasket sealing the oil passages?

Tried to pm you Teflon Mike, but my message was deemed to be spam and now it says I've sent too many messages recently...
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The last post was made 2 years, 182 days ago. Instead of replying here, would creating a new thread be more useful?
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